Identification of auroral emission altitudes associated with relativistic electron precipitation events observed by ISS-CALET

Identification of auroral emission altitudes associated with relativistic electron precipitation events observed by ISS-CALET | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Identification of auroral emission altitudes associated with relativistic electron precipitation events observed by ISS-CALET KYUTARO YANAGISAWA, Ryuho Kataoka, Kanako Seki, Daniel Whiter, and 9 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8545933/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 5 You are reading this latest preprint version Abstract We report a multi-MeV relativistic electron precipitation (REP) event as observed by the CALorimetric Electron Telescope and the Monitor of All-sky X-ray Image aboard the International Space Station together with low-cost twin all-sky auroral imagers installed at Athabasca, Canada. We find that visible aurora associated with REP is not frequent—observed in only two of 10 conjugate REP events during our survey period from September 2024 to September 2025—and that one case, on 3 May 2025, featured diffuse aurora near the ISS magnetic footprint. Using two independent stereoscopic methods, we estimate the auroral emission height to be ~ 90 km, which is typically due to tens-of-keV electron precipitation. Combined with the MeV electron precipitation detected by CALET, such broadband electron precipitation from tens-of-keV to MeV is consistent with the hypothesis of chorus-driven REP events. The emission altitude shows no systematic latitudinal variation, which is not consistent with field-line curvature scattering type energy-dispersed precipitation. The combined space–ground conjunction observations contribute to better understanding of the spatial context of wave–particle interaction associated with the REP events. Electron precipitation atmospheric ionization aurora stereoscopy space weather forecast Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 1 Introduction Relativistic electrons with energies of several hundred keV to a few MeV occasionally precipitate into the Earth’s atmosphere with various spatial and temporal scales. Such relativistic electron precipitation (REP) events have been first detected by ground-based observations (Anderson and Milton, 1964 ; Bailey & Pomerantz, 1965 ; Rosenberg et al., 1972 ) and later by balloon and satellite measurements (Anderson et al., 1968 ; Imhof et al., 1986 ; Lorentzen et al., 2000 ; Nakamura et al., 1995 , 2000 ; Comess et al., 2013 ; Carson et al., 2013 ). The importance of REP events can be categorized as: 1) Revealing their causes contributes to predictive understanding of the dynamic variation of the outer radiation belt, because the REP events are associated with a major loss process for trapped MeV electrons (Lorentzen et al., 2000 ; Kubota et al., 2015 ; Kurita et al., 2018 ; Millan et al., 2002 ; Miyoshi et al., 2008 , 2015 ). 2) The REP-related ionization of the middle atmosphere causes the production of NOx and HOx and subsequent ozone destruction (Daae et al., 2012 ; Isono et al., 2014a , b ; Kataoka et al., 2019 ; Miyoshi et al., 2015 , 2021 ; Turunen et al., 2016 ; Murase et al., 2022 ). 3) Radiation dose exposure at LEO, including to the body of astronauts, can be caused by REP events. Ueno et al. ( 2019 ) quantitatively evaluated the exposure dose rate of REP events during extravehicular activity at the ISS, although the total effects are not severe even for the largest REP events. At least two major types of plasma waves have been known and well-studied as the main causes of REP events; electromagnetic ion cyclotron (EMIC) waves and whistler-mode chorus waves. EMIC waves can resonate with MeV electrons as well as protons of tens of keV (e.g., Kubota et al., 2015 ; Miyoshi et al., 2008 ), scattering of the latter being able to produce so-called isolated proton aurora (IPA) (Sakaguchi et al., 2016 ). The expected emission altitude of the IPA ranges from 115 km to 135 km (e.g., Liang et al., 2022 ; Shumko et al., 2022 ; Nakamura et al., 2022 ). Chorus waves can also cause REP events. Chorus waves propagating toward the Earth along magnetic field lines can cause broad-band electron precipitation ranging from a few keV to MeV leading to both diffuse aurora and REP events (Miyoshi et al., 2020 ). The chorus model has been confirmed by several conjunction observations between optical images from the ground and satellite/sounding rockets (Miyoshi et al., 2015 ; Kurita et al., 2015 , Kawamura et al., 2021 , Shumko et al., 2021 , Namekawa et al., 2023 ). Although these observations were not directly related to the REP events, the emission altitude of such a diffuse aurora is well below 100 km (Brown et al., 1976 ; Hosokawa et al., 2024 ). Kataoka et al. ( 2016 ) first utilized the CALorimetric Electron Telescope (CALET) aboard the ISS to perform observations of the REP events. Since then, this instrument has been used to characterize the temporal and spatial properties of REP events (e.g., Kataoka et al., 2020 ; Bruno et al., 2022 ; Blum et al., 2024 , Freund et al., 2024 , 2025 ; Vidal-Luengo et al., 2024a , b ). Further, Zhang et al. ( 2025 ) conducted a statistical classification of the REP events driven by different causes such as chorus, EMIC, and field-line curvature scattering (FLCS). While EMIC-driven REP events have been associated with proton aurora with emission altitudes exceeding 100 km, as expected from IPA (Liang et al., 2022 ), the emission altitude of aurora directly linked to chorus-driven REP events has not yet been quantitatively determined. In the case of broadband electron precipitation from tens-of-keV to MeV energies driven by propagating chorus waves, the lower-energy electrons are expected to cause optical emission at below 100 km. The purpose of this paper is to spatially characterize the mechanisms of REP events by identifying the footprint auroras of the conjugate REP events and estimating the emission altitude. By doing so we also verify the effectiveness of altitude estimation as a diagnostic tool for better understanding the mechanisms of REP events and demonstrate that ground-based stereoscopic imaging can complement point-like satellite observations by providing additional spatial and temporal context. Section 2 describes the instrumentation and data used in this study. Section 3 explains the methods for identifying REP–aurora conjunction events and estimating auroral emission altitudes. Section 4 presents the results of these analyses. Section 5 discusses the implications of the findings in the context of REP drivers and wave–particle interactions. Section 6 summarizes the conclusions of this study. 2 Instrumentation and Data In this study, we investigated the characteristics and altitudes of auroral emissions associated with REP by conducting conjunction observations using space-based measurements from the ISS and ground-based data from all-sky imagers installed in Athabasca, Canada. The ISS carries several scientific instruments capable of detecting energetic particles. Among them, CALET and Monitor of All-sky X-ray Image (MAXI) are suitable for identifying REP events (Kataoka et al., 2016 ). 2.1 Instruments on the International Space Station (ISS) CALET has been operating onboard the ISS since 2015 to measure high-energy cosmic-ray electrons and nuclei (Asaoka et al., 2018 ; Torii, 2017 ). The instrument includes the Charge Detector (CHD), composed of segmented plastic scintillators arranged in two layers (CHD-X and CHD-Y) to measure the electric charge. The trigger counter signals from these layers are accumulated every 1 s. Although CHD was originally designed for cosmic-ray observations, its 1 s count rates can also be used to monitor MeV electron precipitation events in the radiation environment (Kataoka et al., 2016 ). In this study, we utilize the count rates of CHD-X and CHD-Y, which are sensitive to electrons above 1.5 MeV and 3.4 MeV, respectively. The Radiation Belt Monitor (RBM) of the MAXI instrument (Matsuoka et al., 2009 ) detects electrons and protons above 0.3 and 3 MeV, respectively, with a 1s time resolution. Two identical sensors are installed: RBM-H, directed horizontally (limbward), and RBM-Z, directed vertically (zenith). RBM data therefore complements CALET by providing directional information and coverage of lower-energy precipitation (0.3–1.5 MeV). Following the method of Bruno et al. ( 2022 ), REP events were identified based on the ratio of count rates in the two CHD layers: R xy = N x / N y > 1 + 3σ Rxy where N X and N Y denote 1-s count rates from CHD-X and CHD-Y, respectively, and σ Rxy is the standard deviation of R XY computed over 10 min time window. In addition, to confirm that the observed CHD ratio enhancements were caused by actual REP rather than orbital effects (e.g., variations associated with the ISS trajectory through the radiation belts), we imposed further criterion on the RBM data. The criterion is that the RBM-Z counts, sensitive to energetic electron precipitation, exhibit a burst-like increase relative to the RBM-H counts, as identified by visual inspection. Since RBM-Z is oriented perpendicular to the Earth's surface, an enhancement in RBM-Z relative to RBM-H indicates precipitation directed earthward, thus supporting the identification of REP. 2.2 Ground-Based All-Sky Camera Observations in Athabasca To investigate visible auroral emissions associated with REP events, we installed two identical all-sky cameras, ZWO ASI 183mm Pro equipped with Fujino fish-eye lens (FF185C086HA-1) at observation sites ~ 24.5 km apart, AUGO (54.71N, 113.31W) and AUGSO (54.60N, 113.64W) in Athabasca, Canada. To control the camera, we use a single-board computer (Raspberry Pi 4). Figure 1 shows the appearance of the camera configuration. We set a 427.8 nm optical filter between the lens and sensor. The selected wavelength corresponds to N₂⁺ first negative band, because auroral emissions at this wavelength are mainly produced by precipitating electrons with energies of several tens of keV. We set the exposure time to be 18 seconds, with images taken every 20 seconds. This stereoscopic configuration, combined with long-exposure imaging and high-sensitivity optics, enabled the detection of faint diffuse auroral emissions above 1.6 Rayleigh, potentially associated with REP events. The camera was configured with a 4×4 on-chip binning mode and an effective sensor ROI of 3648 × 3648 pixels, producing 912 × 912 pixel 16-bit images. 3 Methods 3.1 Selection and Classification of REP Events From the available dataset, we selected conjugate REP events that met magnetic conjugacy with Athabasca observation sites by tracing the ISS orbit using the International Geomagnetic Reference Field (IGRF)-13 geomagnetic field model. We required that the magnetic footprints be observed with elevation angles greater than 30° from both all-sky imagers, and that the events also satisfied and temporal coincidence with auroral emissions observed by the cameras. We analyzed the REP events observed during the period from September 2024, when our observations started, to September 2025. We identified a total of 23 conjugate REP events, among which 10 occurred under clear-sky conditions. The observation of the remaining 13 REP events was affected by clouds, snow, or strong moonlight, preventing reliable auroral imaging. Among the selected 10 events, two REP events showed auroral emissions near the ISS footprint. Another five REP events exhibited auroral activity in regions spatially displaced from the footprint, while the remaining three REP events showed emission levels below 1.6 Rayleigh, due to low intensity or background noise. 3.2 Estimation of Auroral Emission Altitude For analysis of stereoscopic observations from two all-sky cameras (AUGO and AUGSO) in Athabasca with overlapping all-sky fields of view, we use two different altitude estimation methods, which complement each other as explained below. The first method is based on finding the geographical transform which gives the maximum correlation (Kataoka et al., 2013 ). We projected the all-sky images from both sites onto a horizontal plane at a series of assumed emission altitudes in geographic coordinates. For each assumed altitude, we clipped the projected images into regions, we calculated the cross-correlation coefficient between the image pairs from the two sites for each region. We interpreted the altitude that gave the maximum correlation as the auroral emission height. We applied this geographical transform method within a restricted region of 52°-56° N and 111°-116° W to minimize geographic distortion in the projection. To ensure the reliability of the estimated emission altitude, we retained only the solutions that satisfied two quality criteria: (i) a peak correlation coefficient greater than 0.7, and (ii) a full width at 95% maximum of the correlation curve less than 50 km. These criteria allowed us to remove poorly constrained or multi-peaked solutions and ensured physically meaningful altitude estimates. The second method is based on magnetic field line tracing (Whiter et al., 2013 ): We traced geomagnetic field lines using the IGRF-13 model and overlaid them onto the all-sky images. For each field line, we identified the corresponding pixels and convert their values into auroral emission intensity in Rayleigh. Then, assuming that the emission occurs along the field line, we constructed an altitude profile of aurora emission intensity by mapping the intensity as a function of altitude along each line. The emission altitude was calculated by averaging only the profiles that satisfied our selection criteria at the two observation sites. We imposed five selection criteria. These include: (1) agreement of peak altitudes at the two sites within ± 5 km; (2) exclusion of peaks occurring within 10 km of the prescribed altitude bounds to avoid edge effects; (3) consistency of emission centroids within 20 km; (4) a correlation coefficient greater than 0.8 between the two Rayleigh–altitude profiles; and (5) a full width at 90% maximum of the correlation curve less than 50 km to ensure a well-defined emission layer. The geographical transform method is most effective for diffuse aurora near the magnetic zenith, where auroral structures are thin and horizontally extended. However, this technique is affected by geographic distortion and limited spatial resolution, making it not suitable for tall discrete aurora especially near the edge of all-sky field-of-view. On the contrary, the field-line tracing method is more appropriate for discrete aurora with well-defined vertical structures, even for auroral forms at low elevation angles. Nevertheless, this method becomes unreliable for thin diffuse aurora near the magnetic zenith, as such emissions do not show distinct ray structure associated with the magnetic field lines. By combining the two approaches, we can obtain a more reliable altitude estimate across the entire all-sky field-of-view, for any types of auroral morphology. 