Severe shrinkage of Earth’s magnetosphere during the May 2024 solar storm

Severe shrinkage of Earth’s magnetosphere during the May 2024 solar storm | 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 Article Severe shrinkage of Earth’s magnetosphere during the May 2024 solar storm Kazuhiro Yamamoto, Yoshizumi Miyoshi, Naritoshi Kitamura, Rumi Nakamura, and 15 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8649685/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 10 You are reading this latest preprint version Abstract Earth’s main magnetic field forms the magnetosphere, a barrier against solar wind plasmas. A southward magnetic field carried by the solar wind makes the magnetosphere smaller, which is termed “magnetopause erosion.” This process leads to equatorward extension of the polar region, where solar wind plasmas directly intrude into the magnetosphere. Although previous studies have suggested that magnetopause reconnection causes magnetopause erosion, the erosion mechanism during super geomagnetic storms remains controversial owing to a lack of in situ measurements. Here, we report a magnetopause crossing by the Arase satellite at 4.96 R E during the May 2024 super geomagnetic storm, which is the innermost magnetopause ever detected by magnetospheric spacecraft. An unexpected decrease in the magnetic field intensity extended from the inner magnetosphere to the magnetopause, indicating that magnetospheric currents reduced Earth’s main magnetic field on a global scale. Consequently, the solar wind pushed the weakened magnetosphere further toward the Earth, resulting in the severe shrinkage of the magnetosphere. This shrinkage led to a significant equatorward penetration of solar energetic particles on the dayside, suggesting that the magnetospheric currents can increase the risk of aircraft exposure to solar energetic particles at mid-latitudes. May 2024 super geomagnetic storm solar storm magnetopause solar energetic particles (SEPs) magnetospheric current system Arase (ERG) satellite Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction The Earth’s dayside magnetosphere directly interacts with the solar wind, a supersonic plasma flow ejected from the Sun. This interaction forms a boundary between the Earth’s main magnetic field and the interplanetary magnetic field (IMF) embedded in the solar wind, known as the magnetopause [1] . The location and shape of the magnetopause are primarily controlled by the pressure balance between the magnetic field pressure on the magnetospheric side and the dynamic pressure of the solar wind under a northward IMF [1] . The averaged location of the magnetopause is ~ 10 R E , where R E is the Earth’s radius (6371.2 km) [2] . However, a southward IMF moves the magnetopause closer to the Earth than a northward IMF does [3] , a phenomenon referred to as “magnetopause erosion.” Although numerous space-borne measurements have confirmed magnetopause erosion, the mechanism underlying this phenomenon has been debated for decades. Previous studies initially postulated that a southward IMF leads to an enhanced magnetopause reconnection that removes the magnetic flux from the dayside magnetosphere, and that the solar wind plasma further presses the magnetosphere closer to the Earth owing to the weakened magnetic field pressure [3–7] . Conversely, recent global magnetohydrodynamic (MHD) simulations have demonstrated that magnetospheric current systems, such as the Region-1 field-aligned current [8,9] and the nightside cross-tail current [10] , generate a southward magnetic field (downward light blue arrows in Fig. 1 ) and weaken the Earth’s main magnetic field, according to Ampere’s law. This results in magnetopause erosion as originally proposed by ref. 11,12 with their analytical models of the current systems. Verifying the erosion mechanism during large geomagnetic storms requires in-situ observations of a low-latitude magnetopause at noon inside the geosynchronous orbit (at ~ 6.6 R E ) with simultaneous solar wind measurements. Such a coordinated observation has not previously been achieved during a super geomagnetic storm (i.e., an event with the disturbance storm time index (Dst) [13] of < − 250 nT [14] ), in which space weather effects are significantly enhanced and the location of the magnetopause is essential for the space weather forecasting. For addressing the debate of the magnetopause erosion mechanism, the Exploration of energization and Radiation in Geospace (ERG) mission [15] of the Japan Aerospace Exploration Agency (JAXA) has successfully conducted in-situ observations of an intense magnetopause erosion by the Arase satellite during a super geomagnetic storm in May 2024. Results Firstly, we provide an overview of the super geomagnetic storm that occurred in May 2024. A solar active region produced numerous X-class solar flares and launched multiple Earth-directed coronal mass ejections (CMEs) [16] between May 5 and 15, 2024. The CMEs launched on May 8 and 9 struck the Earth and caused the super geomagnetic storm between May 10 and 12 [17] . The Dst index reached − 406 nT during this storm, which is the largest disturbance in the past 20 years since the November 2003 storm. Here, we examine the observational data of the Arase satellite during its magnetopause crossing on May 11, 2024. Until 06:52:50 UT (vertical line on the left side in Fig. 2 , where UT is Universal Time), the Arase satellite remained in the magnetosphere on its outbound orbit between 11.6 and 11.8 magnetic local time (MLT; in hours) at a magnetic latitude (MLAT) of approximately 25°. After 06:52:50 UT, electron fluxes in a broad energy range below 1 keV increased significantly and ion fluxes subsequently followed the electron variation, while the local magnetic field remained northward ( B Z,GSM > 0, where B Z,GSM is the Z-component of the local magnetic field in the geocentric solar magnetospheric (GSM) coordinate system) (Fig. 2 a-d,f). The flux enhancement indicates that the Arase satellite entered the low-latitude boundary layer (LLBL), where the local magnetic field originates from the Earth’s main magnetic field (a northward magnetic field), but one end of a magnetic field line is connected to the solar wind. In LLBL, Arase observed plasma originating from the magnetosheath, which is the region of solar wind decelerated and heated at the bow shock. After 07:04:37 UT (vertical line at the center of Fig. 2 ), multiple magnetopause crossings occurred at 12.0 MLT under solar wind dynamic pressure from 35 to 39 nPa and IMF- B Z from − 27 to − 31 nT. The positive and negative excursions of B Z,GSM (orange line in Fig. 2 f) indicate that the magnetopause oscillated back and forth around the position of Arase, so that the satellite was intermittently observing the LLBL and then the magnetosheath boundary layer (MSBL, a southward magnetic field originating from IMF). Since the magnetopause is the boundary of the Earth’s main magnetic field and IMF, it is located between LLBL and MSBL. The first magnetopause crossing by the Arase satellite occurred at a radial distance of 4.96 R E , which is the innermost spacecraft observation of the magnetopause ever available in the literature [18–20] . Energetic O + ions exhibited parallel flows in the LLBL and anti-parallel flows in the MSBL with sudden flux enhancements (Fig. 2 e,f). The unidirectional enhanced fluxes represent ion reconnection jets because ions launched from a lower latitude become parallel flows in the northward background magnetic field and anti-parallel flows in the southward background magnetic field in the northern hemisphere. Furthermore, in MSBL, one end of a magnetic field line is connected to the magnetosphere, and thus high-energy ( ≳ 60 keV) protons continue to be detected (Fig. 2 c). After 07:13:05 UT (vertical line on the right side in Fig. 2 ), the Arase satellite finally entered the magnetosheath proper, and the magnetic field was continuously directed southward (Fig. 2 f). Magnetic field lines were no longer connected to the magnetosphere in the magnetosheath, and hence Arase did not observe any enhanced fluxes of O + ions and the high-energy protons, both of which originate from the magnetosphere (Fig. 2 c, e). Figure 1 summarizes the magnetopause crossing by the Arase satellite on May 11, 2024. The magnetopause is represented by a dashed line at ~ 5 R E in Fig. 1 . LLBL and MSBL lie on the earthward side and sunward side of the magnetopause under a southward IMF. Within the LLBL and MSBL, the magnetic