4 Results An overview of the selected REP event is summarized in Fig. 2 . In the top panel, the REP event appeared on 3 May 2025 between 09:33 and 09:39 UT, when the ISS footprint moved from MLAT 48° to 60° across the dawn side sector (01–04 MLT) as shown in the bottom panel. The CHD-X counts began to increase at 09:33:30 UT, reached a maximum of ~ 4.0 × 10³ counts s⁻¹ at 09:34:30 UT, and gradually decreased until 09:37 UT before dropping sharply to the CHD-Y level (~ 2.0 × 10³ counts s⁻¹) by 09:39 UT. The R XY (= CHD-X counts / CHD-Y counts) reached ≈ 2.0 at 09:34:30 UT, and the CHD-Y counts remained nearly constant around ~ 2.0 × 10³ counts s⁻¹ throughout the interval. The RBM-Z channel began to increase at 09:34:00 UT, about 30 s after the CHD-X increase, and showed multiple burst-like spikes between 09:34 and 09:37 UT. The count rate increased from ~ 10² to 10⁴ s⁻¹. The amplitude of the short spikes, each lasting a few seconds, became largest at 09:36 UT, when the RBM-Z/RBM-H count ratio ≈ 1.0. The RBM-H channel maintained a high baseline (10⁴–10⁵ counts s⁻¹) and gradually increased over the same time interval. Count rates in CHD-X and RBM-Z exhibited a rapid increase within approximately one minute around 09:33 UT and returned to their background levels by 09:39 UT, when the ISS footprint was located near MLAT ≈ 58°. These observations suggest that electrons in the 0.3–3.4 MeV energy range precipitated above MLAT 58°. Figure 3 a shows the altitude–correlation profile derived from the geographical transform method applied to the auroral images at 09:33:40 UT. We computed the correlation in the region indicated by the magenta frame in Fig. 3 b (52-54N, 111-112.5W). The profile exhibits a clear peak around 95 km, and we interpret this altitude as the most probable emission height of the diffuse aurora inside the analysis region. Figure 3 b summarizes the spatial distribution of the inferred emission altitude. To enhance the visibility of fine auroral structures, we applied a σ-based contrast enhancement, in which the pixel values were linearly scaled so that the mean and ± 1σ of the clear-sky background correspond to the mid-level and the upper/lower limits of the displayed grayscale range, respectively. The overplotted MLT and MLAT isolines (red and blue) help to identify the geomagnetic context of the emission. The analyzed region lies near ~ 1.2–1.6 MLT and MLAT 60° to 64° at this event. The red dashed line marks the ISS magnetic footprint, which passed near 52.5N, 112W~113W during these 20 s (09:33:40 ~ 09:34:00 UT). The ISS magnetic footprint (black dotted line) passed near to the edge of the diffuse aurora. The auroral emission was spatially extended and modulated. We also note that the all-sky image shows a mixture of diffuse aurora and curtain-like forms. This image shows the best condition in which the ISS passed over the structured aurora at a relatively high elevation angle from the ground-based observation sites. We do not observe a latitudinal trend within the analysis region. However, the emission altitude increases slightly from west (~ 80 km) to east (~ 90–100 km). While the western part of the analysis region is dominated by patchy diffuse structures, the eastern part includes more pronounced curtain-like aurora, which likely contributes to the slightly higher inferred altitudes on the east side. Figure 4 shows the results from the magnetic field line tracing method. In Fig. 4 a, only the field lines that satisfied all criteria in the profiles obtained from AUGSO and AUGO are depicted. Different colors denote the eastern-side field lines in different regions R1–R3. We analyzed magnetic field lines traced over the southeastern part of the image (52°-55° N, 109°-112.5° W). On the western side of the aurora, we do not identify any field lines that meet our criteria, possibly because patchy and horizontally extended structures dominate the morphology and fail to produce well-defined peaks in the Rayleigh–altitude profiles. Figure 4 b shows the averaged emission altitude profile of the eastern-side field lines in regions R1–R3 shown in Fig. 4 a. The obtained peak emission altitudes are ~ 95 km in R1 (52°-53°N, 109°-111°W), ~ 85 km in R2 (53°-54°N, 111°-112.5°W), and ~ 80 km in R3 (54°-55°N, 111°-112.5°W). The emission layer extends over ~ 15 km in altitude below 100 km, consistent with the optical emissions produced by tens-of-keV electron precipitation. From these two different methods, we conclude that the emission altitude is 80–95 km during this particular REP event. Figure 5 shows the auroral emission altitude maps from the geographical transform method at three different times on 3 May 2025: 09:26 UT, 09:28 UT, and 09:48 UT. These maps show both temporal and spatial variations in the estimated emission altitudes. Notably, regions exhibiting patchy aurora—compact and well-defined structures—tend to show lower emission altitudes, typically in the range of 70–85 km. In contrast, areas appearing as diffuse aurora are associated with higher altitudes, generally around 90–100 km. This trend suggests that auroral morphology has a correlation with the estimated altitude. At 09:26 UT, patchy aurora dominated both the western and eastern regions, whereas the southern and near-zenith areas displayed more diffuse structures, corresponding to higher estimated altitudes. A similar pattern was observed at 09:28 UT, where patchy aurora in the western and northeastern sectors were associated with lower altitudes, while more diffuse aurora in the southeastern region exhibited higher emission altitudes. At 09:48 UT, the region near the zenith became dominated by diffuse aurora, leading to higher altitude estimates compared to the more structured patchy aurora in the southwestern to southern areas. These results demonstrate a consistent relationship between auroral morphology and emission altitude, where patchy aurora is linked to lower altitudes and diffuse aurora tends to originate from higher altitudes. 5 Discussion In this study we installed a new stereoscopic all-sky imager system (Fig. 1 ) for investigating possible conjugate footprint aurora under the REP events as observed by CALET onboard ISS. The results shown above can be summarized as follows: We identified a REP event on 3 May 2025, which showed electrons in the 0.3–3.4 MeV energy range precipitating along magnetic field-lines crossed by the ISS orbit between 09:33 and 09:39 UT (Fig. 2 ). Using ground-based optical observations at Athabasca, the ISS magnetic footprint passed near the edge of the diffuse aurora (Fig. 3 b). From the geographic transform method, we estimated the emission altitude to be ~ 95 km (Fig. 3 a). Using the magnetic field line tracing method, we obtained consistent results (Fig. 4 ), typically associated with tens-of-keV electron precipitation. In addition, the auroral emission altitude showed no systematic latitudinal variation (Figs. 3 b and 4 b), while a clear longitudinal trend emerged (Fig. 3 b), with lower altitudes in the west and higher altitudes in the east. Figures 3 and 5 collectively demonstrate a consistent spatial correspondence between auroral morphology and estimated emission altitude. Note first that identifying a chorus-related auroral emission itself is not a new finding in itself. There are many examples of pulsating auroras, which are associated with chorus waves. Kurita et al. ( 2015 ) identified REP event-related pulsating auroras and demonstrated the link between chorus-driven precipitation and auroral modulation. As also summarized in the Introduction, the emission altitude of aurora at the ISS footprint provides a diagnostic for the wave–particle interaction type: chorus-driven REP events tend to produce auroral emissions below 100 km, whereas EMIC-driven REP is typically linked to aurora emissions above 100 km. The results obtained in this study are consistent with the former case (Kurita et al., 2015 ), suggesting that the analyzed event was chorus-driven REP events. The key advance of this study is that we determined the emission altitude of REP-related aurora stereoscopically with two independent methods. We confirmed the corresponding REP events are in the MeV and sub-MeV range, and we identified that the footprint aurora is most likely produced by tens-of-keV electrons. The broadband electron precipitation, spanning tens of keV to a few MeV, can be a consequence of electron resonance with duct-propagating chorus waves (Miyoshi et al., 2020 ). We found no systematic latitudinal variation in the auroral emission altitude. FLCS is expected to produce energy-dispersed precipitation, in which higher-energy electrons precipitate at lower magnetic latitudes and therefore generate lower emission altitudes toward the equator (Sivadas et al., 2019 ; Murase et al., 2022 ). Such a signature was not observed in our data. Instead, the altitude remained nearly uniform at ~ 90 km across latitude, indicating that tens-of-keV electrons precipitated broadly and simultaneously. This uniformity is inconsistent with FLCS-driven precipitation and is instead consistent with chorus-driven broadband precipitation. Patchy aurora, characterized by compact and well-defined structures, tend to occur at lower altitudes (70–85 km), while diffuse aurora—appearing more blurred or faint —are associated with higher emission altitudes (90–100 km). This systematic trend suggests that auroral morphology provides a meaningful indicator of emission altitude. However, this relationship is partially affected by the spectral sensitivity of the imaging system. The 427.8 nm wavelength is primarily sensitive to high-energy electron precipitation (tens of keV), and emissions driven by lower-energy electrons (a few keV), which typically occur at higher altitudes, are less efficiently detected at this wavelength. As a result, such aurora appear more diffuse simply due to reduced optical response—introducing an apparent “morphology–altitude” effect shaped by observational bias. Therefore, our findings highlight both the utility and the limitations of single-wavelength imaging in diagnosing auroral altitude and particle origins. Future studies will pursue coordinated multi-wavelength observations to improve the accuracy of morphological classification and altitude estimation, and to better resolve the underlying energy characteristics of precipitating particles. We also demonstrate that REP-related footprint auroral emissions are not frequent, based on a systematic survey of conjugate REP events. In our year-long observation period, we found that only two out of ten conjugate REP events exhibited visible aurora near the magnetic footprint. The use of a 427.8 nm optical filter is one possible reason to understand the infrequent occurrence of footprint auroras. Note that the 557.7 nm Oxygen line basically provides the strongest optical signature of EMIC-related proton precipitation (Sakaguchi et al., 2007, Liang et al., 2022 ), whereas the 427.8 nm emission used in this study has a weaker response to the proton aurora. Given that our imager system is therefore less sensitive to the EMIC-related proton aurora, our findings that only two out of 10 REP events which exhibited optical footprint counterparts, and both of those events are likely chorus related, is not inconsistent with the recent statistical result of Zhang et al. ( 2025 ) where they noted that EMIC-related REP events are comparable or more frequent than chorus-driven REP events. We modernized the ground-based experimental setup by utilizing a low-cost single-board computer for controlling the all-sky camera, also setting an optical filter between the lens and sensor to make the imager smaller. We demonstrated that the compact and twin imager systems can effectively complement satellite measurements by providing quantitative information on auroral emission altitudes as well as the 2D spatial context which is complemental to in-situ satellite observations. For future work, we aim to further reduce the cost without losing the sensitivity, to expand the all-sky observation network, hopefully via the spreading help of citizen-science participation (Kataoka et al., 2024 ; 2025 ). 