fields are directed northward and southward, respectively. In these regions, the magnetic field lines of the magnetosphere are connected to the magnetosheath, and the plasmas from both the magnetosphere and the magnetosheath are mixed. Thus, energetic O + ions and protons still exist in LLBL and MSBL. The magnetospheric electrons at > 10 keV exhibit a rapid decrease in their flux at the middle of LLBL (at 06:56 UT). This is consistent with the fact that the parallel velocity of the magnetospheric electrons is sufficiently high to escape into interplanetary space along an open magnetic field line. The Arase satellite observed these features of the boundary layers as it moved sunward (the right-hand side of Fig. 1 ). Discussion and Conclusions The in situ observation by the Arase satellite further provides insight into the mechanism of magnetospheric erosion during the May 2024 super geomagnetic storm. Figure 3 a shows the radial distributions of the local magnetic pressure ( P B , blue line), O + thermal pressure ( P O+ , green line), and total pressure ( P B + P O+ , red line) during the interval from 06:11 UT to 07:34 UT on May 11, 2024. In this interval, the O + thermal pressure was a good proxy for the ion thermal pressure on the magnetospheric side because ring current O + ions dominated the ion thermal pressure (80–90% of ion thermal pressure) in the magnetosphere (Supplementary Information Fig. 1 . See Methods for details of the pressure calculation). The O + dominance enabled us to distinguish the magnetospheric ion thermal pressure from the solar wind ion (H + ) thermal pressure. In Fig. 3 a, as the Arase satellite moved outward (sunward) in Earth’s strong main magnetic field, the magnetic pressure and the total pressure decreased in the magnetosphere and LLBL. In the vicinity just inside the magnetopause, the total pressure becomes approximately equal to the solar wind dynamic pressure for the magnetopause crossing at 07:03 UT (dashed horizontal line). Therefore, we consider that the pressure balance between solar wind dynamic pressure and magnetospheric pressure was approximately satisfied during the magnetopause crossing by Arase. As shown in Fig. 3 a, the magnetic field is the primary factor of the pressure balance between the solar wind and the magnetosphere. To examine the details of the magnetic pressure variations, we subtracted the magnetic field calculated by the IGRF-14 geomagnetic field model [21] from the measured local magnetic field (Fig. 3 b). Because the IGRF-14 model describes Earth’s main magnetic field, the difference between the local magnetic field and the IGRF-14 model field represents the external magnetic field generated by magnetospheric current systems and local magnetospheric dynamics, such as magnetic reconnection. At > 4.2 R E , the decrease in the local magnetic field intensity became prominent, but its position was still far from the magnetopause. The magnitude of the decrease reached approximately 60 nT at 4.5 R E . Fluctuations in the magnetic field intensity were then observed at larger radial distances. In the LLBL, sharp transient depletions were repeatedly detected. The white line in Fig. 3 b represents a linear fit to the variation in the magnetic field intensity within the LLBL. The slope of the fitting line shows that the local magnetic field intensity, on average, decreased by ~ 41 nT within LLBL. There is an estimation error of ± 6.5 nT in the amplitude of the decrease, caused by the 1-sigma uncertainty of the inclination of the linear fitting. Fluctuations in the magnetic field intensity intensified in the MSBL, and finally, a strong compressed magnetic field with smaller fluctuations were observed in the magnetosheath. The decrease in the magnetic field intensity deep inside the magnetosphere (Fig. 3 b) clearly shows the dominant contribution of the magnetospheric currents to magnetic field variations. This is consistent with the fact that the magnetopause reconnection is a localized process around the magnetopause and cannot account for the radially-wide decrease in the magnetic field intensity. Moreover, the decrease in the magnetic field intensity beyond the center of the ring current at ~ 3 R E (Supplementary Information Fig. 1 ) indicates that the Region-1 field-aligned current and/or the cross-tail current counteract the northward magnetic field (upward light blue arrows in Fig. 1 ) generated by the magnetopause current and dayside ring current. Therefore, our findings provide the first experimental evidence of the significant contribution of magnetospheric currents to magnetopause erosion around the low-latitude magnetopause during super geomagnetic storms. Given that the decrease in the magnetic field intensity deep inside the magnetosphere is attributed to the magnetospheric currents, the Region-1 field-aligned current and/or the cross-tail current caused a decrease of > 60 nT if we take the magnetopause current and the ring current into account. In addition, the gradual decrease in the magnetic field intensity around the magnetopause also indicates another contributing factor associated with the magnetopause. The transient depletion of the magnetic field intensity in the LLBL was caused by an approach to MSBL, where the magnetic field was much weaker than LLBL. The transient magnetic field depletion and the multiple magnetopause crossings can be attributed to the transient removal of the magnetic flux caused by repeated magnetopause reconnections, as described in previous studies [3–7] . Therefore, we conclude that magnetopause reconnection also contributed to the magnetopause erosion. Given that the northward magnetic field generated by the magnetopause current did not vary largely within the thin LLBL, its average contribution to the decrease in the magnetic field intensity derived from linear fitting (~ 41 [± 6.5] nT) is secondary compared to those of the Region-1 field-aligned current and/or the cross-tail current ( ≳ 60 nT). The summation of the decrease in the magnetic field intensity discussed here is ≳ 100 nT, and hence, ≳60% of the decrease in the magnetic field intensity can be attributed to the Region-1 field-aligned current and/or the cross-tail current on average. The final remarkable feature of the magnetopause crossing is the ring current ions near the magnetopause. In previous studies, ring current ions were considered to play a role in the pressure balance at the magnetopause [22,23] ; however, no studies have directly evaluated the contribution from the ring current thermal pressure during a super geomagnetic storm. In this study, the O + thermal pressure was significantly high (~ 7 nPa) even at the magnetopause (Fig. 3 a), indicating that the magnetospheric ring current ions partially pushed back the solar wind exerted on the magnetopause. Approximately 20% of the solar wind dynamic pressure (~ 35 nPa) was directly balanced by the ring current ion thermal pressure. This result suggests that the heavy ion originating from the planetary atmosphere is one of the key factors controlling the magnetopause location during highly disturbed times. The present study has suggested that magnetospheric currents are the primary cause of intense magnetopause erosion during the May 2024 super geomagnetic storm. The effect of the magnetospheric currents on the magnetopause erosion should be taken into account more seriously because it can increase the risk of solar energetic particle (SEP) exposure at mid-latitudes. The magnetopause is an approximate boundary of open and closed magnetic field lines (Fig. 1 ), and hence intense magnetopause erosion causes an equatorward shift of the penetration region of SEPs coming from the solar wind through an open field line. The solid line in the top panel of Fig. 4 shows the Altitude-Adjusted Corrected Geomagnetic (AACGM) latitude of the magnetic footprint of the magnetopause detected by Arase under a steady IMF- B Z,GSM and P SW (see Method section for details of the determination of the equator boundary of SEPs) during this storm. The magnetic field line was calculated using the Tsyganenko and Sitnov (2005) model [24] . In the dayside topside ionosphere, the Polar Orbiting Environmental Satellite (POES) 18 detected SEPs (> 6.9 MeV protons) in the polar region (color-coded map in Fig. 4 ). The magnetopause footprint shows variations synchronized with the equatorward boundary of SEPs, reaching down to 60° AACGM latitudes during the storm main phase. Therefore, the significant equatorward expansion