6 Conclusions We analyzed the REP event on 3 May 2025 and identified MeV and sub-MeV electron precipitation accompanied by diffuse aurora near the ISS magnetic footprint. We modernized the stereoscopic all-sky camera observations, then we estimated the auroral emission altitude to be ~ 90 km, which is typically due to tens-of-keV electrons. Such broadband electron precipitation from tens-of-keV to MeV is consistent with the chorus-driven REP events, and the absence of systematic latitudinal trend in the emission altitude is not consistent with FLCS-driven REP events. Our results demonstrate that compact and low-cost all-sky imagers can effectively complement the in-situ satellite observations by providing auroral altitude and spatial context. Abbreviations REP Relativistic electron precipitation EMIC Electromagnetic ion-cyclotron IPA Isolated proton aurora CALET CALorimetric Electron Telescope CHD CHarge Detector MAXI Monitor of All-sky X-ray Imag RBM Radiation Belt Monitor ISS International Space Station LEO Low-Earth Orbit FLCS Field-Line Curvature Scattering IGRF International Geomagnetic Reference Field Declarations Ethics approval and consent to participate Not applicable Consent for publication Not applicable Availability of data and materials The datasets analyzed in this study are publicly available through the Data Archives and Transmission System (DARTS) operated by the Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA). The CALET Charge Detector (CHD) Level 1.1 data are available at: https://darts.isas.jaxa.jp/pub/calet/cal-v1.1/CHD/level1.1/obs/2025/ The MAXI Radiation Belt Monitor (RBM) data are available at: https://darts.isas.jaxa.jp/pub/maxi/rbm/2025/ Ground-based auroral images obtained at Athabasca are not publicly archived due to limited storage and data privacy policies, but they are available from the corresponding author upon reasonable request. Competing interests The authors declare that they have no competing interests. Funding R.K. and S. T. are supported by JSPS-KAKENHI 24H00025, R. K. and K.S. are supported by JSPS-KAKENHI (22K21345 (PBASE program)). D. W. was supported by the Natural Environment Research Council (NERC) of the UK under grant NE/V012541/1. Initial construction and operation of Athabasca University observatory facilities were supported by the Canada Foundation for Innovation. M.C. holds NSERC funding. The development and calibration of the camera were supported by the Hoso Bunka Foundation. Authors' contributions K.Y. conducted the related research activity under the supervision of R.K. and K.S. S. T. advised the use of CALET CHD data, while S. N. advised the use of MAXI RBM data. D. W. advised the field-line mapping method. R. K. advised the geographic transform method. Y. M. and K. S. supported the installation of the auroral cameras at AUGSO and AUGO, with help from M. C. and R. A. at Athabasca University. T. S. designed the imager system and provided the observation software used in this study. AB and LB discussed the CALET observation of REP events and the mechanisms. Acknowledgements YK thanks the student support program of the PBASE project (PI: K. Shiokawa) for observation trips to Canada and research visits to Southampton University as well as to OIST. YK also acknowledges the technical support for the all-sky camera development and calibration works under the JARE AJ1007 Project (PI: R. Kataoka). Authors' information (1 Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan , (2 Okinawa Institute of Science and Technology, 1919-1 Tancha, Onna, Kunigami District, Okinawa 904-0495, Japan, (3 Research Center for Advanced Science and Technology (RCAST), The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan, (4 Department of Physics and Astronomy, University of Southampton, University Road, Southampton SO17 1BJ, United Kingdom, (5 Institute for Space-Earth Environmental Research, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8601, Japan, (6 Department of Physics & Astronomy, Athabasca University, Athabasca, Alberta, Canada T9S 3A3, (7 Waseda Research Institute for Science and Engineering, Waseda University, 17 Kikui-cho, Shinjuku-ku, Tokyo 162-0044, Japan (8 Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA), 3-1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 252-5210, Japan, (9 Graduate School of Science, Tohoku University, 6-3 Aramaki-aza Aoba, Aoba-ku, Sendai, Miyagi 980-8578, Japan, (10 Heliophysics Science Division, NASA Goddard Space Flight Center, 8800 Greenbelt Rd, Greenbelt, MD 20771, United States, (11 Laboratory for Atmospheric and Space Physics and Department of Aerospace Engineering Sciences, University of Colorado Boulder, 1234 Innovation Drive, Boulder, CO 80303-7814, United States References Anderson, K. A., D. W. Milton (1964). Balloon observations of X rays in the auroral zone, 3, High time resolution studies. Journal of Geophysical Research, 69, 4457–4479. https://doi.org/10.1029/JZ069i021p04457 Anderson, H. R., Hudson, P. D., & McCoy, J. E. (1968). Observations of POGO ion chamber experiment in the outer radiation zone. Journal of Geophysical Research, 73(19), 6285–6297. https://doi.org/10.1029/JA073i019p06285 Asaoka, Y., Ozawa, S., Torii, S., Adriani, O., Akaike, Y., Asano, K., et al. (2018). On‐orbit operations and offline data processing of CALET onboard the ISS. Astroparticle Physics, 100, 29–37. https://doi.org/10.1016/j.astropartphys.2018.02.010 Bailey, D. K., & Pomerantz, M. A. (1965). Relativistic electron precipitation into the mesosphere at subauroral latitudes. Journal of Geophysical Research, 70(23), 5823–5830. https://doi.org/10.1029/JZ070i023p05823 Blum, L. W., Bruno, A., Capannolo, L., Ma, Q., Kataoka, R., Torii, S., & Baishev, D. (2024). On the spatial and temporal evolution of EMIC wave-driven relativistic electron precipitation: Magnetically conjugate observations from the Van Allen probes and CALET. Geophysical Research Letters, 51(5), e2023GL107087. https://doi.org/10.1029/2023GL107087 Brown, N. B., Davis, T. N., Hallinan, T. J., & Stenbaek-Nielsen, H. C. (1976). Altitude of pulsating aurora determined by a new instrumental technique. Geophysical Research Letters, 3(7), 403–404. https://doi.org/10.1029/gl003i007p00403 Bruno, A., Blum, L. W., de Nolfo, G. A., Kataoka, R., Torii, S., Greeley, A. D., et al. (2022). EMIC-wave driven electron precipitation observed by CALET on the international space station. Geophysical Research Letters, 49(6), e97529. https://doi.org/10.1029/2021GL097529 Carson, B. R., Rodger, C. J., & Clilverd, M. A. (2013). POES satellite observations of EMIC‐wave driven relativistic electron precipitation during 1998–2010. Journal of Geophysical Research: Space Physics, 118, 232–243. https://doi.org/10.1029/2012JA017998 Comess, M. D., Smith, D. M., Selesnick, R. S., Millan, R. M., & Sample, J. G. (2013). Duskside relativistic electron precipitation as measured by SAMPEX: A statistical survey. Journal of Geophysical Research: Space Physics, 118, 5050–5058. https://doi.org/10.1002/jgra.50481 Daae, M., Espy, P., Nesse Tyssoy, H., Newnham, D., Stadsnes, J., & Soraas, F. (2012). The effect of energetic electron precipitation on middle mesospheric night‐time ozone during and after a moderate geomagnetic storm. Geophysical Research Letters, 39, L21811. https://doi.org/10.1029/2012GL053787 Freund, D., Blum, L., Huang, J., Vidal-Luengo, S., Bruno, A., & Kataoka, R. (2025). Radiation-belt dropouts: Relationship with geomagnetic storms and MeV precipitation. Geophysical Research Letters, 52(4), e2024GL113660. https://doi.org/10.1029/2024GL113660 Freund, D., Blum, L., Vidal-Luengo, S., Bruno, A., & Kataoka, R. (2024). MeV electron precipitation during radiation belt dropouts. Journal of Geophysical Research: Space Physics, 129(8), e2024JA032759. https://doi.org/10.1029/2024JA032759 Hosokawa, K., Miyoshi, Y., Mcharg, M., Ledvina, V., Hampton, D., Lessard, M., et al. (2024). Variation of the altitude of auroral emission during a substorm cycle: Stereoscopic optical observations during the lamp rocket experiment. Journal of Geophysical Research: Space Physics, 129(11), e2024JA033036. https://doi.org/10.1029/2024JA033036 Imhof, W. L., Voss, H. D., Reagan, J. B., Datlowe, D. W., Gaines, E. E., Mobilia, J., & Evans, D. S. (1986). Relativistic electron and energetic ion precipitation spikes near the plasmapause. Journal of Geophysical Research, 91(A3), 3077–3088. https://doi.org/10.1029/JA091iA03p03077 Isono, Y., Mizuno, A., Nagahama, T., Miyoshi, Y., Nakamura, T., Kataoka, R., et al. (2014a). Variations of nitric oxide in the mesosphere and lower thermosphere over Antarctica associated with a magnetic storm in April 2012. Geophysical Research Letters, 41, 2568–2574. https://doi.org/10.1002/2014GL059360 Isono, Y., Mizuno, A., Nagahama, T., Miyoshi, Y., Nakamura, T., Kataoka, R., et al. (2014b). Ground‐based observations of nitric oxide in the mesosphere and lower thermosphere over Antarctica in 2012–2013. Journal of Geophysical Research: Space Physics, 119, 7745–7761. https://doi.org/10.1002/2014JA019881 Kataoka, R., Miyoshi, Y., Shigematsu, K., Hampton, D., Mori, Y., Kubo, T., et al. (2013). Stereoscopic determination of all‐sky altitude map of aurora using two ground‐based Nikon DSLR cameras. Annals of Geophysics, 31(9), 1543–1548. https://doi.org/10.5194/angeo‐31‐1543‐2013 Kataoka, R., Asaoka, Y., Torii, S., Terasawa, T., Ozawa, S., Tamura, T., et al. (2016). Relativistic electron precipitation at International Space Station: Space weather monitoring by Calorimetric Electron Telescope. Geophysical Research Letters, 43, 4119–4125. http://doi.org/10.1002/2016GL068930 Kataoka, R., Nishiyama, T., Tanaka, Y., Kadokura, A., Uchida, H. A., Ebihara, Y., et al. (2019). Transient ionization of the mesosphere during auroral breakup: Arase satellite and ground‐based conjugate observations at Syowa Station. Earth Planets and Space, 71(1), 9. https://doi.org/10.1186/s40623‐019‐0989‐7 Kataoka, R., Asaoka, Y., Torii, S., Nakahira, S., Ueno, H., Miyake, S., et al. (2020). Plasma waves causing relativistic electron precipitation events at international space station: Lessons from conjunction observations with arase satellite. Journal of Geophysical Research: Space Physics, 125(9), e27875. https://doi.org10.1029/2020JA027875 Kataoka, R., Reddy, S.A., Nakano, S. et al. Extended magenta aurora as revealed by citizen science. Sci Rep 14, 25849 (2024). https://doi.org/10.1038/s41598-024-75184-9 Kataoka, R., Nakano, S., Uchino, S. et al. Extended red aurora associated with super substorm igniting the October 10, 2024 magnetic storm as revealed by citizen science. Earth Planets Space 77, 64 (2025). https://doi.org/10.1186/s40623-025-02178-w Kawamura, M., Sakanoi, T., Fukizawa, M., Miyoshi, Y., Hosokawa, K., Tsuchiya, F., et al. (2021). Simultaneous pulsating aurora and microburst observations with ground-based fast auroral imagers and CubeSat FIREBIRD-II. Geophysical Research Letters, 48(18), e2021GL094494. https://doi.org/10.1029/2021GL094494 Kubota, Y., Omura, Y., & Summers, D. (2015). Relativistic electron precipitation induced by EMIC‐triggered emissions in a dipole magnetosphere. Journal of Geophysical Research: Space Physics, 120, 4384–4399. https://doi.org/10.1002/2015JA021017 Kurita, S., Kadokura, A., Miyoshi, Y., Morioka, A., Sato, Y., & Misawa, H. (2015). Relativistic electron precipitations in association with diffuse aurora: Conjugate observation of SAMPEX and the all sky TV camera at Syowa station. Geophysical Research Letters, 42(12), 4702–4708. https://doi.org/10.1002/2015GL064564 Kurita, S., Miyoshi, Y., Shiokawa, K., Higashio, N., Mitani, T., Takashima, T., et al. (2018). Rapid loss of relativistic electrons by EMIC waves in the outer radiation belt observed by Arase Van Allen Probes, and the PWING ground stations. Geophysical Research Letters , 45 , 12,720–12,729. https://doi.org/10.1029/2018GL080262 Liang J, Gillies D, Donovan E, Parry H, Mann I, Connors M and Spanswick E (2022) On the green isolated proton auroras during Canada thanksgiving geomagnetic storm. Front. Astron. Space Sci. 9:1040092. https://doi.org/10.3389/fspas.2022.1040092 Lorentzen, K. R., McCarthy, M. P., Parks, G. K., Foat, J. E., Millan, R. M., Smith, D. M., et al. (2000). Precipitation of relativistic electrons by interaction with electromagnetic ion cyclotron waves. Geophysical Research Letters, 105(A3), 5381–5389. https://doi.org/10.1029/1999JA000283 Matsuoka, M., Kawasaki, K., Ueno, S., Tomida, H., Kohama, M., Suzuki, M., et al. (2009). The MAXI mission on the ISS: Science and instruments for monitoring all‐sky X‐ray images. Publications of the Astronomical Society of Japan, 61(5), 999–1010. https://doi.org/10.1093/pasj/61.5.999 Millan, R. M., Lin, R. P., Smith, D. M., Lorentzen, K. R., & McCarthy, M. P. (2002). X‐ray observations of MeV electron precipitation with a balloon‐borne germanium spectrometer. Geophysical Research Letters, 29(24), 2194. https://doi.org/10.1029/2002GL015922 Miyoshi, Y., Sakaguchi, K., Shiokawa, K., Evans, D., Albert, J., Connors, M., & Jordanova, V. (2008). Precipitation of radiation belt electrons by EMIC waves, observed from ground and space. Geophysical Research Letters, 35, L23101. https://doi.org/10.1029/2008GL035727 Miyoshi, Y., Saito, S., Seki, K., Nishiyama, T., Kataoka, R., Asamura, K., et al. (2015). Relation between fine structure of energy spectra for pulsating aurora electrons and frequency spectra of whistler mode chorus waves. Journal of Geophysical Research: Space Physics, 120, 7728–7736. https://doi.org/10.1002/2015JA021562 Miyoshi, Y., Saito, S., Kurita, S., Asamura, K., Hosokawa, K., Sakanoi, T., et al. (2020). Relativistic electron microbursts as high-energy tail of pulsating aurora electrons. Geophysical Research Letters, 47(21), e90360. https://doi.org/10.1029/2020GL090360 Miyoshi, Y., Hosokawa, K., Kurita, S. e., Oyama, S.-I., Ogawa, Y., Saito, S., et al. (2021). Penetration of MeV electrons into the mesosphere accompanying pulsating aurorae. Scientific Reports, 11(1), 13724. https://doi.org/10.1038/s41598-021-92611-3 Murase, K., Kataoka, R., Nishiyama, T., Nishimura, K., Hashimoto, T., Tanaka, Y., et al. (2022). Mesospheric ionization during substorm growth phase. Journal of Space Weather and Space Climate, 12, 18. https://doi.org/10.1051/swsc/2022012 Nakamura, R., Bake, D. N., Blake, J. B., Kanekal, S., Klecker, B., & Hovesta, D. (1995). Relativistic electron precipitation enhancements near the outer edge of the radiation belt. Geophysical Research Letters, 22. https://doi.org/10.1029/95GL00378 Nakamura, R., Isowa, M., Kamide, Y., Baker, D. N., Blake, J. B., & Looper, M. (2000). SAMPEX observations of precipitation bursts in the outer radiation belt. Journal of Geophysical Research, 105(A7), 15875–15885. https://doi.org/10.1029/2000JA900018 Nakamura, S., Miyoshi, Y., Shiokawa, K., Omura, Y., Mitani, T., Takashima, T., et al. (2022). Simultaneous observations of EMIC-induced drifting electron holes (EDEHs) in the earth’s radiation belt by the arase satellite, Van Allen probes, and THEMIS. Geophysical Research Letters, 49(5), e2021GL095194. https://doi.org/10.1029/2021GL095194 Namekawa, T., Mitani, T., Asamura, K., Miyoshi, Y., Hosokawa, K., Lessard, M., et al. (2023). Simultaneous precipitation of sub‐relativistic electron microburst and pulsating aurora electrons. Geophysical Research Letters, 50(24), e2023GL104001. https://doi.org/10.1029/2023GL104001 Rosenberg, T. J., Lanzerotti, L. J., Bailey, D. K., & Pierson, J. D. (1972). Energy spectra in relativistic electron precipitation events. Journal of Atmospheric and Terrestrial Physics, 34(12), 1977–1990. https://doi.org/10.1016/0021-9169(72)90179-1 Sakaguchi, K., K. Shiokawa, A. Ieda, Y. Miyoshi, Y. Otsuka, T. Ogawa, M. G. Connors, E. Donovan, and F. Rich, Simultaneous ground and satellite observations of an isolated proton arc at subauroral latitudes, J. Geophys. Res., 112, A04202, https://doi.org/0.1029/2006JA012135, 2007. Sakaguchi, K., K. Shiokawa, Y. Miyoshi, and M. Connors, Isoleted proton aurora and Pc1/EMIC waves at subauroral latitudes, in Auroral Dynamics and Space Weather, Geophysical Monograph 215, First Edition. Edited by Y. Zhang and L. J. Paxton. John Wiley & Sons, Inc. https://doi.org/10.1002/9781118978719.ch5, pp.59-70, 2016. Shumko, M., Gallardo-Lacourt, B., Halford, A. J., Liang, J., Blum, L. W., Donovan, E., et al. (2021). A strong correlation between relativistic electron microbursts and patchy Aurora. Geophysical Research Letters, 48(18), e2021GL094696. https://doi.org/10.1029/2021gl094696 Shumko, M., Gallardo-Lacourt, B., Halford, A. J., Blum, L. W., Liang, J., Miyoshi, Y., et al. (2022). Proton Aurora and relativistic electron microbursts scattered by electromagnetic ion cyclotron waves. Frontiers in Astronomy and Space Sciences, 9, 975123. https://doi.org/10.3389/fspas.2022.975123 Sivadas N, Semeter J, Nishimura YT, Mrak S. 2019. Optical signatures of the outer radiation belt boundary. Geophys Res Lett 46: 8588–8596. https://doi.org/10.1029/2019GL083908. Torii, S. (2017). The CALorimetric Electron Telescope (CALET) on the ISS: Preliminary results from on‐orbit observations since October, 2015. In Proceeding of Science (ICRC2017). https://doi.org/10.22323/1.301.1092 Turunen, E., Kero, A., Verronen, P. T., Miyoshi, Y., Oyama, S.