of the SEPs indicates that the magnetospheric currents can surely increase the risk of aircraft exposure to SEPs, which leads to glitches in flight-control data and radiation exposure for the crew at mid-latitudes during super geomagnetic storms. Methods Arase Satellite Data The Arase satellite is equipped with several instruments for the measurement of charged particles. MEP-e [25] measures energetic electrons between 7.0 keV and 87.5 keV with 15 energy steps. LEP-e [26] measures lower energy electrons between 0.067 keV and 19.1 keV with 28 energy steps. MEP-e and LEP-e cover a solid angle of 4π sr using spacecraft spin motion and provide 8-sec (spin-averaged) omnidirectional electron flux data. We used MEP-i [27] and LEP-i [28] data for the analysis of ions. MEP-i measures H + , He ++ , He + , O ++ , O + , and N 2 + /NO + /O 2 + between 9.6 keV/q and 184.2 keV/q with 15 energy steps. LEP-i measures H + , He + , and O + between 0.0039 keV and 25.5 keV with 29 energy steps. Both instruments have two observational modes, normal mode and time-of-flight (TOF) mode. In the normal mode, 3D velocity distributions are provided with an angular resolution of 22.5° in the azimuthal and polar directions. While LEP-i was operating in the normal mode during the period considered in this study, MEP-i was in the TOF mode, in which 3D velocity distributions of ions are not provided. Because MEP-i in the TOF mode provides energy spectra of omnidirectional ion fluxes as well as TOF data, we used the omnidirectional ion flux data to calculate ion thermal pressure by combining the MEP-i and LEP-i data obtained in the two different modes. In the calculation of ion thermal pressure, we used MEP-i 32-s TOF-mode omnidirectional fluxes between 22.1-184.2 keV/q and LEP-i 8-s normal-mode omnidirectional fluxes between 0.035 keV and 19.2 keV with an assumption of an isotropic pitch angle distribution: $$\:{P}_{s,\text{L}\text{E}\text{P}-\text{i}}\simeq\:\frac{4\pi\:}{3}\:\sqrt{2{m}_{s}}\sum\:_{0.035\:\text{k}\text{e}\text{V}}^{19.2\:\text{k}\text{e}\text{V}}\sqrt{E}{J}_{s,\text{L}\text{E}\text{P}-\text{i}}\left(E\right){\Delta\:}E,$$ $$\:{P}_{s,\text{M}\text{E}\text{P}-\text{i}}\simeq\:\frac{4\pi\:}{3}\:\sqrt{2{m}_{s}}\sum\:_{22.1\:\text{k}\text{e}\text{V}/\text{q}}^{184.2\:\text{k}\text{e}\text{V}/\text{q}}\sqrt{E}{J}_{s,\text{M}\text{E}\text{P}-\text{i}}\left(E\right){\Delta\:}E,$$ $$\:{P}_{s}={P}_{s,\text{L}\text{E}\text{P}-\text{i}}+{P}_{s,\text{M}\text{E}\text{P}-\text{i}},$$ where m s is the mass of ion species s , E is the ion energy, and Δ E is the energy step width. To apply the trapezoidal rule to the pressure calculation, E and Δ E were obtained from log 10 E = (log 10 E i + log 10 E i+ 1 )/2, Δ E = E i +1 – E i , where i represents the i -th energy step. J s ,LEP−i/MEP−i is the omnidirectional differential number flux of LEP-i/MEP-i obtained from J s ,LEP−i/MEP−i ( E ) = ( J s ,LEP−i/MEP−i ( E i ) + J s ,LEP−i/MEP−i ( E i +1 ))/2. P s is the ion thermal pressure obtained by combining the LEP-i pressure ( P s ,LEP−i ) and MEP-i pressure ( P s ,MEP−i ) of ion species s . Magnetopause Location under a Steady Solar Wind To exclude dynamic variations of the magnetopause location in the comparison analysis between the Arase and POES observations, we used the magnetopause location under a steady solar wind. The criteria for a steady solar wind are as follows. For the solar wind data during the past 10 min, (i) IMF- B Z,GSM and P SW variation is less than 50% of an average over the past 10 min, (ii) more than 5 data points (> 5 min) of IMF- B Z and P SW pressure have finite values, (iii) the northward-southward direction of IMF does not change. In Supplementary Information Fig. 2 , the steady solar wind selected with these criteria is represented in blue-filled circles. The magenta-filled circles represent the period of an unsteady solar wind. During the steady solar wind, Arase crossed the magnetopause six times, as shown in Supplementary Information Fig. 2 . Because IMF-B Z was southward during these periods, we considered that the magnetopause corresponds to a step-like variation between the northward/southward magnetic field at Arase. POES-18 Satellite Data and Equator Boundary of SEPs The POES-18 satellite is a Low Earth Orbit (LEO) satellite at polar orbits with an orbital period of ~ 100 min. It is equipped with the MEPED instrument of the Solar Environment Monitor (SEM-2) instrument package, which measures trapped and precipitating fluxes of protons from 30 keV to > 6,900 keV [29] . The equatorward boundary of SEPs was determined from the lowest latitude of the latitudinal bins with proton fluxes of > 0.1 s −1 cm −2 sr −1 keV − 1 on each spacecraft orbit at AACGM latitudes of > 55° in the northern hemisphere. Declarations Data Availability The ERG Science Center [3 0 ] , which is operated by ISAS/JAXA and Nagoya University, archives the science data of the Arase satellite. The data used in the present study are publicly available from the ERG Science Center website (https://ergsc.isee.nagoya-u.ac.jp/index.shtml.en). This study used the MGF Level-2 8 sec spin-averaged v04.06 data [3 1 ,3 2 ] , MEP-i Level-2 omnidirectional flux v02_02 data [ 27 ,3 3 ] , MEP-e Level-2 omnidirectional flux v01_02 data [28,37] , LEP-i Level-2 omnidirectional flux v03_00 data [ 28 ,3 5 ] , LEP-e Level-2 omnidirectional flux v04_01 data [2 6 ,3 6 ] , Level-2 definitive orbital v05 data [ 37 ] , and Level-3 TS04 model v01 data [ 38 ] . The OMNI 1-min solar wind v01 data set [ 39 ] was obtained from the Goddard Space Flight Center Space Physics Data Facility OMNIWeb (http://omniweb.gsfc.nasa.gov). The Dst index [1 3 ] was provided by the World Data Center for Geomagnetism, Kyoto (http://wdc.kugi.kyoto-u.ac.jp/wdc/Sec3.html). The POES-18 SEM-2 MEPED 2-s flux data (v01), which are originally provided by National Oceanic and Atmospheric Administration (NOAA), are available at Coordinated Data Analysis Web (CDAWeb) https://cdaweb.gsfc.nasa.gov/istp_public/data/noaa/noaa18/sem2_fluxes-2sec/. Code Availability The Space Physics Environment Data Analysis System (SPEDAS) [40] software (version 6.1) was used in the analysis. The software is publicly available at https://themis.ssl.berkeley.edu/socware/. Acknowledgements We thank T. Segawa (Technical Center, Nagoya University) for maintaining the data archiving system at the ERG Science Center. We thank Editage for English language editing. Funding Declaration The LEPe development is supported by the Academia Sinica, National Cheng Kung University, and the Ministry of Science and Technology of Taiwan under Contract No. MOST 105-3111-Y-001-042 and MOST 106-2111-M-001-011. A.S. was supported by JSPS KAKENHI Grant Numbers 24K07112, 24K00898, and 23K22555. Contributions K.Y., Y.M., and N.K. conceived this study. K.Y. collected, analyzed data, and prepared the manuscript. K.Y., Y.M., N.K., A.S., and R.N. interpreted the result. A.M., M.T., K.Y. (MGF), S.Y., S.K., K.K., T.H. (MEP-e and MEP-i), K.A., Y.M. 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Res. 115 , A04207 (2010). Additional Declarations No competing interests reported. Supplementary Files 2026SciRepMPCSupplementaryInformationv2.0.0.docx Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Revision requested 09 Mar, 2026 Reviews received at journal 06 Mar, 2026 Reviews received at journal 27 Feb, 2026 Reviewers agreed at journal 17 Feb, 2026 Reviewers agreed at journal 17 Feb, 2026 Reviewers agreed at journal 07 Feb, 2026 Reviewers invited by journal 05 Feb, 2026 Editor assigned by journal 05 Feb, 2026 Submission checks completed at journal 01 Feb, 2026 First submitted to journal 01 Feb, 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. 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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-8649685","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":623805765,"identity":"2ceb8fe5-1221-4fc4-bd7c-6c9684a9049f","order_by":0,"name":"Kazuhiro 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Astrophysics","correspondingAuthor":false,"prefix":"","firstName":"Shiang-Yu","middleName":"","lastName":"Wang","suffix":""},{"id":623805779,"identity":"46763381-6a23-447a-b539-15b3d053f7ea","order_by":14,"name":"Sunny Wing-Yee Tam","email":"","orcid":"","institution":"National Cheng Kung University","correspondingAuthor":false,"prefix":"","firstName":"Sunny","middleName":"Wing-Yee","lastName":"Tam","suffix":""},{"id":623805780,"identity":"eec4d8eb-08b9-4bc8-84be-080f4b36e9c3","order_by":15,"name":"Tzu-Fang Chang","email":"","orcid":"","institution":"National Cheng Kung University","correspondingAuthor":false,"prefix":"","firstName":"Tzu-Fang","middleName":"","lastName":"Chang","suffix":""},{"id":623805781,"identity":"9a6d3c45-ff35-4a36-80ae-cc12c2cdb3e1","order_by":16,"name":"Bo-Jhou Wang","email":"","orcid":"","institution":"Academia Sinica Institute of Astronomy and Astrophysics","correspondingAuthor":false,"prefix":"","firstName":"Bo-Jhou","middleName":"","lastName":"Wang","suffix":""},{"id":623805782,"identity":"6d2ac710-c3b8-4529-b0eb-fe1e24f6f2dc","order_by":17,"name":"Chae-Woo Jun","email":"","orcid":"","institution":"Nagoya University","correspondingAuthor":false,"prefix":"","firstName":"Chae-Woo","middleName":"","lastName":"Jun","suffix":""},{"id":623805783,"identity":"49ba8cda-9f9b-4384-9993-57fa8ebcbc5d","order_by":18,"name":"Iku Shinohara","email":"","orcid":"","institution":"Japan Aerospace Exploration Agency","correspondingAuthor":false,"prefix":"","firstName":"Iku","middleName":"","lastName":"Shinohara","suffix":""}],"badges":[],"createdAt":"2026-01-20 13:35:48","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8649685/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8649685/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":107868982,"identity":"fde416b3-771f-40f7-81f8-6f7a6bb3f706","added_by":"auto","created_at":"2026-04-27 07:35:29","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":9671157,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSchematic view of the Arase satellite observation of the magnetopause during the May 2024 super geomagnetic storm. \u003c/strong\u003eLight\u003cstrong\u003e \u003c/strong\u003eblue arrows represent the magnetic field perturbation generated by magnetospheric currents, which are indicated by yellow arrows.\u003c/p\u003e","description":"","filename":"fig1.png","url":"https://assets-eu.researchsquare.com/files/rs-8649685/v1/e8b139604ddd1306e0e45e9d.png"},{"id":107640609,"identity":"9a374a78-309e-43f5-90fb-767b02fcb025","added_by":"auto","created_at":"2026-04-23 13:23:03","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":443255,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003e\u003cstrong\u003eIn situ\u003c/strong\u003e\u003c/em\u003e \u003cstrong\u003eobservation of the intensely eroded magnetopause by the Arase satellite at 5 \u003c/strong\u003e\u003cem\u003e\u003cstrong\u003eR\u003c/strong\u003e\u003c/em\u003e\u003csub\u003e\u003cstrong\u003eE\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e during a super geomagnetic storm on May 11, 2024. a, b,\u003c/strong\u003e Energy-time spectrograms of electron omnidirectional differential fluxes measured by MEP-e\u003csup\u003e[25]\u003c/sup\u003e and LEP-e\u003csup\u003e[26]\u003c/sup\u003e. \u003cstrong\u003ec, d,\u003c/strong\u003e Energy-time spectrograms of proton omnidirectional differential fluxes measured by MEP-i\u003csup\u003e[27]\u003c/sup\u003e and LEP-i\u003csup\u003e[28]\u003c/sup\u003e. \u003cstrong\u003ee,\u003c/strong\u003e Pitch angle distributions of differential fluxes of 8.16 keV O\u003csup\u003e+\u003c/sup\u003e ion measured by LEP-i. \u003cstrong\u003ef,\u003c/strong\u003e Magnetic field vectors measured by MGF\u003csup\u003e[31]\u003c/sup\u003e in the GSM coordinates.\u003c/p\u003e","description":"","filename":"fig2rev.png","url":"https://assets-eu.researchsquare.com/files/rs-8649685/v1/096f4071be7b3055e777594f.png"},{"id":107640606,"identity":"4ad8e60f-58ca-4100-bcbb-ab46c86e616e","added_by":"auto","created_at":"2026-04-23 13:23:01","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":489114,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eRadial distribution of magnetic field intensity, showing large decreases in the magnetosphere and the boundary layers.\u003c/strong\u003e \u003cstrong\u003ea,\u003c/strong\u003e Magnetic field pressure (blue line), O\u003csup\u003e+\u003c/sup\u003e ion thermal pressure (green line), and the sum of them (red line) calculated from LEP-i (0.035–19.2 keV) and MEP-i (22.1–184.2 keV) measurements. Magnetopause locations predicted by empirical models\u003csup\u003e[5,6,41]\u003c/sup\u003e are shown at the top for reference. \u003cstrong\u003eb,\u003c/strong\u003e Differences between the observed magnetic field intensity and the IGRF-14 model. The white line in panel b represents the linear fit in the low-latitude boundary layer, showing the baseline of the magnetic field variations.\u003c/p\u003e","description":"","filename":"fig3rev.png","url":"https://assets-eu.researchsquare.com/files/rs-8649685/v1/dee0e0e5b61b76a819312f6e.png"},{"id":107640604,"identity":"0488bdb8-7dfa-4b54-8a02-7c00ecd9f8e1","added_by":"auto","created_at":"2026-04-23 13:23:01","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":290604,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eThe latitudinal distribution of SEP proton fluxes of \u0026gt; 6.9 MeV (color-coded map) measured by Medium Energy Proton and Electron Detector (MEPED)\u003c/strong\u003e\u003csup\u003e [\u003c/sup\u003e\u003csup\u003e\u003cstrong\u003e29]\u003c/strong\u003e\u003c/sup\u003e\u003cstrong\u003e onboard POES-18. \u003c/strong\u003eThe ionospheric footprint of the magnetopause detected by Arase under a relatively steady solar wind (purple solid circles) is also shown.\u003cstrong\u003e \u003c/strong\u003eThe MLTs of the magnetopause and the equatorward boundary of SEPs are presented in the lower panel.\u003c/p\u003e","description":"","filename":"fig4.png","url":"https://assets-eu.researchsquare.com/files/rs-8649685/v1/7e1aed8104b74c5fcb12952a.png"},{"id":108006533,"identity":"ca04f3ed-3d76-4049-8cf6-fbf70fea3fa5","added_by":"auto","created_at":"2026-04-28 12:55:53","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":12638882,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8649685/v1/ca4016a1-07d5-4b32-9c17-ef96f0b7ce92.pdf"},{"id":107707392,"identity":"92e042ca-05cf-4690-8b09-c6f89e0c4f85","added_by":"auto","created_at":"2026-04-24 09:20:13","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":621972,"visible":true,"origin":"","legend":"","description":"","filename":"2026SciRepMPCSupplementaryInformationv2.0.0.docx","url":"https://assets-eu.researchsquare.com/files/rs-8649685/v1/22dd6fc1180bbae956771166.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Severe shrinkage of Earth’s magnetosphere during the May 2024 solar storm","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe Earth\u0026rsquo;s dayside magnetosphere directly interacts with the solar wind, a supersonic plasma flow ejected from the Sun. This interaction forms a boundary between the Earth\u0026rsquo;s main magnetic field and the interplanetary magnetic field (IMF) embedded in the solar wind, known as the magnetopause\u003csup\u003e[1]\u003c/sup\u003e. The location and shape of the magnetopause are primarily controlled by the pressure balance between the magnetic field pressure on the magnetospheric side and the dynamic pressure of the solar wind under a northward IMF\u003csup\u003e[1]\u003c/sup\u003e. The averaged location of the magnetopause is ~\u0026thinsp;10 \u003cem\u003eR\u003c/em\u003e\u003csub\u003eE\u003c/sub\u003e, where \u003cem\u003eR\u003c/em\u003e\u003csub\u003eE\u003c/sub\u003e is the Earth\u0026rsquo;s radius (6371.2 km)\u003csup\u003e[2]\u003c/sup\u003e. However, a southward IMF moves the magnetopause closer to the Earth than a northward IMF does\u003csup\u003e[3]\u003c/sup\u003e, a phenomenon referred to as \u0026ldquo;magnetopause erosion.\u0026rdquo;\u003c/p\u003e \u003cp\u003eAlthough numerous space-borne measurements have confirmed magnetopause erosion, the mechanism underlying this phenomenon has been debated for decades. Previous studies initially postulated that a southward IMF leads to an enhanced magnetopause reconnection that removes the magnetic flux from the dayside magnetosphere, and that the solar wind plasma further presses the magnetosphere closer to the Earth owing to the weakened magnetic field pressure\u003csup\u003e[3\u0026ndash;7]\u003c/sup\u003e. Conversely, recent global magnetohydrodynamic (MHD) simulations have demonstrated that magnetospheric current systems, such as the Region-1 field-aligned current\u003csup\u003e[8,9]\u003c/sup\u003e and the nightside cross-tail current\u003csup\u003e[10]\u003c/sup\u003e, generate a southward magnetic field (downward light blue arrows in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) and weaken the Earth\u0026rsquo;s main magnetic field, according to Ampere\u0026rsquo;s law. This results in magnetopause erosion as originally proposed by ref. 11,12 with their analytical models of the current systems. Verifying the erosion mechanism during large geomagnetic storms requires \u003cem\u003ein-situ\u003c/em\u003e observations of a low-latitude magnetopause at noon inside the geosynchronous orbit (at ~\u0026thinsp;6.6 \u003cem\u003eR\u003c/em\u003e\u003csub\u003eE\u003c/sub\u003e) with simultaneous solar wind measurements. Such a coordinated observation has not previously been achieved during a super geomagnetic storm (i.e., an event with the disturbance storm time index (Dst) \u003csup\u003e[13]\u003c/sup\u003e of \u0026lt;\u0026thinsp;\u0026minus;\u0026thinsp;250 nT\u003csup\u003e[14]\u003c/sup\u003e), in which space weather effects are significantly enhanced and the location of the magnetopause is essential for the space weather forecasting.