‐I., & Saito, S. (2016). Mesospheric ozone destruction by high‐energy electron precipitation associated with pulsating aurora. Journal of Geophysical Research: Atmospheres, 121, 11,852–11,861. https://doi.org/10.1002/2016JD025015 Ueno, H., Nakahira, S., Kataoka, R., Asaoka, Y., Torii, S., & Ozawa, S. (2019). Radiation dose during relativistic electron precipitation events at the International Space Station. Space Weather, 17. https://doi.org/10.1029/2019SW002280 Vidal-Luengo, S. E., Blum, L. W., Bruno, A., Ficklin, A. W., de Nolfo, G., Guzik, T. G., et al. (2024a). Characterization of relativistic electron precipitation events observed by the calet experiment using self-organizing-maps. Journal of Geophysical Research: Space Physics, 129(5), e2024JA032481. https://doi.org/10.1029/2024ja032481 Vidal-Luengo, S. E., Blum, L. W., Bruno, A., Ficklin, A. W., de Nolfo, G., Guzik, T. G., et al. (2024b). Comparative observations of the outer belt electron fluxes and precipitated relativistic electrons. Geophysical Research Letters, 51, e2024GL109673. https://doi.org/10.1029/2024gl109673 Whiter, D. K., Gustavsson, B., Partamies, N., & Sangalli, L. (2013). A new automatic method for estimating the peak auroral emission height from all‐sky camera image. Geosci. Instrum. Methods Data Syst., 2(1), 131–144. https://doi.org/10.5194/gi‐2‐131‐2013 Zhang, X., Artemyev, A., Katoh, Y., Hsieh, Y., Angelopoulos, V., Torii, S., Kataoka, R., Akaike, Y., Nakahira, S. (2025). Exploring Outer Radiation Belt Losses From the International Space Station. Geophysical Research Letters, 52(13), e2025GL116966. https://doi.org/10.1029/2025GL116966 Supplementary Files graphicalabstract01.jpeg Cite Share Download PDF Status: Under Review Version 1 posted Reviewers agreed at journal 09 Mar, 2026 Reviewers invited by journal 05 Mar, 2026 Editor assigned by journal 02 Mar, 2026 First submitted to journal 01 Mar, 2026 Editorial decision: Major Revision 22 Jan, 2026 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8545933","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":601000518,"identity":"ebc32231-97f1-42f2-8b96-c39692e32b15","order_by":0,"name":"KYUTARO YANAGISAWA","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABCUlEQVRIiWNgGAWjYDADNnYg8cFAQg7EOfAAr1IYzczAwDijwsIYrCWBGC0MQC3MHGcqEhtAHHxa+O43P/vwgaEumo+Z+QAzY5tE+vywww+BttjJ6TZg1yJ5jM145gyGw7ltzGwJzIVtErkbb6cZALUkG5sdwK7F4BiDMTMPwwGgFh4D5pkgLbMTQFoOJG7DqYX9M1BLHVAL/wdmXqDDDGenfyCghQdkCzPIFgZmnjMSCfLSOfhtkTyWU8w4wwDsF4ODMyokDDdI5xQcSDDA7Re+w8c3M3yoqMud39788MEHgzp5+dnpmz98qLCTw6WFASxugMxGFsGtBRnIN+BRPQpGwSgYBSMSAAC3eFseUtJM5AAAAABJRU5ErkJggg==","orcid":"https://orcid.org/0009-0000-6159-4891","institution":"The University of Tokyo: Tokyo Daigaku","correspondingAuthor":true,"prefix":"","firstName":"KYUTARO","middleName":"","lastName":"YANAGISAWA","suffix":""},{"id":601000519,"identity":"55e1b41e-37fe-4a61-813c-515397506fb1","order_by":1,"name":"Ryuho Kataoka","email":"","orcid":"","institution":"Okinawa Institute of Science and Technology Research Laboratories: Gakko Hojin Okinawa Kagaku Gijutsu Daigakuin Daigaku Gakuen","correspondingAuthor":false,"prefix":"","firstName":"Ryuho","middleName":"","lastName":"Kataoka","suffix":""},{"id":601000520,"identity":"89277153-bc17-47ea-a67f-1f1429575036","order_by":2,"name":"Kanako Seki","email":"","orcid":"https://orcid.org/0000-0001-5557-9062","institution":"RCAST: Tokyo Daigaku Sentan Kagaku Gijutsu Kenkyu Center","correspondingAuthor":false,"prefix":"","firstName":"Kanako","middleName":"","lastName":"Seki","suffix":""},{"id":601000521,"identity":"5636a995-736c-4f2e-b55c-14a63324eaa8","order_by":3,"name":"Daniel Whiter","email":"","orcid":"","institution":"University of Southampton - Highfield Campus: University of Southampton","correspondingAuthor":false,"prefix":"","firstName":"Daniel","middleName":"","lastName":"Whiter","suffix":""},{"id":601000522,"identity":"9b90e71b-d41d-4371-bc70-85cb1d627c9b","order_by":4,"name":"Yoshizumi Miyoshi","email":"","orcid":"https://orcid.org/0000-0001-7998-1240","institution":"Nagoya University: Nagoya Daigaku","correspondingAuthor":false,"prefix":"","firstName":"Yoshizumi","middleName":"","lastName":"Miyoshi","suffix":""},{"id":601000523,"identity":"631e7fc5-83cb-4f1d-aada-8096a90e180e","order_by":5,"name":"Kazuo Shiokawa","email":"","orcid":"","institution":"Nagoya University: Nagoya Daigaku","correspondingAuthor":false,"prefix":"","firstName":"Kazuo","middleName":"","lastName":"Shiokawa","suffix":""},{"id":601000524,"identity":"f13ff5e2-5b59-4d00-8bf7-c08eb49e28dc","order_by":6,"name":"Martin Connors","email":"","orcid":"","institution":"Athabasca University","correspondingAuthor":false,"prefix":"","firstName":"Martin","middleName":"","lastName":"Connors","suffix":""},{"id":601000525,"identity":"41694193-a7b1-4f35-b393-7ecf8e742649","order_by":7,"name":"Shoji Torii","email":"","orcid":"","institution":"Waseda University - Waseda Campus: Waseda Daigaku","correspondingAuthor":false,"prefix":"","firstName":"Shoji","middleName":"","lastName":"Torii","suffix":""},{"id":601000526,"identity":"ad49a092-c7f4-4705-a0fb-675e47cad2bd","order_by":8,"name":"Satoshi Nakahira","email":"","orcid":"","institution":"JAXA ISAS: Uchu Koku Kenkyu Kaihatsu Kiko Uchu Kagaku Kenkyujo","correspondingAuthor":false,"prefix":"","firstName":"Satoshi","middleName":"","lastName":"Nakahira","suffix":""},{"id":601000527,"identity":"a674f27e-b5bf-4a2c-a62c-5ce7fbd43042","order_by":9,"name":"Takeshi Sakanoi","email":"","orcid":"","institution":"Tohoku University: Tohoku Daigaku","correspondingAuthor":false,"prefix":"","firstName":"Takeshi","middleName":"","lastName":"Sakanoi","suffix":""},{"id":601000528,"identity":"4327c2ac-7343-47a3-b31d-940cb6a33a41","order_by":10,"name":"Raju Aryal","email":"","orcid":"","institution":"Athabasca University","correspondingAuthor":false,"prefix":"","firstName":"Raju","middleName":"","lastName":"Aryal","suffix":""},{"id":601000529,"identity":"3399b9b5-bc94-44ca-9985-28bf5d8675b1","order_by":11,"name":"Alessandro Bruno","email":"","orcid":"","institution":"NASA Science Mission Directorate Heliophysics Division","correspondingAuthor":false,"prefix":"","firstName":"Alessandro","middleName":"","lastName":"Bruno","suffix":""},{"id":601000530,"identity":"009fc646-55f8-42ef-bc72-c69c2601b752","order_by":12,"name":"Lauren Blum","email":"","orcid":"","institution":"University of Colorado at Boulder: University of Colorado Boulder","correspondingAuthor":false,"prefix":"","firstName":"Lauren","middleName":"","lastName":"Blum","suffix":""}],"badges":[],"createdAt":"2026-01-08 01:58:07","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8545933/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8545933/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":104343325,"identity":"ba7e1790-e086-4566-bcd6-7b8c6fcb552e","added_by":"auto","created_at":"2026-03-10 17:10:26","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":154170,"visible":true,"origin":"","legend":"\u003cp\u003eAll-sky camera system (ZWO ASI 183MM Pro with a Fujinon FF185C086HA-1 fish-eye lens) installed at Athabasca.\u003c/p\u003e","description":"","filename":"figure1.png","url":"https://assets-eu.researchsquare.com/files/rs-8545933/v1/adb39357fdb8db5a50bbb3ad.png"},{"id":104343274,"identity":"49ad16c5-bfdc-4128-8598-912a3c582535","added_by":"auto","created_at":"2026-03-10 17:10:13","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":513722,"visible":true,"origin":"","legend":"\u003cp\u003eTime series of the REP event observed on 3 May 2025 from 09:28 to 09:40 UT, (a) CHD-X and CHD-Y count rates from CALET, (b) RBM-Z and RBM-H count rates from MAXI, and (c) the ISS sub-satellite magnetic latitude (MLAT) and magnetic local time (MLT) , together with the elevation angle of the ISS (right axis).\u003c/p\u003e","description":"","filename":"figure2.png","url":"https://assets-eu.researchsquare.com/files/rs-8545933/v1/4ab64d447cdd2d3dd462228a.png"},{"id":104343280,"identity":"a3ca66c5-1419-43c9-a509-bf0abf952064","added_by":"auto","created_at":"2026-03-10 17:10:13","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":1014217,"visible":true,"origin":"","legend":"\u003cp\u003eGeographical transform analysis of the 427.8 nm auroral emission observed at 09:33:40 UT on 3 May 2025, showing (a) the image-correlation coefficients between AUGO and AUGSO as a function of assumed emission altitude (60–140 km) of aurora in the magenta-boxed region of (b)(52°–54° N, 111°–112.5° W), and (b) an AUGSO all-sky image mapped emission altitude obtained by the geographical transform method. The projected all-sky images were divided into 1°×1° regions, sampled every 0.5° in both latitude and longitude, and the correlation coefficient was calculated for each region. Solid red (dashed) curves denote major (minor) MLT isolines, and solid (dashed) blue curves denote major (minor) MLAT isolines. The red dashed line shows the ISS magnetic footprint, and the magenta frame the analysis region used in panel (a). Longitudes are given in degrees west, and the map is oriented with north upward and east to the right.\u003c/p\u003e","description":"","filename":"figure3.png","url":"https://assets-eu.researchsquare.com/files/rs-8545933/v1/86ee90b52ca175cea6791bb2.png"},{"id":104343335,"identity":"eec8a488-558a-4db9-aae1-7cd5e0fb0a22","added_by":"auto","created_at":"2026-03-10 17:10:28","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":790952,"visible":true,"origin":"","legend":"\u003cp\u003eField-line tracing analysis of the 427.8 nm auroral emission observed at 09:33:40 UT on 3 May 2025.\u003cstrong\u003e (a) A\u003c/strong\u003ell-sky images mapped along magnetic field lines that satisfied all selection criteria, grouped into three analysis regions: \u003cstrong\u003eR1\u003c/strong\u003e(red; 52°–53° N, 109°-111° W), \u003cstrong\u003eR2\u003c/strong\u003e (green; 53°–54° N, 111°-113° W), and \u003cstrong\u003eR3\u003c/strong\u003e (blue; 54–55° N, 111°-113° W). The black dashed line shows the ISS magnetic footprint.\u003cstrong\u003e (b)\u003c/strong\u003e The mean Rayleigh–altitude profiles (±1σ) derived from the accepted field lines in each region.\u003c/p\u003e","description":"","filename":"figure4.png","url":"https://assets-eu.researchsquare.com/files/rs-8545933/v1/4b78ff3d78afacb1993679ac.png"},{"id":104343276,"identity":"fa1c75d6-1bc9-4311-81cb-7deeeed7163e","added_by":"auto","created_at":"2026-03-10 17:10:13","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":1338445,"visible":true,"origin":"","legend":"\u003cp\u003eSnapshots of auroral emission altitude maps derived using the geographical transform method at three selected times on 3 May 2025: (a) 09:26 UT, (b) 09:28 UT, and (c) 09:48 UT. The projection is the same as in Figure 3(b). Solid red (dashed) curves indicate major (minor) magnetic local time (MLT) isolines, and solid (dashed) blue curves indicate major (minor) magnetic latitude (MLAT) isolines.\u003c/p\u003e","description":"","filename":"figure5.png","url":"https://assets-eu.researchsquare.com/files/rs-8545933/v1/26bcbaa484cf8ab37dde405f.png"},{"id":104405588,"identity":"7aa46b79-ca19-49b8-a5bf-5caaa770dc1c","added_by":"auto","created_at":"2026-03-11 12:23:21","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":4815649,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8545933/v1/1eb6ae7a-4874-41a0-961c-19d57cf41bd8.pdf"},{"id":104343253,"identity":"b130e65a-d836-4ffd-a404-ce1bde0f43ae","added_by":"auto","created_at":"2026-03-10 17:10:06","extension":"jpeg","order_by":9,"title":"","display":"","copyAsset":false,"role":"supplement","size":213539,"visible":true,"origin":"","legend":"","description":"","filename":"graphicalabstract01.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8545933/v1/1b81ea6ff735263f7b87790f.jpeg"}],"financialInterests":"","formattedTitle":"Identification of auroral emission altitudes associated with relativistic electron precipitation events observed by ISS-CALET","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003eRelativistic electrons with energies of several hundred keV to a few MeV occasionally precipitate into the Earth\u0026rsquo;s atmosphere with various spatial and temporal scales. Such relativistic electron precipitation (REP) events have been first detected by ground-based observations (Anderson and Milton, \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1964\u003c/span\u003e; Bailey \u0026amp; Pomerantz, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e1965\u003c/span\u003e; Rosenberg et al., \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e1972\u003c/span\u003e) and later by balloon and satellite measurements (Anderson et al., \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e1968\u003c/span\u003e; Imhof et al., \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e1986\u003c/span\u003e; Lorentzen et al., \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2000\u003c/span\u003e; Nakamura et al., \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e1995\u003c/span\u003e, \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2000\u003c/span\u003e; Comess et al., \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Carson et al., \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). The importance of REP events can be categorized as: 1) Revealing their causes contributes to predictive understanding of the dynamic variation of the outer radiation belt, because the REP events are associated with a major loss process for trapped MeV electrons (Lorentzen et al., \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2000\u003c/span\u003e; Kubota et al., \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Kurita et al., \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Millan et al., \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2002\u003c/span\u003e; Miyoshi et al., \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2008\u003c/span\u003e, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). 