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFor addressing the debate of the magnetopause erosion mechanism, the Exploration of energization and Radiation in Geospace (ERG) mission\u003csup\u003e[15]\u003c/sup\u003e of the Japan Aerospace Exploration Agency (JAXA) has successfully conducted \u003cem\u003ein-situ\u003c/em\u003e observations of an intense magnetopause erosion by the Arase satellite during a super geomagnetic storm in May 2024.\u003c/p\u003e"},{"header":"Results","content":"\u003cp\u003eFirstly, we provide an overview of the super geomagnetic storm that occurred in May 2024. A solar active region produced numerous X-class solar flares and launched multiple Earth-directed coronal mass ejections (CMEs)\u003csup\u003e[16]\u003c/sup\u003e between May 5 and 15, 2024. The CMEs launched on May 8 and 9 struck the Earth and caused the super geomagnetic storm between May 10 and 12\u003csup\u003e[17]\u003c/sup\u003e. The Dst index reached − 406 nT during this storm, which is the largest disturbance in the past 20 years since the November 2003 storm.\u003c/p\u003e \u003cp\u003eHere, we examine the observational data of the Arase satellite during its magnetopause crossing on May 11, 2024. Until 06:52:50 UT (vertical line on the left side in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e, where UT is Universal Time), the Arase satellite remained in the magnetosphere on its outbound orbit between 11.6 and 11.8 magnetic local time (MLT; in hours) at a magnetic latitude (MLAT) of approximately 25°. After 06:52:50 UT, electron fluxes in a broad energy range below 1 keV increased significantly and ion fluxes subsequently followed the electron variation, while the local magnetic field remained northward (\u003cem\u003eB\u003c/em\u003e\u003csub\u003eZ,GSM\u003c/sub\u003e \u0026gt; 0, where \u003cem\u003eB\u003c/em\u003e\u003csub\u003eZ,GSM\u003c/sub\u003e is the Z-component of the local magnetic field in the geocentric solar magnetospheric (GSM) coordinate system) (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ea-d,f). The flux enhancement indicates that the Arase satellite entered the low-latitude boundary layer (LLBL), where the local magnetic field originates from the Earth’s main magnetic field (a northward magnetic field), but one end of a magnetic field line is connected to the solar wind. In LLBL, Arase observed plasma originating from the magnetosheath, which is the region of solar wind decelerated and heated at the bow shock. After 07:04:37 UT (vertical line at the center of Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e), multiple magnetopause crossings occurred at 12.0 MLT under solar wind dynamic pressure from 35 to 39 nPa and IMF-\u003cem\u003eB\u003c/em\u003e\u003csub\u003eZ\u003c/sub\u003e from − 27 to − 31 nT. The positive and negative excursions of \u003cem\u003eB\u003c/em\u003e\u003csub\u003eZ,GSM\u003c/sub\u003e (orange line in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ef) indicate that the magnetopause oscillated back and forth around the position of Arase, so that the satellite was intermittently observing the LLBL and then the magnetosheath boundary layer (MSBL, a southward magnetic field originating from IMF). Since the magnetopause is the boundary of the Earth’s main magnetic field and IMF, it is located between LLBL and MSBL. The first magnetopause crossing by the Arase satellite occurred at a radial distance of 4.96 \u003cem\u003eR\u003c/em\u003e\u003csub\u003eE\u003c/sub\u003e, which is the innermost spacecraft observation of the magnetopause ever available in the literature\u003csup\u003e[18–20]\u003c/sup\u003e. Energetic O\u003csup\u003e+\u003c/sup\u003e ions exhibited parallel flows in the LLBL and anti-parallel flows in the MSBL with sudden flux enhancements (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ee,f). The unidirectional enhanced fluxes represent ion reconnection jets because ions launched from a lower latitude become parallel flows in the northward background magnetic field and anti-parallel flows in the southward background magnetic field in the northern hemisphere. Furthermore, in MSBL, one end of a magnetic field line is connected to the magnetosphere, and thus high-energy ( ≳ 60 keV) protons continue to be detected (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ec). After 07:13:05 UT (vertical line on the right side in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e), the Arase satellite finally entered the magnetosheath proper, and the magnetic field was continuously directed southward (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ef). Magnetic field lines were no longer connected to the magnetosphere in the magnetosheath, and hence Arase did not observe any enhanced fluxes of O\u003csup\u003e+\u003c/sup\u003e ions and the high-energy protons, both of which originate from the magnetosphere (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ec, e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e summarizes the magnetopause crossing by the Arase satellite on May 11, 2024. The magnetopause is represented by a dashed line at ~ 5 \u003cem\u003eR\u003c/em\u003e\u003csub\u003eE\u003c/sub\u003e in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e. LLBL and MSBL lie on the earthward side and sunward side of the magnetopause under a southward IMF. Within the LLBL and MSBL, the magnetic fields are directed northward and southward, respectively. In these regions, the magnetic field lines of the magnetosphere are connected to the magnetosheath, and the plasmas from both the magnetosphere and the magnetosheath are mixed. Thus, energetic O\u003csup\u003e+\u003c/sup\u003e ions and protons still exist in LLBL and MSBL. The magnetospheric electrons at \u0026gt; 10 keV exhibit a rapid decrease in their flux at the middle of LLBL (at 06:56 UT). This is consistent with the fact that the parallel velocity of the magnetospheric electrons is sufficiently high to escape into interplanetary space along an open magnetic field line. The Arase satellite observed these features of the boundary layers as it moved sunward (the right-hand side of Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e).\u003c/p\u003e "},{"header":"Discussion and Conclusions","content":"\u003cp\u003eThe \u003cem\u003ein situ\u003c/em\u003e observation by the Arase satellite further provides insight into the mechanism of magnetospheric erosion during the May 2024 super geomagnetic storm. Figure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ea shows the radial distributions of the local magnetic pressure (\u003cem\u003eP\u003c/em\u003e\u003csub\u003e\u003cem\u003eB\u003c/em\u003e\u003c/sub\u003e, blue line), O\u003csup\u003e+\u003c/sup\u003e thermal pressure (\u003cem\u003eP\u003c/em\u003e\u003csub\u003eO+\u003c/sub\u003e, green line), and total pressure (\u003cem\u003eP\u003c/em\u003e\u003csub\u003e\u003cem\u003eB\u003c/em\u003e\u003c/sub\u003e + \u003cem\u003eP\u003c/em\u003e\u003csub\u003eO+\u003c/sub\u003e, red line) during the interval from 06:11 UT to 07:34 UT on May 11, 2024. In this interval, the O\u003csup\u003e+\u003c/sup\u003e thermal pressure was a good proxy for the ion thermal pressure on the magnetospheric side because ring current O\u003csup\u003e+\u003c/sup\u003e ions dominated the ion thermal pressure (80–90% of ion thermal pressure) in the magnetosphere (Supplementary Information Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e. See Methods for details of the pressure calculation). The O\u003csup\u003e+\u003c/sup\u003e dominance enabled us to distinguish the magnetospheric ion thermal pressure from the solar wind ion (H\u003csup\u003e+\u003c/sup\u003e) thermal pressure. In Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ea, as the Arase satellite moved outward (sunward) in Earth’s strong main magnetic field, the magnetic pressure and the total pressure decreased in the magnetosphere and LLBL. In the vicinity just inside the magnetopause, the total pressure becomes approximately equal to the solar wind dynamic pressure for the magnetopause crossing at 07:03 UT (dashed horizontal line). Therefore, we consider that the pressure balance between solar wind dynamic pressure and magnetospheric pressure was approximately satisfied during the magnetopause crossing by Arase.\u003c/p\u003e\u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ea, the magnetic field is the primary factor of the pressure balance between the solar wind and the magnetosphere. To examine the details of the magnetic pressure variations, we subtracted the magnetic field calculated by the IGRF-14 geomagnetic field model\u003csup\u003e[21]\u003c/sup\u003e from the measured local magnetic field (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eb). Because the IGRF-14 model describes Earth’s main magnetic field, the difference between the local magnetic field and the IGRF-14 model field represents the external magnetic field generated by magnetospheric current systems and local magnetospheric dynamics, such as magnetic reconnection. At \u0026gt; 4.2 \u003cem\u003eR\u003c/em\u003e\u003csub\u003eE\u003c/sub\u003e, the decrease in the local magnetic field intensity became prominent, but its position was still far from the magnetopause. The magnitude of the decrease reached approximately 60 nT at 4.5 \u003cem\u003eR\u003c/em\u003e\u003csub\u003eE\u003c/sub\u003e. Fluctuations in the magnetic field intensity were then observed at larger radial distances. In the LLBL, sharp transient depletions were repeatedly detected. The white line in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eb represents a linear fit to the variation in the magnetic field intensity within the LLBL. The slope of the fitting line shows that the local magnetic field intensity, on average, decreased by ~ 41 nT within LLBL. There is an estimation error of ± 6.5 nT in the amplitude of the decrease, caused by the 1-sigma uncertainty of the inclination of the linear fitting. Fluctuations in the magnetic field intensity intensified in the MSBL, and finally, a strong compressed magnetic field with smaller fluctuations were observed in the magnetosheath.\u003c/p\u003e\u003cp\u003eThe decrease in the magnetic field intensity deep inside the magnetosphere (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eb) clearly shows the dominant contribution of the magnetospheric currents to magnetic field variations. This is consistent with the fact that the magnetopause reconnection is a localized process around the magnetopause and cannot account for the radially-wide decrease in the magnetic field intensity. Moreover, the decrease in the magnetic field intensity beyond the center of the ring current at ~ 3 \u003cem\u003eR\u003c/em\u003e\u003csub\u003eE\u003c/sub\u003e (Supplementary Information Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e) indicates that the Region-1 field-aligned current and/or the cross-tail current counteract the northward magnetic field (upward light blue arrows in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e) generated by the magnetopause current and dayside ring current. Therefore, our findings provide the first experimental evidence of the significant contribution of magnetospheric currents to magnetopause erosion around the low-latitude magnetopause during super geomagnetic storms. Given that the decrease in the magnetic field intensity deep inside the magnetosphere is attributed to the magnetospheric currents, the Region-1 field-aligned current and/or the cross-tail current caused a decrease of \u0026gt; 60 nT if we take the magnetopause current and the ring current into account.\u003c/p\u003e\u003cp\u003eIn addition, the gradual decrease in the magnetic field intensity around the magnetopause also indicates another contributing factor associated with the magnetopause. The transient depletion of the magnetic field intensity in the LLBL was caused by an approach to MSBL, where the magnetic field was much weaker than LLBL. The transient magnetic field depletion and the multiple magnetopause crossings can be attributed to the transient removal of the magnetic flux caused by repeated magnetopause reconnections, as described in previous studies\u003csup\u003e[3–7]\u003c/sup\u003e. Therefore, we conclude that magnetopause reconnection also contributed to the magnetopause erosion. Given that the northward magnetic field generated by the magnetopause current did not vary largely within the thin LLBL, its average contribution to the decrease in the magnetic field intensity derived from linear fitting (~ 41 [± 6.5] nT) is secondary compared to those of the Region-1 field-aligned current and/or the cross-tail current ( ≳ 60 nT). The summation of the decrease in the magnetic field intensity discussed here is ≳ 100 nT, and hence, ≳60% of the decrease in the magnetic field intensity can be attributed to the Region-1 field-aligned current and/or the cross-tail current on average.\u003c/p\u003e\u003cp\u003eThe final remarkable feature of the magnetopause crossing is the ring current ions near the magnetopause. In previous studies, ring current ions were considered to play a role in the pressure balance at the magnetopause\u003csup\u003e[22,23]\u003c/sup\u003e; however, no studies have directly evaluated the contribution from the ring current thermal pressure during a super geomagnetic storm. In this study, the O\u003csup\u003e+\u003c/sup\u003e thermal pressure was significantly high (~ 7 nPa) even at the magnetopause (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ea), indicating that the magnetospheric ring current ions partially pushed back the solar wind exerted on the magnetopause. Approximately 20% of the solar wind dynamic pressure (~ 35 nPa) was directly balanced by the ring current ion thermal pressure. This result suggests that the heavy ion originating from the planetary atmosphere is one of the key factors controlling the magnetopause location during highly disturbed times.\u003c/p\u003e\u003cp\u003eThe present study has suggested that magnetospheric currents are the primary cause of intense magnetopause erosion during the May 2024 super geomagnetic storm. The effect of the magnetospheric currents on the magnetopause erosion should be taken into account more seriously because it can increase the risk of solar energetic particle (SEP) exposure at mid-latitudes. The magnetopause is an approximate boundary of open and closed magnetic field lines (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e), and hence intense magnetopause erosion causes an equatorward shift of the penetration region of SEPs coming from the solar wind through an open field line. The solid line in the top panel of Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e shows the Altitude-Adjusted Corrected Geomagnetic (AACGM) latitude of the magnetic footprint of the magnetopause detected by Arase under a steady IMF-\u003cem\u003eB\u003c/em\u003e\u003csub\u003eZ,GSM\u003c/sub\u003e and \u003cem\u003eP\u003c/em\u003e\u003csub\u003eSW\u003c/sub\u003e (see Method section for details of the determination of the equator boundary of SEPs) during this storm. The magnetic field line was calculated using the Tsyganenko and Sitnov (2005) model\u003csup\u003e[24]\u003c/sup\u003e. In the dayside topside ionosphere, the Polar Orbiting Environmental Satellite (POES) 18 detected SEPs (\u0026gt; 6.9 MeV protons) in the polar region (color-coded map in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e). The magnetopause footprint shows variations synchronized with the equatorward boundary of SEPs, reaching down to 60° AACGM latitudes during the storm main phase. Therefore, the significant equatorward expansion of the SEPs indicates that the magnetospheric currents can surely increase the risk of aircraft exposure to SEPs, which leads to glitches in flight-control data and radiation exposure for the crew at mid-latitudes during super geomagnetic storms.