2) The REP-related ionization of the middle atmosphere causes the production of NOx and HOx and subsequent ozone destruction (Daae et al., \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Isono et al., \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2014a\u003c/span\u003e,\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003eb\u003c/span\u003e; Kataoka et al., \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Miyoshi et al., \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2015\u003c/span\u003e, \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Turunen et al., \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Murase et al., \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). 3) Radiation dose exposure at LEO, including to the body of astronauts, can be caused by REP events. Ueno et al. (\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2019\u003c/span\u003e) quantitatively evaluated the exposure dose rate of REP events during extravehicular activity at the ISS, although the total effects are not severe even for the largest REP events.\u003c/p\u003e \u003cp\u003eAt least two major types of plasma waves have been known and well-studied as the main causes of REP events; electromagnetic ion cyclotron (EMIC) waves and whistler-mode chorus waves.\u003c/p\u003e \u003cp\u003eEMIC waves can resonate with MeV electrons as well as protons of tens of keV (e.g., Kubota et al., \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Miyoshi et al., \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2008\u003c/span\u003e), scattering of the latter being able to produce so-called isolated proton aurora (IPA) (Sakaguchi et al., \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). The expected emission altitude of the IPA ranges from 115 km to 135 km (e.g., Liang et al., \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Shumko et al., \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Nakamura et al., \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2022\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eChorus waves can also cause REP events. Chorus waves propagating toward the Earth along magnetic field lines can cause broad-band electron precipitation ranging from a few keV to MeV leading to both diffuse aurora and REP events (Miyoshi et al., \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). The chorus model has been confirmed by several conjunction observations between optical images from the ground and satellite/sounding rockets (Miyoshi et al., \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Kurita et al., \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2015\u003c/span\u003e, Kawamura et al., \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2021\u003c/span\u003e, Shumko et al., \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2021\u003c/span\u003e, Namekawa et al., \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Although these observations were not directly related to the REP events, the emission altitude of such a diffuse aurora is well below 100 km (Brown et al., \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e1976\u003c/span\u003e; Hosokawa et al., \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2024\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eKataoka et al. (\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2016\u003c/span\u003e) first utilized the CALorimetric Electron Telescope (CALET) aboard the ISS to perform observations of the REP events. Since then, this instrument has been used to characterize the temporal and spatial properties of REP events (e.g., Kataoka et al., \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Bruno et al., \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Blum et al., \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2024\u003c/span\u003e, Freund et al., \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2024\u003c/span\u003e, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2025\u003c/span\u003e; Vidal-Luengo et al., \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e2024a\u003c/span\u003e, \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003eb\u003c/span\u003e). Further, Zhang et al. (\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e2025\u003c/span\u003e) conducted a statistical classification of the REP events driven by different causes such as chorus, EMIC, and field-line curvature scattering (FLCS).\u003c/p\u003e \u003cp\u003eWhile EMIC-driven REP events have been associated with proton aurora with emission altitudes exceeding 100 km, as expected from IPA (Liang et al., \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2022\u003c/span\u003e), the emission altitude of aurora directly linked to chorus-driven REP events has not yet been quantitatively determined. In the case of broadband electron precipitation from tens-of-keV to MeV energies driven by propagating chorus waves, the lower-energy electrons are expected to cause optical emission at below 100 km.\u003c/p\u003e \u003cp\u003eThe purpose of this paper is to spatially characterize the mechanisms of REP events by identifying the footprint auroras of the conjugate REP events and estimating the emission altitude. By doing so we also verify the effectiveness of altitude estimation as a diagnostic tool for better understanding the mechanisms of REP events and demonstrate that ground-based stereoscopic imaging can complement point-like satellite observations by providing additional spatial and temporal context. Section 2 describes the instrumentation and data used in this study. Section 3 explains the methods for identifying REP\u0026ndash;aurora conjunction events and estimating auroral emission altitudes. Section 4 presents the results of these analyses. Section 5 discusses the implications of the findings in the context of REP drivers and wave\u0026ndash;particle interactions. Section 6 summarizes the conclusions of this study.\u003c/p\u003e"},{"header":"2 Instrumentation and Data","content":"\u003cp\u003eIn this study, we investigated the characteristics and altitudes of auroral emissions associated with REP by conducting conjunction observations using space-based measurements from the ISS and ground-based data from all-sky imagers installed in Athabasca, Canada. The ISS carries several scientific instruments capable of detecting energetic particles. Among them, CALET and Monitor of All-sky X-ray Image (MAXI) are suitable for identifying REP events (Kataoka et al., \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2016\u003c/span\u003e).\u003c/p\u003e \u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Instruments on the International Space Station (ISS)\u003c/h2\u003e \u003cp\u003eCALET has been operating onboard the ISS since 2015 to measure high-energy cosmic-ray electrons and nuclei (Asaoka et al., \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Torii, \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). The instrument includes the Charge Detector (CHD), composed of segmented plastic scintillators arranged in two layers (CHD-X and CHD-Y) to measure the electric charge. The trigger counter signals from these layers are accumulated every 1 s. Although CHD was originally designed for cosmic-ray observations, its 1 s count rates can also be used to monitor MeV electron precipitation events in the radiation environment (Kataoka et al., \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). In this study, we utilize the count rates of CHD-X and CHD-Y, which are sensitive to electrons above 1.5 MeV and 3.4 MeV, respectively.\u003c/p\u003e \u003cp\u003eThe Radiation Belt Monitor (RBM) of the MAXI instrument (Matsuoka et al., \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2009\u003c/span\u003e) detects electrons and protons above 0.3 and 3 MeV, respectively, with a 1s time resolution. Two identical sensors are installed: RBM-H, directed horizontally (limbward), and RBM-Z, directed vertically (zenith). RBM data therefore complements CALET by providing directional information and coverage of lower-energy precipitation (0.3\u0026ndash;1.5 MeV).\u003c/p\u003e \u003cp\u003eFollowing the method of Bruno et al. (\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2022\u003c/span\u003e), REP events were identified based on the ratio of count rates in the two CHD layers: R\u003csub\u003exy\u003c/sub\u003e = N\u003csub\u003ex\u003c/sub\u003e / N\u003csub\u003ey\u003c/sub\u003e \u0026gt; 1\u0026thinsp;+\u0026thinsp;3σ\u003csub\u003eRxy\u003c/sub\u003e where N\u003csub\u003eX\u003c/sub\u003e and N\u003csub\u003eY\u003c/sub\u003e denote 1-s count rates from CHD-X and CHD-Y, respectively, and σ\u003csub\u003eRxy\u003c/sub\u003e is the standard deviation of R\u003csub\u003eXY\u003c/sub\u003e computed over 10 min time window. In addition, to confirm that the observed CHD ratio enhancements were caused by actual REP rather than orbital effects (e.g., variations associated with the ISS trajectory through the radiation belts), we imposed further criterion on the RBM data. The criterion is that the RBM-Z counts, sensitive to energetic electron precipitation, exhibit a burst-like increase relative to the RBM-H counts, as identified by visual inspection. Since RBM-Z is oriented perpendicular to the Earth's surface, an enhancement in RBM-Z relative to RBM-H indicates precipitation directed earthward, thus supporting the identification of REP.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Ground-Based All-Sky Camera Observations in Athabasca\u003c/h2\u003e \u003cp\u003eTo investigate visible auroral emissions associated with REP events, we installed two identical all-sky cameras, ZWO ASI 183mm Pro equipped with Fujino fish-eye lens (FF185C086HA-1) at observation sites\u0026thinsp;~\u0026thinsp;24.5 km apart, AUGO (54.71N, 113.31W) and AUGSO (54.60N, 113.64W) in Athabasca, Canada. To control the camera, we use a single-board computer (Raspberry Pi 4). Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e shows the appearance of the camera configuration. We set a 427.8 nm optical filter between the lens and sensor. The selected wavelength corresponds to N₂⁺ first negative band, because auroral emissions at this wavelength are mainly produced by precipitating electrons with energies of several tens of keV. We set the exposure time to be 18 seconds, with images taken every 20 seconds.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThis stereoscopic configuration, combined with long-exposure imaging and high-sensitivity optics, enabled the detection of faint diffuse auroral emissions above 1.6 Rayleigh, potentially associated with REP events. The camera was configured with a 4\u0026times;4 on-chip binning mode and an effective sensor ROI of 3648 \u0026times; 3648 pixels, producing 912 \u0026times; 912 pixel 16-bit images.\u003c/p\u003e \u003c/div\u003e"},{"header":"3 Methods","content":"\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Selection and Classification of REP Events\u003c/h2\u003e \u003cp\u003eFrom the available dataset, we selected conjugate REP events that met magnetic conjugacy with Athabasca observation sites by tracing the ISS orbit using the International Geomagnetic Reference Field (IGRF)-13 geomagnetic field model. We required that the magnetic footprints be observed with elevation angles greater than 30\u0026deg; from both all-sky imagers, and that the events also satisfied and temporal coincidence with auroral emissions observed by the cameras. We analyzed the REP events observed during the period from September 2024, when our observations started, to September 2025. We identified a total of 23 conjugate REP events, among which 10 occurred under clear-sky conditions. The observation of the remaining 13 REP events was affected by clouds, snow, or strong moonlight, preventing reliable auroral imaging. Among the selected 10 events, two REP events showed auroral emissions near the ISS footprint. Another five REP events exhibited auroral activity in regions spatially displaced from the footprint, while the remaining three REP events showed emission levels below 1.6 Rayleigh, due to low intensity or background noise.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Estimation of Auroral Emission Altitude\u003c/h2\u003e \u003cp\u003eFor analysis of stereoscopic observations from two all-sky cameras (AUGO and AUGSO) in Athabasca with overlapping all-sky fields of view, we use two different altitude estimation methods, which complement each other as explained below.\u003c/p\u003e \u003cp\u003eThe first method is based on finding the geographical transform which gives the maximum correlation (Kataoka et al., \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). We projected the all-sky images from both sites onto a horizontal plane at a series of assumed emission altitudes in geographic coordinates. For each assumed altitude, we clipped the projected images into regions, we calculated the cross-correlation coefficient between the image pairs from the two sites for each region. We interpreted the altitude that gave the maximum correlation as the auroral emission height. We applied this geographical transform method within a restricted region of 52\u0026deg;-56\u0026deg; N and 111\u0026deg;-116\u0026deg; W to minimize geographic distortion in the projection. To ensure the reliability of the estimated emission altitude, we retained only the solutions that satisfied two quality criteria: (i) a peak correlation coefficient greater than 0.7, and (ii) a full width at 95% maximum of the correlation curve less than 50 km. These criteria allowed us to remove poorly constrained or multi-peaked solutions and ensured physically meaningful altitude estimates.\u003c/p\u003e \u003cp\u003eThe second method is based on magnetic field line tracing (Whiter et al., \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e2013\u003c/span\u003e): We traced geomagnetic field lines using the IGRF-13 model and overlaid them onto the all-sky images. For each field line, we identified the corresponding pixels and convert their values into auroral emission intensity in Rayleigh. Then, assuming that the emission occurs along the field line, we constructed an altitude profile of aurora emission intensity by mapping the intensity as a function of altitude along each line. The emission altitude was calculated by averaging only the profiles that satisfied our selection criteria at the two observation sites. We imposed five selection criteria. These include: (1) agreement of peak altitudes at the two sites within \u0026plusmn;\u0026thinsp;5 km; (2) exclusion of peaks occurring within 10 km of the prescribed altitude bounds to avoid edge effects; (3) consistency of emission centroids within 20 km; (4) a correlation coefficient greater than 0.8 between the two Rayleigh\u0026ndash;altitude profiles; and (5) a full width at 90% maximum of the correlation curve less than 50 km to ensure a well-defined emission layer.