\u003c/p\u003e"},{"header":"Methods","content":"\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003eArase Satellite Data\u003c/h2\u003e \u003cp\u003eThe Arase satellite is equipped with several instruments for the measurement of charged particles. MEP-e\u003csup\u003e[25]\u003c/sup\u003e measures energetic electrons between 7.0 keV and 87.5 keV with 15 energy steps. LEP-e\u003csup\u003e[26]\u003c/sup\u003e measures lower energy electrons between 0.067 keV and 19.1 keV with 28 energy steps. MEP-e and LEP-e cover a solid angle of 4π sr using spacecraft spin motion and provide 8-sec (spin-averaged) omnidirectional electron flux data. We used MEP-i\u003csup\u003e[27]\u003c/sup\u003e and LEP-i\u003csup\u003e[28]\u003c/sup\u003e data for the analysis of ions. MEP-i measures H\u003csup\u003e+\u003c/sup\u003e, He\u003csup\u003e++\u003c/sup\u003e, He\u003csup\u003e+\u003c/sup\u003e, O\u003csup\u003e++\u003c/sup\u003e, O\u003csup\u003e+\u003c/sup\u003e, and N\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e/NO\u003csup\u003e+\u003c/sup\u003e/O\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e between 9.6 keV/q and 184.2 keV/q with 15 energy steps. LEP-i measures H\u003csup\u003e+\u003c/sup\u003e, He\u003csup\u003e+\u003c/sup\u003e, and O\u003csup\u003e+\u003c/sup\u003e between 0.0039 keV and 25.5 keV with 29 energy steps. Both instruments have two observational modes, normal mode and time-of-flight (TOF) mode. In the normal mode, 3D velocity distributions are provided with an angular resolution of 22.5\u0026deg; in the azimuthal and polar directions. While LEP-i was operating in the normal mode during the period considered in this study, MEP-i was in the TOF mode, in which 3D velocity distributions of ions are not provided. Because MEP-i in the TOF mode provides energy spectra of omnidirectional ion fluxes as well as TOF data, we used the omnidirectional ion flux data to calculate ion thermal pressure by combining the MEP-i and LEP-i data obtained in the two different modes. In the calculation of ion thermal pressure, we used MEP-i 32-s TOF-mode omnidirectional fluxes between 22.1-184.2 keV/q and LEP-i 8-s normal-mode omnidirectional fluxes between 0.035 keV and 19.2 keV with an assumption of an isotropic pitch angle distribution:\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$$\\:{P}_{s,\\text{L}\\text{E}\\text{P}-\\text{i}}\\simeq\\:\\frac{4\\pi\\:}{3}\\:\\sqrt{2{m}_{s}}\\sum\\:_{0.035\\:\\text{k}\\text{e}\\text{V}}^{19.2\\:\\text{k}\\text{e}\\text{V}}\\sqrt{E}{J}_{s,\\text{L}\\text{E}\\text{P}-\\text{i}}\\left(E\\right){\\Delta\\:}E,$$\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equb\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equb\" name=\"EquationSource\"\u003e\n$$\\:{P}_{s,\\text{M}\\text{E}\\text{P}-\\text{i}}\\simeq\\:\\frac{4\\pi\\:}{3}\\:\\sqrt{2{m}_{s}}\\sum\\:_{22.1\\:\\text{k}\\text{e}\\text{V}/\\text{q}}^{184.2\\:\\text{k}\\text{e}\\text{V}/\\text{q}}\\sqrt{E}{J}_{s,\\text{M}\\text{E}\\text{P}-\\text{i}}\\left(E\\right){\\Delta\\:}E,$$\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equc\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equc\" name=\"EquationSource\"\u003e\n$$\\:{P}_{s}={P}_{s,\\text{L}\\text{E}\\text{P}-\\text{i}}+{P}_{s,\\text{M}\\text{E}\\text{P}-\\text{i}},$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere \u003cem\u003em\u003c/em\u003e\u003csub\u003e\u003cem\u003es\u003c/em\u003e\u003c/sub\u003e is the mass of ion species \u003cem\u003es\u003c/em\u003e, \u003cem\u003eE\u003c/em\u003e is the ion energy, and Δ\u003cem\u003eE\u003c/em\u003e is the energy step width. To apply the trapezoidal rule to the pressure calculation, \u003cem\u003eE\u003c/em\u003e and Δ\u003cem\u003eE\u003c/em\u003e were obtained from log\u003csub\u003e10\u003c/sub\u003e\u003cem\u003eE\u003c/em\u003e = (log\u003csub\u003e10\u003c/sub\u003e\u003cem\u003eE\u003c/em\u003e\u003csub\u003e\u003cem\u003ei\u003c/em\u003e\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;log\u003csub\u003e10\u003c/sub\u003e\u003cem\u003eE\u003c/em\u003e\u003csub\u003e\u003cem\u003ei+\u003c/em\u003e1\u003c/sub\u003e)/2, Δ\u003cem\u003eE\u003c/em\u003e\u0026thinsp;=\u0026thinsp;\u003cem\u003eE\u003c/em\u003e\u003csub\u003e\u003cem\u003ei\u003c/em\u003e+1\u003c/sub\u003e \u0026ndash; \u003cem\u003eE\u003c/em\u003e\u003csub\u003e\u003cem\u003ei\u003c/em\u003e\u003c/sub\u003e, where \u003cem\u003ei\u003c/em\u003e represents the \u003cem\u003ei\u003c/em\u003e-th energy step. \u003cem\u003eJ\u003c/em\u003e\u003csub\u003e\u003cem\u003es\u003c/em\u003e,LEP\u0026minus;i/MEP\u0026minus;i\u003c/sub\u003e is the omnidirectional differential number flux of LEP-i/MEP-i obtained from \u003cem\u003eJ\u003c/em\u003e\u003csub\u003e\u003cem\u003es\u003c/em\u003e,LEP\u0026minus;i/MEP\u0026minus;i\u003c/sub\u003e(\u003cem\u003eE\u003c/em\u003e) = (\u003cem\u003eJ\u003c/em\u003e\u003csub\u003e\u003cem\u003es\u003c/em\u003e,LEP\u0026minus;i/MEP\u0026minus;i\u003c/sub\u003e (\u003cem\u003eE\u003c/em\u003e\u003csub\u003e\u003cem\u003ei\u003c/em\u003e\u003c/sub\u003e) + \u003cem\u003eJ\u003c/em\u003e\u003csub\u003e\u003cem\u003es\u003c/em\u003e,LEP\u0026minus;i/MEP\u0026minus;i\u003c/sub\u003e (\u003cem\u003eE\u003c/em\u003e\u003csub\u003e\u003cem\u003ei\u003c/em\u003e+1\u003c/sub\u003e))/2. \u003cem\u003eP\u003c/em\u003e\u003csub\u003e\u003cem\u003es\u003c/em\u003e\u003c/sub\u003e is the ion thermal pressure obtained by combining the LEP-i pressure (\u003cem\u003eP\u003c/em\u003e\u003csub\u003e\u003cem\u003es\u003c/em\u003e,LEP\u0026minus;i\u003c/sub\u003e) and MEP-i pressure (\u003cem\u003eP\u003c/em\u003e\u003csub\u003e\u003cem\u003es\u003c/em\u003e,MEP\u0026minus;i\u003c/sub\u003e) of ion species \u003cem\u003es\u003c/em\u003e.\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eMagnetopause Location under a Steady Solar Wind\u003c/h3\u003e\n\u003cp\u003eTo exclude dynamic variations of the magnetopause location in the comparison analysis between the Arase and POES observations, we used the magnetopause location under a steady solar wind. The criteria for a steady solar wind are as follows. For the solar wind data during the past 10 min, (i) IMF-\u003cem\u003eB\u003c/em\u003e\u003csub\u003eZ,GSM\u003c/sub\u003e and \u003cem\u003eP\u003c/em\u003e\u003csub\u003eSW\u003c/sub\u003e variation is less than 50% of an average over the past 10 min, (ii) more than 5 data points (\u0026gt;\u0026thinsp;5 min) of IMF-\u003cem\u003eB\u003c/em\u003e\u003csub\u003eZ\u003c/sub\u003e and \u003cem\u003eP\u003c/em\u003e\u003csub\u003eSW\u003c/sub\u003e pressure have finite values, (iii) the northward-southward direction of IMF does not change. In Supplementary Information Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, the steady solar wind selected with these criteria is represented in blue-filled circles. The magenta-filled circles represent the period of an unsteady solar wind. During the steady solar wind, Arase crossed the magnetopause six times, as shown in Supplementary Information Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. Because IMF-B\u003csub\u003eZ\u003c/sub\u003e was southward during these periods, we considered that the magnetopause corresponds to a step-like variation between the northward/southward magnetic field at Arase.\u003c/p\u003e\n\u003ch3\u003ePOES-18 Satellite Data and Equator Boundary of SEPs\u003c/h3\u003e\n\u003cp\u003eThe POES-18 satellite is a Low Earth Orbit (LEO) satellite at polar orbits with an orbital period of ~\u0026thinsp;100 min. It is equipped with the MEPED instrument of the Solar Environment Monitor (SEM-2) instrument package, which measures trapped and precipitating fluxes of protons from 30 keV to \u0026gt;\u0026thinsp;6,900 keV\u003csup\u003e[29]\u003c/sup\u003e. The equatorward boundary of SEPs was determined from the lowest latitude of the latitudinal bins with proton fluxes of \u0026gt;\u0026thinsp;0.1 s\u003csup\u003e\u0026minus;1\u003c/sup\u003ecm\u003csup\u003e\u0026minus;2\u003c/sup\u003esr\u003csup\u003e\u0026minus;1\u003c/sup\u003ekeV\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e on each spacecraft orbit at AACGM latitudes of \u0026gt;\u0026thinsp;55\u0026deg; in the northern hemisphere.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eData Availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe ERG Science Center\u003csup\u003e[3\u003c/sup\u003e\u003csup\u003e0\u003c/sup\u003e\u003csup\u003e]\u003c/sup\u003e, which is operated by ISAS/JAXA and Nagoya University, archives the science data of the Arase satellite. The data used in the present study are publicly available from the ERG Science Center website (https://ergsc.isee.nagoya-u.ac.jp/index.shtml.en).