\u003c/p\u003e \u003cp\u003eThe geographical transform method is most effective for diffuse aurora near the magnetic zenith, where auroral structures are thin and horizontally extended. However, this technique is affected by geographic distortion and limited spatial resolution, making it not suitable for tall discrete aurora especially near the edge of all-sky field-of-view. On the contrary, the field-line tracing method is more appropriate for discrete aurora with well-defined vertical structures, even for auroral forms at low elevation angles. Nevertheless, this method becomes unreliable for thin diffuse aurora near the magnetic zenith, as such emissions do not show distinct ray structure associated with the magnetic field lines. By combining the two approaches, we can obtain a more reliable altitude estimate across the entire all-sky field-of-view, for any types of auroral morphology.\u003c/p\u003e \u003c/div\u003e"},{"header":"4 Results","content":"\u003cp\u003eAn overview of the selected REP event is summarized in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. In the top panel, the REP event appeared on 3 May 2025 between 09:33 and 09:39 UT, when the ISS footprint moved from MLAT 48\u0026deg; to 60\u0026deg; across the dawn side sector (01\u0026ndash;04 MLT) as shown in the bottom panel. The CHD-X counts began to increase at 09:33:30 UT, reached a maximum of ~\u0026thinsp;4.0 \u0026times; 10\u0026sup3; counts s⁻\u0026sup1; at 09:34:30 UT, and gradually decreased until 09:37 UT before dropping sharply to the CHD-Y level (~\u0026thinsp;2.0 \u0026times; 10\u0026sup3; counts s⁻\u0026sup1;) by 09:39 UT. The R\u003csub\u003eXY\u003c/sub\u003e (=\u0026thinsp;CHD-X counts / CHD-Y counts) reached\u0026thinsp;\u0026asymp;\u0026thinsp;2.0 at 09:34:30 UT, and the CHD-Y counts remained nearly constant around ~\u0026thinsp;2.0 \u0026times; 10\u0026sup3; counts s⁻\u0026sup1; throughout the interval.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe RBM-Z channel began to increase at 09:34:00 UT, about 30 s after the CHD-X increase, and showed multiple burst-like spikes between 09:34 and 09:37 UT. The count rate increased from ~\u0026thinsp;10\u0026sup2; to 10⁴ s⁻\u0026sup1;. The amplitude of the short spikes, each lasting a few seconds, became largest at 09:36 UT, when the RBM-Z/RBM-H count ratio\u0026thinsp;\u0026asymp;\u0026thinsp;1.0. The RBM-H channel maintained a high baseline (10⁴\u0026ndash;10⁵ counts s⁻\u0026sup1;) and gradually increased over the same time interval.\u003c/p\u003e \u003cp\u003eCount rates in CHD-X and RBM-Z exhibited a rapid increase within approximately one minute around 09:33 UT and returned to their background levels by 09:39 UT, when the ISS footprint was located near MLAT\u0026thinsp;\u0026asymp;\u0026thinsp;58\u0026deg;. These observations suggest that electrons in the 0.3\u0026ndash;3.4 MeV energy range precipitated above MLAT 58\u0026deg;.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea shows the altitude\u0026ndash;correlation profile derived from the geographical transform method applied to the auroral images at 09:33:40 UT. We computed the correlation in the region indicated by the magenta frame in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb (52-54N, 111-112.5W). The profile exhibits a clear peak around 95 km, and we interpret this altitude as the most probable emission height of the diffuse aurora inside the analysis region.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb summarizes the spatial distribution of the inferred emission altitude. To enhance the visibility of fine auroral structures, we applied a σ-based contrast enhancement, in which the pixel values were linearly scaled so that the mean and \u0026plusmn;\u0026thinsp;1σ of the clear-sky background correspond to the mid-level and the upper/lower limits of the displayed grayscale range, respectively. The overplotted MLT and MLAT isolines (red and blue) help to identify the geomagnetic context of the emission. The analyzed region lies near ~\u0026thinsp;1.2\u0026ndash;1.6 MLT and MLAT 60\u0026deg; to 64\u0026deg; at this event. The red dashed line marks the ISS magnetic footprint, which passed near 52.5N, 112W~113W during these 20 s (09:33:40\u0026thinsp;~\u0026thinsp;09:34:00 UT). The ISS magnetic footprint (black dotted line) passed near to the edge of the diffuse aurora. The auroral emission was spatially extended and modulated. We also note that the all-sky image shows a mixture of diffuse aurora and curtain-like forms. This image shows the best condition in which the ISS passed over the structured aurora at a relatively high elevation angle from the ground-based observation sites. We do not observe a latitudinal trend within the analysis region. However, the emission altitude increases slightly from west (~\u0026thinsp;80 km) to east (~\u0026thinsp;90\u0026ndash;100 km). While the western part of the analysis region is dominated by patchy diffuse structures, the eastern part includes more pronounced curtain-like aurora, which likely contributes to the slightly higher inferred altitudes on the east side.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e shows the results from the magnetic field line tracing method. In Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea, only the field lines that satisfied all criteria in the profiles obtained from AUGSO and AUGO are depicted. Different colors denote the eastern-side field lines in different regions R1\u0026ndash;R3. We analyzed magnetic field lines traced over the southeastern part of the image (52\u0026deg;-55\u0026deg; N, 109\u0026deg;-112.5\u0026deg; W). On the western side of the aurora, we do not identify any field lines that meet our criteria, possibly because patchy and horizontally extended structures dominate the morphology and fail to produce well-defined peaks in the Rayleigh\u0026ndash;altitude profiles. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb shows the averaged emission altitude profile of the eastern-side field lines in regions R1\u0026ndash;R3 shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea. The obtained peak emission altitudes are ~\u0026thinsp;95 km in R1 (52\u0026deg;-53\u0026deg;N, 109\u0026deg;-111\u0026deg;W), ~\u0026thinsp;85 km in R2 (53\u0026deg;-54\u0026deg;N, 111\u0026deg;-112.5\u0026deg;W), and ~\u0026thinsp;80 km in R3 (54\u0026deg;-55\u0026deg;N, 111\u0026deg;-112.5\u0026deg;W). The emission layer extends over ~\u0026thinsp;15 km in altitude below 100 km, consistent with the optical emissions produced by tens-of-keV electron precipitation. From these two different methods, we conclude that the emission altitude is 80\u0026ndash;95 km during this particular REP event.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e shows the auroral emission altitude maps from the geographical transform method at three different times on 3 May 2025: 09:26 UT, 09:28 UT, and 09:48 UT. These maps show both temporal and spatial variations in the estimated emission altitudes. Notably, regions exhibiting patchy aurora\u0026mdash;compact and well-defined structures\u0026mdash;tend to show lower emission altitudes, typically in the range of 70\u0026ndash;85 km. In contrast, areas appearing as diffuse aurora are associated with higher altitudes, generally around 90\u0026ndash;100 km. This trend suggests that auroral morphology has a correlation with the estimated altitude. At 09:26 UT, patchy aurora dominated both the western and eastern regions, whereas the southern and near-zenith areas displayed more diffuse structures, corresponding to higher estimated altitudes. A similar pattern was observed at 09:28 UT, where patchy aurora in the western and northeastern sectors were associated with lower altitudes, while more diffuse aurora in the southeastern region exhibited higher emission altitudes. At 09:48 UT, the region near the zenith became dominated by diffuse aurora, leading to higher altitude estimates compared to the more structured patchy aurora in the southwestern to southern areas. These results demonstrate a consistent relationship between auroral morphology and emission altitude, where patchy aurora is linked to lower altitudes and diffuse aurora tends to originate from higher altitudes.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"5 Discussion","content":"\u003cp\u003eIn this study we installed a new stereoscopic all-sky imager system (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) for investigating possible conjugate footprint aurora under the REP events as observed by CALET onboard ISS. The results shown above can be summarized as follows: We identified a REP event on 3 May 2025, which showed electrons in the 0.3\u0026ndash;3.4 MeV energy range precipitating along magnetic field-lines crossed by the ISS orbit between 09:33 and 09:39 UT (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Using ground-based optical observations at Athabasca, the ISS magnetic footprint passed near the edge of the diffuse aurora (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb). From the geographic transform method, we estimated the emission altitude to be ~\u0026thinsp;95 km (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea). Using the magnetic field line tracing method, we obtained consistent results (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e), typically associated with tens-of-keV electron precipitation. In addition, the auroral emission altitude showed no systematic latitudinal variation (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb and \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb), while a clear longitudinal trend emerged (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb), with lower altitudes in the west and higher altitudes in the east. Figures\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e collectively demonstrate a consistent spatial correspondence between auroral morphology and estimated emission altitude.\u003c/p\u003e \u003cp\u003eNote first that identifying a chorus-related auroral emission itself is not a new finding in itself. There are many examples of pulsating auroras, which are associated with chorus waves. Kurita et al. (\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2015\u003c/span\u003e) identified REP event-related pulsating auroras and demonstrated the link between chorus-driven precipitation and auroral modulation. As also summarized in the Introduction, the emission altitude of aurora at the ISS footprint provides a diagnostic for the wave\u0026ndash;particle interaction type: chorus-driven REP events tend to produce auroral emissions below 100 km, whereas EMIC-driven REP is typically linked to aurora emissions above 100 km. The results obtained in this study are consistent with the former case (Kurita et al., \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2015\u003c/span\u003e), suggesting that the analyzed event was chorus-driven REP events. The key advance of this study is that we determined the emission altitude of REP-related aurora stereoscopically with two independent methods.\u003c/p\u003e \u003cp\u003eWe confirmed the corresponding REP events are in the MeV and sub-MeV range, and we identified that the footprint aurora is most likely produced by tens-of-keV electrons. The broadband electron precipitation, spanning tens of keV to a few MeV, can be a consequence of electron resonance with duct-propagating chorus waves (Miyoshi et al., \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2020\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eWe found no systematic latitudinal variation in the auroral emission altitude. FLCS is expected to produce energy-dispersed precipitation, in which higher-energy electrons precipitate at lower magnetic latitudes and therefore generate lower emission altitudes toward the equator (Sivadas et al., \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Murase et al., \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Such a signature was not observed in our data. Instead, the altitude remained nearly uniform at ~\u0026thinsp;90 km across latitude, indicating that tens-of-keV electrons precipitated broadly and simultaneously. This uniformity is inconsistent with FLCS-driven precipitation and is instead consistent with chorus-driven broadband precipitation.\u003c/p\u003e \u003cp\u003ePatchy aurora, characterized by compact and well-defined structures, tend to occur at lower altitudes (70\u0026ndash;85 km), while diffuse aurora\u0026mdash;appearing more blurred or faint \u0026mdash;are associated with higher emission altitudes (90\u0026ndash;100 km). This systematic trend suggests that auroral morphology provides a meaningful indicator of emission altitude. However, this relationship is partially affected by the spectral sensitivity of the imaging system. The 427.8 nm wavelength is primarily sensitive to high-energy electron precipitation (tens of keV), and emissions driven by lower-energy electrons (a few keV), which typically occur at higher altitudes, are less efficiently detected at this wavelength. As a result, such aurora appear more diffuse simply due to reduced optical response\u0026mdash;introducing an apparent \u0026ldquo;morphology\u0026ndash;altitude\u0026rdquo; effect shaped by observational bias. Therefore, our findings highlight both the utility and the limitations of single-wavelength imaging in diagnosing auroral altitude and particle origins. Future studies will pursue coordinated multi-wavelength observations to improve the accuracy of morphological classification and altitude estimation, and to better resolve the underlying energy characteristics of precipitating particles.\u003c/p\u003e \u003cp\u003eWe also demonstrate that REP-related footprint auroral emissions are not frequent, based on a systematic survey of conjugate REP events. In our year-long observation period, we found that only two out of ten conjugate REP events exhibited visible aurora near the magnetic footprint.\u003c/p\u003e \u003cp\u003eThe use of a 427.8 nm optical filter is one possible reason to understand the infrequent occurrence of footprint auroras. Note that the 557.7 nm Oxygen line basically provides the strongest optical signature of EMIC-related proton precipitation (Sakaguchi et al., 2007, Liang et al., \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2022\u003c/span\u003e), whereas the 427.8 nm emission used in this study has a weaker response to the proton aurora. Given that our imager system is therefore less sensitive to the EMIC-related proton aurora, our findings that only two out of 10 REP events which exhibited optical footprint counterparts, and both of those events are likely chorus related, is not inconsistent with the recent statistical result of Zhang et al. (\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e2025\u003c/span\u003e) where they noted that EMIC-related REP events are comparable or more frequent than chorus-driven REP events.\u003c/p\u003e \u003cp\u003eWe modernized the ground-based experimental setup by utilizing a low-cost single-board computer for controlling the all-sky camera, also setting an optical filter between the lens and sensor to make the imager smaller. We demonstrated that the compact and twin imager systems can effectively complement satellite measurements by providing quantitative information on auroral emission altitudes as well as the 2D spatial context which is complemental to in-situ satellite observations. For future work, we aim to further reduce the cost without losing the sensitivity, to expand the all-sky observation network, hopefully via the spreading help of citizen-science participation (Kataoka et al., \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2025\u003c/span\u003e).\u003c/p\u003e"},{"header":"6 Conclusions","content":"\u003cp\u003eWe analyzed the REP event on 3 May 2025 and identified MeV and sub-MeV electron precipitation accompanied by diffuse aurora near the ISS magnetic footprint. We modernized the stereoscopic all-sky camera observations, then we estimated the auroral emission altitude to be ~\u0026thinsp;90 km, which is typically due to tens-of-keV electrons. Such broadband electron precipitation from tens-of-keV to MeV is consistent with the chorus-driven REP events, and the absence of systematic latitudinal trend in the emission altitude is not consistent with FLCS-driven REP events. Our results demonstrate that compact and low-cost all-sky imagers can effectively complement the in-situ satellite observations by providing auroral altitude and spatial context.\u003c/p\u003e"},{"header":"Abbreviations","content":"\u003cdiv class=\"DefinitionList\"\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eREP\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eRelativistic electron precipitation\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eEMIC\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eElectromagnetic ion-cyclotron\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eIPA\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eIsolated proton aurora\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eCALET\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eCALorimetric Electron Telescope\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eCHD\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eCHarge Detector\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eMAXI\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eMonitor of All-sky X-ray Imag\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eRBM\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eRadiation Belt Monitor\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eISS\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eInternational Space Station\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eLEO\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eLow-Earth Orbit\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eFLCS\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eField-Line Curvature Scattering\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eIGRF\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eInternational Geomagnetic Reference Field\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003c/div\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eEthics approval and consent to participate\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent for publication\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAvailability of data and materials\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe datasets analyzed in this study are publicly available through the Data Archives and Transmission System (DARTS) operated by the Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA).\u003cbr\u003e\u0026nbsp;The CALET Charge Detector (CHD) Level 1.1 data are available at:\u003cbr\u003e\u0026nbsp;https://darts.isas.jaxa.jp/pub/calet/cal-v1.1/CHD/level1.1/obs/2025/\u003cbr\u003e\u0026nbsp;The MAXI Radiation Belt Monitor (RBM) data are available at:\u003cbr\u003e\u0026nbsp;https://darts.isas.jaxa.jp/pub/maxi/rbm/2025/\u003c/p\u003e\n\u003cp\u003eGround-based auroral images obtained at Athabasca are not publicly archived due to limited storage and data privacy policies, but they are available from the corresponding author upon reasonable request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no competing interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eR.K. and S. T. are supported by JSPS-KAKENHI 24H00025, R. K. and K.S. are supported by JSPS-KAKENHI (22K21345 (PBASE program)).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eD. W. was supported by the Natural Environment Research Council\u003c/p\u003e\n\u003cp\u003e(NERC) of the UK under grant NE/V012541/1.\u003c/p\u003e\n\u003cp\u003eInitial construction and operation of Athabasca University observatory facilities were supported by the Canada Foundation for Innovation. M.C. holds NSERC funding.\u003c/p\u003e\n\u003cp\u003eThe development and calibration of the camera were supported by the Hoso Bunka Foundation.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthors\u0026apos; contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eK.Y. conducted the related research activity under the supervision of R.K. and K.S.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eS. T. advised the use of CALET CHD data, while S. N. advised the use of MAXI RBM data.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eD. W. advised the field-line mapping method.\u003c/p\u003e\n\u003cp\u003eR. K. advised the geographic transform method.\u003c/p\u003e\n\u003cp\u003eY. M. and K. S. supported the installation of the auroral cameras at AUGSO and AUGO, with help from M. C. and R. A. at Athabasca University.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eT. S. designed the imager system and provided the observation software used in this study.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eAB and LB discussed the CALET observation of REP events and the mechanisms.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eYK thanks the student support program of the PBASE project (PI: K. Shiokawa) for observation trips to Canada and research visits to Southampton University as well as to OIST.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eYK also acknowledges the technical support for the all-sky camera development and calibration works under the JARE AJ1007 Project (PI: R. Kataoka).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthors\u0026apos; information\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003csup\u003e(1\u003c/sup\u003eGraduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan , \u003csup\u003e(2\u003c/sup\u003eOkinawa Institute of Science and Technology, 1919-1 Tancha, Onna, Kunigami District, Okinawa 904-0495, Japan, \u003csup\u003e(3\u003c/sup\u003eResearch Center for Advanced Science and Technology (RCAST), The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan, \u003csup\u003e(4\u003c/sup\u003eDepartment of Physics and Astronomy, University of Southampton, University Road, Southampton SO17 1BJ, United Kingdom, \u003csup\u003e(5\u003c/sup\u003eInstitute for Space-Earth Environmental Research, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8601, Japan, \u003csup\u003e(6\u003c/sup\u003eDepartment of Physics \u0026amp; Astronomy, Athabasca University, Athabasca, Alberta, Canada T9S 3A3,\u003csup\u003e\u0026nbsp;(7\u003c/sup\u003eWaseda Research Institute for Science and Engineering, Waseda University, 17 Kikui-cho, Shinjuku-ku, Tokyo 162-0044, Japan \u003csup\u003e(8\u003c/sup\u003eInstitute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA), 3-1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 252-5210, Japan, \u003csup\u003e(9\u003c/sup\u003eGraduate School of Science, Tohoku University, 6-3 Aramaki-aza Aoba, Aoba-ku, Sendai, Miyagi 980-8578, Japan, \u003csup\u003e(10\u003c/sup\u003e Heliophysics Science Division, NASA Goddard Space Flight Center, 8800 Greenbelt Rd, Greenbelt, MD 20771, United States, \u003csup\u003e(11\u003c/sup\u003e Laboratory for Atmospheric and Space Physics and Department of Aerospace Engineering Sciences, University of Colorado Boulder, 1234 Innovation Drive, Boulder, CO 80303-7814, United States\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eAnderson, K. A., D. W. Milton (1964). Balloon observations of X rays in the auroral zone, 3, High time resolution studies. Journal of Geophysical Research, 69, 4457\u0026ndash;4479. https://doi.org/10.1029/JZ069i021p04457\u003c/li\u003e\n\u003cli\u003eAnderson, H. R., Hudson, P. D., \u0026amp; McCoy, J. E. (1968). Observations of POGO ion chamber experiment in the outer radiation zone. Journal of Geophysical Research, 73(19), 6285\u0026ndash;6297. https://doi.org/10.1029/JA073i019p06285\u003c/li\u003e\n\u003cli\u003eAsaoka, Y., Ozawa, S., Torii, S., Adriani, O., Akaike, Y., Asano, K., et al. (2018). On‐orbit operations and offline data processing of CALET onboard the ISS. Astroparticle Physics, 100, 29\u0026ndash;37. https://doi.org/10.1016/j.astropartphys.2018.02.010\u003c/li\u003e\n\u003cli\u003eBailey, D. K., \u0026amp; Pomerantz, M. A. (1965). Relativistic electron precipitation into the mesosphere at subauroral latitudes. Journal of Geophysical Research, 70(23), 5823\u0026ndash;5830. https://doi.org/10.1029/JZ070i023p05823\u003c/li\u003e\n\u003cli\u003eBlum, L. W., Bruno, A., Capannolo, L., Ma, Q., Kataoka, R., Torii, S., \u0026amp; Baishev, D. (2024). On the spatial and temporal evolution of EMIC wave-driven relativistic electron precipitation: Magnetically conjugate observations from the Van Allen probes and CALET. Geophysical Research Letters, 51(5), e2023GL107087. https://doi.org/10.1029/2023GL107087\u003c/li\u003e\n\u003cli\u003eBrown, N. B., Davis, T. N., Hallinan, T. J., \u0026amp; Stenbaek-Nielsen, H. C. (1976). Altitude of pulsating aurora determined by a new instrumental technique. Geophysical Research Letters, 3(7), 403\u0026ndash;404. https://doi.org/10.1029/gl003i007p00403\u003c/li\u003e\n\u003cli\u003eBruno, A., Blum, L. W., de Nolfo, G. A., Kataoka, R., Torii, S., Greeley, A. D., et al. (2022). EMIC-wave driven electron precipitation observed by CALET on the international space station. Geophysical Research Letters, 49(6), e97529. https://doi.org/10.1029/2021GL097529\u003c/li\u003e\n\u003cli\u003eCarson, B. R., Rodger, C. J., \u0026amp; Clilverd, M. A. (2013). POES satellite observations of EMIC‐wave driven relativistic electron precipitation during 1998\u0026ndash;2010. Journal of Geophysical Research: Space Physics, 118, 232\u0026ndash;243. https://doi.org/10.1029/2012JA017998\u003c/li\u003e\n\u003cli\u003eComess, M. D., Smith, D. M., Selesnick, R. S., Millan, R. M., \u0026amp; Sample, J. G. (2013). Duskside relativistic electron precipitation as measured by SAMPEX: A statistical survey. Journal of Geophysical Research: Space Physics, 118, 5050\u0026ndash;5058. https://doi.org/10.1002/jgra.50481\u003c/li\u003e\n\u003cli\u003eDaae, M., Espy, P., Nesse Tyssoy, H., Newnham, D., Stadsnes, J., \u0026amp; Soraas, F. (2012). The effect of energetic electron precipitation on middle mesospheric night‐time ozone during and after a moderate geomagnetic storm. Geophysical Research Letters, 39, L21811. https://doi.org/10.1029/2012GL053787\u003c/li\u003e\n\u003cli\u003eFreund, D., Blum, L., Huang, J., Vidal-Luengo, S., Bruno, A., \u0026amp; Kataoka, R. (2025). Radiation-belt dropouts: Relationship with geomagnetic storms and MeV precipitation. Geophysical Research Letters, 52(4), e2024GL113660. https://doi.org/10.1029/2024GL113660\u003c/li\u003e\n\u003cli\u003eFreund, D., Blum, L., Vidal-Luengo, S., Bruno, A., \u0026amp; Kataoka, R. (2024). MeV electron precipitation during radiation belt dropouts. Journal of Geophysical Research: Space Physics, 129(8), e2024JA032759. https://doi.org/10.1029/2024JA032759\u003c/li\u003e\n\u003cli\u003eHosokawa, K., Miyoshi, Y., Mcharg, M., Ledvina, V., Hampton, D., Lessard, M., et al. (2024). Variation of the altitude of auroral emission during a substorm cycle: Stereoscopic optical observations during the lamp rocket experiment. Journal of Geophysical Research: Space Physics, 129(11), e2024JA033036. https://doi.org/10.1029/2024JA033036\u003c/li\u003e\n\u003cli\u003eImhof, W. L., Voss, H. D., Reagan, J. B., Datlowe, D. W., Gaines, E. E., Mobilia, J., \u0026amp; Evans, D. S. (1986). Relativistic electron and energetic ion precipitation spikes near the plasmapause. Journal of Geophysical Research, 91(A3), 3077\u0026ndash;3088. https://doi.org/10.1029/JA091iA03p03077\u003c/li\u003e\n\u003cli\u003eIsono, Y., Mizuno, A., Nagahama, T., Miyoshi, Y., Nakamura, T., Kataoka, R., et al. (2014a). Variations of nitric oxide in the mesosphere and lower thermosphere over Antarctica associated with a magnetic storm in April 2012. Geophysical Research Letters, 41, 2568\u0026ndash;2574. https://doi.org/10.1002/2014GL059360\u003c/li\u003e\n\u003cli\u003eIsono, Y., Mizuno, A., Nagahama, T., Miyoshi, Y., Nakamura, T., Kataoka, R., et al. (2014b). Ground‐based observations of nitric oxide in the mesosphere and lower thermosphere over Antarctica in 2012\u0026ndash;2013. Journal of Geophysical Research: Space Physics, 119, 7745\u0026ndash;7761. https://doi.org/10.1002/2014JA019881\u003c/li\u003e\n\u003cli\u003eKataoka, R., Miyoshi, Y., Shigematsu, K., Hampton, D., Mori, Y., Kubo, T., et al. (2013). Stereoscopic determination of all‐sky altitude map of aurora using two ground‐based Nikon DSLR cameras. Annals of Geophysics, 31(9), 1543\u0026ndash;1548. https://doi.org/10.5194/angeo‐31‐1543‐2013\u003c/li\u003e\n\u003cli\u003eKataoka, R., Asaoka, Y., Torii, S., Terasawa, T., Ozawa, S., Tamura, T., et al. (2016). Relativistic electron precipitation at International Space Station: Space weather monitoring by Calorimetric Electron Telescope. Geophysical Research Letters, 43, 4119\u0026ndash;4125. http://doi.org/10.1002/2016GL068930\u003c/li\u003e\n\u003cli\u003eKataoka, R., Nishiyama, T., Tanaka, Y., Kadokura, A., Uchida, H. A., Ebihara, Y., et al. (2019). Transient ionization of the mesosphere during auroral breakup: Arase satellite and ground‐based conjugate observations at Syowa Station. Earth Planets and Space, 71(1), 9. https://doi.org/10.1186/s40623‐019‐0989‐7\u003c/li\u003e\n\u003cli\u003eKataoka, R., Asaoka, Y., Torii, S., Nakahira, S., Ueno, H., Miyake, S., et al. (2020). Plasma waves causing relativistic electron precipitation events at international space station: Lessons from conjunction observations with arase satellite. Journal of Geophysical Research: Space Physics, 125(9), e27875. https://doi.org10.1029/2020JA027875\u003c/li\u003e\n\u003cli\u003eKataoka, R., Reddy, S.A., Nakano, S. \u003cem\u003eet al.