\u0026nbsp;This study used the MGF Level-2 8 sec spin-averaged v04.06 data\u003csup\u003e[3\u003c/sup\u003e\u003csup\u003e1\u003c/sup\u003e\u003csup\u003e,3\u003c/sup\u003e\u003csup\u003e2\u003c/sup\u003e\u003csup\u003e]\u003c/sup\u003e, MEP-i Level-2 omnidirectional flux v02_02\u0026nbsp;data\u003csup\u003e[\u003c/sup\u003e\u003csup\u003e27\u003c/sup\u003e\u003csup\u003e,3\u003c/sup\u003e\u003csup\u003e3\u003c/sup\u003e\u003csup\u003e]\u003c/sup\u003e, MEP-e Level-2 omnidirectional flux v01_02 data\u003csup\u003e[28,37]\u003c/sup\u003e, LEP-i Level-2 omnidirectional flux v03_00 data\u003csup\u003e[\u003c/sup\u003e\u003csup\u003e28\u003c/sup\u003e\u003csup\u003e,3\u003c/sup\u003e\u003csup\u003e5\u003c/sup\u003e\u003csup\u003e]\u003c/sup\u003e, LEP-e Level-2 omnidirectional flux v04_01 data\u003csup\u003e[2\u003c/sup\u003e\u003csup\u003e6\u003c/sup\u003e\u003csup\u003e,3\u003c/sup\u003e\u003csup\u003e6\u003c/sup\u003e\u003csup\u003e]\u003c/sup\u003e, Level-2 definitive orbital v05 data\u003csup\u003e[\u003c/sup\u003e\u003csup\u003e37\u003c/sup\u003e\u003csup\u003e]\u003c/sup\u003e, and Level-3 TS04 model v01 data\u003csup\u003e[\u003c/sup\u003e\u003csup\u003e38\u003c/sup\u003e\u003csup\u003e]\u003c/sup\u003e. The OMNI 1-min solar wind v01 data set\u003csup\u003e[\u003c/sup\u003e\u003csup\u003e39\u003c/sup\u003e\u003csup\u003e]\u003c/sup\u003e was obtained from the Goddard Space Flight Center Space Physics Data Facility OMNIWeb (http://omniweb.gsfc.nasa.gov). The Dst index\u003csup\u003e[1\u003c/sup\u003e\u003csup\u003e3\u003c/sup\u003e\u003csup\u003e]\u003c/sup\u003e was provided by the World Data Center for Geomagnetism, Kyoto (http://wdc.kugi.kyoto-u.ac.jp/wdc/Sec3.html). The POES-18 \u0026nbsp;SEM-2\u0026nbsp;MEPED 2-s flux\u0026nbsp;data\u0026nbsp;(v01), which are originally provided by National Oceanic and Atmospheric Administration (NOAA),\u0026nbsp;are available at\u0026nbsp;Coordinated Data Analysis Web (CDAWeb) https://cdaweb.gsfc.nasa.gov/istp_public/data/noaa/noaa18/sem2_fluxes-2sec/.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCode Availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe Space Physics Environment Data Analysis System (SPEDAS)\u003csup\u003e[40]\u003c/sup\u003e software (version 6.1) was used in the analysis. The software is publicly available at https://themis.ssl.berkeley.edu/socware/.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe thank T.\u0026nbsp;Segawa (Technical Center, Nagoya University) for maintaining the data archiving system at the ERG Science Center. We thank Editage for English language editing.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding Declaration\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe LEPe development is supported by the Academia Sinica, National Cheng Kung University, and the Ministry of Science and Technology of Taiwan under Contract No. MOST 105-3111-Y-001-042 and MOST 106-2111-M-001-011. A.S. was supported by JSPS KAKENHI Grant Numbers 24K07112, 24K00898, and 23K22555.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eContributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eK.Y., Y.M., and N.K. conceived this study. K.Y. collected, analyzed data, and prepared the manuscript. K.Y., Y.M., N.K., A.S., and R.N. interpreted the result. A.M., M.T., K.Y. (MGF), S.Y., S.K., K.K., T.H. (MEP-e and MEP-i), K.A., Y.M. (LEP-i), Y.K., S.Y.W., S.W.Y.T., T.F.C., B.J.W., and C.W.J. (LEP-e) contributed to the instrument development, operation, data processing, and data quality assurance. I.S. and Y.M. led the ERG mission. All authors reviewed and improved the manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAdditional Information\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eChapman, S. \u0026amp; Ferraro, V. C. A. A new theory of magnetic storms. \u003cem\u003eTerr. Magn. Atmos. Electr.\u003c/em\u003e \u003cstrong\u003e38\u003c/strong\u003e, 79\u0026ndash;96 (1933).\u003c/li\u003e\n\u003cli\u003eFairfield, D. H. Average and unusual locations of the Earth\u0026apos;s magnetopause and bow shock. \u003cem\u003eJ. Geophys. Res.\u003c/em\u003e \u003cstrong\u003e76\u003c/strong\u003e, 6700\u0026ndash;6716 (1971).\u003c/li\u003e\n\u003cli\u003eAubry, M. P., Russell, C. T. \u0026amp; Kivelson, M. G. Inward motion of the magnetopause before a substorm. \u003cem\u003eJ. Geophys. 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The Level-3 orbit data of Exploration of energization and Radiation in Geospace (ERG) Arase satellite, (2018d). [Data set]. Accessed on2026-January 20.\u003c/li\u003e\n\u003cli\u003ePapitashvili, N. E. \u0026amp; King, J. H. OMNI 1-min Data Set. [Data set] (NASA Space Physics Data Facility, 2020). Accessed on2026-January 20.\u003c/li\u003e\n\u003cli\u003eAngelopoulos, V. et al. The Space Physics Environment Data Analysis System (SPEDAS). \u003cem\u003eSpace Sci. Rev.\u003c/em\u003e \u003cstrong\u003e215\u003c/strong\u003e, 9 (2019).\u003c/li\u003e\n\u003cli\u003eLin, R. L., Zhang, X. X., Liu, S. Q., Wang, Y. L. \u0026amp; Gong, J. C. A three-dimensional asymmetric magnetopause model. \u003cem\u003eJ. Geophys. Res.\u003c/em\u003e \u003cstrong\u003e115\u003c/strong\u003e, A04207 (2010).\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":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"May 2024 super geomagnetic storm, solar storm, magnetopause, solar energetic particles (SEPs), magnetospheric current system, Arase (ERG) satellite","lastPublishedDoi":"10.21203/rs.3.rs-8649685/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8649685/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eEarth\u0026rsquo;s main magnetic field forms the magnetosphere, a barrier against solar wind plasmas. A southward magnetic field carried by the solar wind makes the magnetosphere smaller, which is termed \u0026ldquo;magnetopause erosion.\u0026rdquo; This process leads to equatorward extension of the polar region, where solar wind plasmas directly intrude into the magnetosphere. Although previous studies have suggested that magnetopause reconnection causes magnetopause erosion, the erosion mechanism during super geomagnetic storms remains controversial owing to a lack of \u003cem\u003ein situ\u003c/em\u003e measurements. Here, we report a magnetopause crossing by the Arase satellite at 4.96 \u003cem\u003eR\u003c/em\u003e\u003csub\u003eE\u003c/sub\u003e during the May 2024 super geomagnetic storm, which is the innermost magnetopause ever detected by magnetospheric spacecraft. An unexpected decrease in the magnetic field intensity extended from the inner magnetosphere to the magnetopause, indicating that magnetospheric currents reduced Earth\u0026rsquo;s main magnetic field on a global scale. Consequently, the solar wind pushed the weakened magnetosphere further toward the Earth, resulting in the severe shrinkage of the magnetosphere. This shrinkage led to a significant equatorward penetration of solar energetic particles on the dayside, suggesting that the magnetospheric currents can increase the risk of aircraft exposure to solar energetic particles at mid-latitudes.\u003c/p\u003e","manuscriptTitle":"Severe shrinkage of Earth’s magnetosphere during the May 2024 solar storm","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-04-23 13:22:53","doi":"10.21203/rs.3.rs-8649685/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-03-09T09:50:55+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-03-07T01:21:46+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-02-28T04:04:13+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"313696736223277366207133403892371240572","date":"2026-02-18T01:14:51+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"45779724753056979601789021660743953973","date":"2026-02-17T13:30:13+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"206673321133575750467700810517613782855","date":"2026-02-07T16:13:04+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-02-05T16:23:49+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-02-05T16:00:29+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-02-02T04:22:18+00:00","index":"","fulltext":""},{"type":"submitted","content":"Scientific Reports","date":"2026-02-02T04:15:26+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"c31b9959-2c4d-477d-8394-756f2654bfab","owner":[],"postedDate":"April 23rd, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-05-09T14:53:13+00:00","versionOfRecord":[],"versionCreatedAt":"2026-04-23 13:22:53","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8649685","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8649685","identity":"rs-8649685","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}