\u003c/em\u003e Extended magenta aurora as revealed by citizen science. \u003cem\u003eSci Rep\u003c/em\u003e 14, 25849 (2024). https://doi.org/10.1038/s41598-024-75184-9\u003c/li\u003e\n\u003cli\u003eKataoka, R., Nakano, S., Uchino, S. \u003cem\u003eet al.\u003c/em\u003e Extended red aurora associated with super substorm igniting the October 10, 2024 magnetic storm as revealed by citizen science. \u003cem\u003eEarth Planets Space\u003c/em\u003e 77, 64 (2025). https://doi.org/10.1186/s40623-025-02178-w\u003c/li\u003e\n\u003cli\u003eKawamura, M., Sakanoi, T., Fukizawa, M., Miyoshi, Y., Hosokawa, K., Tsuchiya, F., et al. (2021). Simultaneous pulsating aurora and microburst observations with ground-based fast auroral imagers and CubeSat FIREBIRD-II. Geophysical Research Letters, 48(18), e2021GL094494. https://doi.org/10.1029/2021GL094494\u003c/li\u003e\n\u003cli\u003eKubota, Y., Omura, Y., \u0026amp; Summers, D. (2015). Relativistic electron precipitation induced by EMIC‐triggered emissions in a dipole magnetosphere. Journal of Geophysical Research: Space Physics, 120, 4384\u0026ndash;4399. https://doi.org/10.1002/2015JA021017\u003c/li\u003e\n\u003cli\u003eKurita, S., Kadokura, A., Miyoshi, Y., Morioka, A., Sato, Y., \u0026amp; Misawa, H. (2015). Relativistic electron precipitations in association with diffuse aurora: Conjugate observation of SAMPEX and the all sky TV camera at Syowa station. Geophysical Research Letters, 42(12), 4702\u0026ndash;4708. https://doi.org/10.1002/2015GL064564\u003c/li\u003e\n\u003cli\u003eKurita, S., Miyoshi, Y., Shiokawa, K., Higashio, N., Mitani, T., Takashima, T., et al. (2018). Rapid loss of relativistic electrons by EMIC waves in the outer radiation belt observed by Arase Van Allen Probes, and the PWING ground stations. \u003cem\u003eGeophysical Research Letters\u003c/em\u003e, \u003cem\u003e45\u003c/em\u003e, 12,720\u0026ndash;12,729. https://doi.org/10.1029/2018GL080262 \u003c/li\u003e\n\u003cli\u003eLiang J, Gillies D, Donovan E, Parry H, Mann I, Connors M and Spanswick E (2022) On the green isolated proton auroras during Canada thanksgiving geomagnetic storm. Front. Astron. Space Sci. 9:1040092. https://doi.org/10.3389/fspas.2022.1040092\u003c/li\u003e\n\u003cli\u003eLorentzen, K. R., McCarthy, M. P., Parks, G. K., Foat, J. E., Millan, R. M., Smith, D. M., et al. (2000). Precipitation of relativistic electrons by interaction with electromagnetic ion cyclotron waves. Geophysical Research Letters, 105(A3), 5381\u0026ndash;5389. https://doi.org/10.1029/1999JA000283\u003c/li\u003e\n\u003cli\u003eMatsuoka, M., Kawasaki, K., Ueno, S., Tomida, H., Kohama, M., Suzuki, M., et al. (2009). The MAXI mission on the ISS: Science and instruments for monitoring all‐sky X‐ray images. Publications of the Astronomical Society of Japan, 61(5), 999\u0026ndash;1010. https://doi.org/10.1093/pasj/61.5.999\u003c/li\u003e\n\u003cli\u003eMillan, R. M., Lin, R. P., Smith, D. M., Lorentzen, K. R., \u0026amp; McCarthy, M. P. (2002). X‐ray observations of MeV electron precipitation with a balloon‐borne germanium spectrometer. Geophysical Research Letters, 29(24), 2194. https://doi.org/10.1029/2002GL015922\u003c/li\u003e\n\u003cli\u003eMiyoshi, Y., Sakaguchi, K., Shiokawa, K., Evans, D., Albert, J., Connors, M., \u0026amp; Jordanova, V. (2008). Precipitation of radiation belt electrons by EMIC waves, observed from ground and space. Geophysical Research Letters, 35, L23101. https://doi.org/10.1029/2008GL035727\u003c/li\u003e\n\u003cli\u003eMiyoshi, Y., Saito, S., Seki, K., Nishiyama, T., Kataoka, R., Asamura, K., et al. (2015). Relation between fine structure of energy spectra for pulsating aurora electrons and frequency spectra of whistler mode chorus waves. Journal of Geophysical Research: Space Physics, 120, 7728\u0026ndash;7736. https://doi.org/10.1002/2015JA021562\u003c/li\u003e\n\u003cli\u003eMiyoshi, Y., Saito, S., Kurita, S., Asamura, K., Hosokawa, K., Sakanoi, T., et al. (2020). Relativistic electron microbursts as high-energy tail of pulsating aurora electrons. Geophysical Research Letters, 47(21), e90360. https://doi.org/10.1029/2020GL090360\u003c/li\u003e\n\u003cli\u003eMiyoshi, Y., Hosokawa, K., Kurita, S. e., Oyama, S.-I., Ogawa, Y., Saito, S., et al. (2021). Penetration of MeV electrons into the mesosphere accompanying pulsating aurorae. Scientific Reports, 11(1), 13724. https://doi.org/10.1038/s41598-021-92611-3\u003c/li\u003e\n\u003cli\u003eMurase, K., Kataoka, R., Nishiyama, T., Nishimura, K., Hashimoto, T., Tanaka, Y., et al. (2022). Mesospheric ionization during substorm growth phase. Journal of Space Weather and Space Climate, 12, 18. https://doi.org/10.1051/swsc/2022012\u003c/li\u003e\n\u003cli\u003eNakamura, R., Bake, D. N., Blake, J. B., Kanekal, S., Klecker, B., \u0026amp; Hovesta, D. (1995). Relativistic electron precipitation enhancements near the outer edge of the radiation belt. Geophysical Research Letters, 22. https://doi.org/10.1029/95GL00378\u2028\u003c/li\u003e\n\u003cli\u003eNakamura, R., Isowa, M., Kamide, Y., Baker, D. N., Blake, J. B., \u0026amp; Looper, M. (2000). SAMPEX observations of precipitation bursts in the outer radiation belt. Journal of Geophysical Research, 105(A7), 15875\u0026ndash;15885. https://doi.org/10.1029/2000JA900018\u003c/li\u003e\n\u003cli\u003eNakamura, S., Miyoshi, Y., Shiokawa, K., Omura, Y., Mitani, T., Takashima, T., et al. (2022). Simultaneous observations of EMIC-induced drifting electron holes (EDEHs) in the earth\u0026rsquo;s radiation belt by the arase satellite, Van Allen probes, and THEMIS. Geophysical Research Letters, 49(5), e2021GL095194. https://doi.org/10.1029/2021GL095194\u003c/li\u003e\n\u003cli\u003eNamekawa, T., Mitani, T., Asamura, K., Miyoshi, Y., Hosokawa, K., Lessard, M., et al. (2023). Simultaneous precipitation of sub‐relativistic electron microburst and pulsating aurora electrons. Geophysical Research Letters, 50(24), e2023GL104001. https://doi.org/10.1029/2023GL104001\u003c/li\u003e\n\u003cli\u003eRosenberg, T. J., Lanzerotti, L. J., Bailey, D. K., \u0026amp; Pierson, J. D. (1972). Energy spectra in relativistic electron precipitation events. Journal of Atmospheric and Terrestrial Physics, 34(12), 1977\u0026ndash;1990. https://doi.org/10.1016/0021-9169(72)90179-1\u003c/li\u003e\n\u003cli\u003eSakaguchi, K., K. Shiokawa, A. Ieda, Y. Miyoshi, Y. Otsuka, T. Ogawa, M. G. Connors, E. Donovan, and F. Rich, Simultaneous ground and satellite observations of an isolated proton arc at subauroral latitudes, J. Geophys. Res., 112, A04202, https://doi.org/0.1029/2006JA012135, 2007.\u003c/li\u003e\n\u003cli\u003eSakaguchi, K., K. Shiokawa, Y. Miyoshi, and M. Connors, Isoleted proton aurora and Pc1/EMIC waves at subauroral latitudes, in Auroral Dynamics and Space Weather, Geophysical Monograph 215, First Edition. Edited by Y. Zhang and L. J. Paxton. John Wiley \u0026amp; Sons, Inc. https://doi.org/10.1002/9781118978719.ch5, pp.59-70, 2016.\u003c/li\u003e\n\u003cli\u003eShumko, M., Gallardo-Lacourt, B., Halford, A. J., Liang, J., Blum, L. W., Donovan, E., et al. (2021). A strong correlation between relativistic electron microbursts and patchy Aurora. Geophysical Research Letters, 48(18), e2021GL094696. https://doi.org/10.1029/2021gl094696\u003c/li\u003e\n\u003cli\u003eShumko, M., Gallardo-Lacourt, B., Halford, A. J., Blum, L. W., Liang, J., Miyoshi, Y., et al. (2022). Proton Aurora and relativistic electron microbursts scattered by electromagnetic ion cyclotron waves. Frontiers in Astronomy and Space Sciences, 9, 975123. https://doi.org/10.3389/fspas.2022.975123\u003c/li\u003e\n\u003cli\u003eSivadas N, Semeter J, Nishimura YT, Mrak S. 2019. Optical signatures of the outer radiation belt boundary. Geophys Res Lett 46: 8588\u0026ndash;8596. https://doi.org/10.1029/2019GL083908.\u003c/li\u003e\n\u003cli\u003eTorii, S. (2017). The CALorimetric Electron Telescope (CALET) on the ISS: Preliminary results from on‐orbit observations since October, 2015. In Proceeding of Science (ICRC2017). https://doi.org/10.22323/1.301.1092\u003c/li\u003e\n\u003cli\u003eTurunen, E., Kero, A., Verronen, P. T., Miyoshi, Y., Oyama, S.‐I., \u0026amp; Saito, S. (2016). Mesospheric ozone destruction by high‐energy electron precipitation associated with pulsating aurora. Journal of Geophysical Research: Atmospheres, 121, 11,852\u0026ndash;11,861. https://doi.org/10.1002/2016JD025015\u003c/li\u003e\n\u003cli\u003eUeno, H., Nakahira, S., Kataoka, R., Asaoka, Y., Torii, S., \u0026amp; Ozawa, S. (2019). Radiation dose during relativistic electron precipitation events at the International Space Station. Space Weather, 17. https://doi.org/10.1029/2019SW002280\u003c/li\u003e\n\u003cli\u003eVidal-Luengo, S. E., Blum, L. W., Bruno, A., Ficklin, A. W., de Nolfo, G., Guzik, T. G., et al. (2024a). Characterization of relativistic electron precipitation events observed by the calet experiment using self-organizing-maps. Journal of Geophysical Research: Space Physics, 129(5), e2024JA032481. https://doi.org/10.1029/2024ja032481\u003c/li\u003e\n\u003cli\u003eVidal-Luengo, S. E., Blum, L. W., Bruno, A., Ficklin, A. W., de Nolfo, G., Guzik, T. G., et al. (2024b). Comparative observations of the outer belt electron fluxes and precipitated relativistic electrons. Geophysical Research Letters, 51, e2024GL109673. https://doi.org/10.1029/2024gl109673\u003c/li\u003e\n\u003cli\u003eWhiter, D. K., Gustavsson, B., Partamies, N., \u0026amp; Sangalli, L. (2013). A new automatic method for estimating the peak auroral emission height from all‐sky camera image. Geosci. Instrum. Methods Data Syst., 2(1), 131\u0026ndash;144. https://doi.org/10.5194/gi‐2‐131‐2013\u003c/li\u003e\n\u003cli\u003eZhang, X., Artemyev, A., Katoh, Y., Hsieh, Y., Angelopoulos, V., Torii, S., Kataoka, R., Akaike, Y., Nakahira, S. (2025). Exploring Outer Radiation Belt Losses From the International Space Station. Geophysical Research Letters, 52(13), e2025GL116966. https://doi.org/10.1029/2025GL116966\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":true,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"earth-planets-and-space","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"epsp","sideBox":"Learn more about [Earth, Planets and Space](http://earth-planets-space.springeropen.com)","snPcode":"","submissionUrl":"https://www.editorialmanager.com/epsp/default.aspx","title":"Earth, Planets and Space","twitterHandle":"@SpringerOpen","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"BMC/SO AJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"Electron precipitation, atmospheric ionization, aurora, stereoscopy, space weather forecast","lastPublishedDoi":"10.21203/rs.3.rs-8545933/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8545933/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eWe report a multi-MeV relativistic electron precipitation (REP) event as observed by the CALorimetric Electron Telescope and the Monitor of All-sky X-ray Image aboard the International Space Station together with low-cost twin all-sky auroral imagers installed at Athabasca, Canada. We find that visible aurora associated with REP is not frequent\u0026mdash;observed in only two of 10 conjugate REP events during our survey period from September 2024 to September 2025\u0026mdash;and that one case, on 3 May 2025, featured diffuse aurora near the ISS magnetic footprint. Using two independent stereoscopic methods, we estimate the auroral emission height to be ~\u0026thinsp;90 km, which is typically due to tens-of-keV electron precipitation. Combined with the MeV electron precipitation detected by CALET, such broadband electron precipitation from tens-of-keV to MeV is consistent with the hypothesis of chorus-driven REP events. The emission altitude shows no systematic latitudinal variation, which is not consistent with field-line curvature scattering type energy-dispersed precipitation. The combined space\u0026ndash;ground conjunction observations contribute to better understanding of the spatial context of wave\u0026ndash;particle interaction associated with the REP events.\u003c/p\u003e","manuscriptTitle":"Identification of auroral emission altitudes associated with relativistic electron precipitation events observed by ISS-CALET","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-03-10 17:09:11","doi":"10.21203/rs.3.rs-8545933/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"reviewerAgreed","content":"","date":"2026-03-09T08:26:03+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-03-05T05:29:00+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-03-02T10:10:24+00:00","index":"","fulltext":""},{"type":"submitted","content":"Earth, Planets and Space","date":"2026-03-01T20:24:09+00:00","index":"","fulltext":""},{"type":"decision","content":"Major Revision","date":"2026-01-23T00:07:42+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"earth-planets-and-space","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"epsp","sideBox":"Learn more about [Earth, Planets and Space](http://earth-planets-space.springeropen.com)","snPcode":"","submissionUrl":"https://www.editorialmanager.com/epsp/default.aspx","title":"Earth, Planets and Space","twitterHandle":"@SpringerOpen","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"BMC/SO AJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"7ccbebc9-c6dc-4be9-9b75-b42d64f22949","owner":[],"postedDate":"March 10th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-05-12T11:03:02+00:00","versionOfRecord":[],"versionCreatedAt":"2026-03-10 17:09:11","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8545933","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8545933","identity":"rs-8545933","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}