Asymmetry in the Morning and Evening Occurrence of Noctilucent Clouds in Japan

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In recent years, NLC occurrences have been reported to expand toward lower latitudes. Particularly, the number of NLC detections from midlatitudes (below 50°) is increasing. In Japan, the NLC imaging network has been updated since the first NLC detection from Hokkaido in 2015. Four more events were observed in 2020, one in 2021, and four in 2025, resulting in a total of 10 NLC observations at present. The event on June 14, 2020, was particularly notable, reaching a southernmost latitude of 44.6°N, which is the first observation of NLCs extending over the main island of Hokkaido. Except for one case, all events in Hokkaido occurred in the early morning hours (2:00–3:00 LST), indicating a strong local-time dependence on the occurrence. In this study, we investigated the occurrence characteristics of NLCs from Hokkaido and their relationship with the background atmospheric conditions. By analyzing the NLC formation environment and advection history using temperature and water vapor data from Aura/Microwave Limb Sounder (MLS) and temperature data from the JAGUAR-DAS Whole neutral Atmosphere Reanalysis (JAWARA), we obtained the following key findings. In the longitudinal sector including Hokkaido, a low-temperature condition favorable for NLC conservation is formed by atmospheric tidal waves prior to the morning hours. This creates a favorable environment for NLC survival during its advection from high latitudes. This tidal activity creates a distinct observational advantage for morning NLC observations, consistent with the fact that most events in this study were observed in the morning. Noctilucent clouds (NLCs) Mesopause region Upper mesosphere Middle latitudes NLCs Atmospheric tides JAWARA Aura/MLS Hokkaido Japan Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 1. Introduction Noctilucent clouds (NLCs) are the highest clouds in the Earth’s atmosphere, occurring at altitudes of approximately 82–85 km in polar regions, and are typically observed during summer in high-latitude regions (50°–60° in latitude). NLCs are composed of ice particles with mean radius of approximately 50 nm. Because NLCs become visible by scattering sunlight from below the horizon, their typical observation windows occur during the twilight periods after sunset and before sunrise, when the solar zenith angle ranges from approximately 96° and 106° (Fogle and Haurwitz, 1966 ). NLC observations are effective for monitoring atmospheric dynamics in the mesopause region with high spatial and temporal resolution. In recent years, NLC occurrences have been reported to expand toward lower latitudes. Particularly, the number of NLC detections from midlatitudes (below 50°) is increasing, and these NLCs are generally interpreted as being advected from higher latitudes (Kaifler et al., 2018 ). For example, 36 events were reported in North America between 2003 and 2011 (Russell III et al., 2014 ). In Europe, more than five events per year were recorded from 2005 to 2021, with as many as 22 cases in 2009 (Dalin et al., 2023 ). As an example of observations at extremely low latitudes, NLCs were observed at the Calar Alto Observatory, Spain (37°N, 3°W) in 2012, 2019, and 2021 (Russell III et al., 2014 ; Gerding et al., 2021 ; Lange et al., 2022 ). In Asia, NLCs were observed in Hokkaido, Japan (42°N) in 2015 (Suzuki et al., 2016 ) and in Beijing, China (40°N) in 2020 (Miao et al., 2022 ). In the summer (June–August) of 2020, numerous NLCs were observed at middle latitudes of the Northern Hemisphere. The number of NLCs in 2020 was unprecedented, indicating that 2020 was an NLC outbreak year, as noted in Dalin et al. ( 2023 ). In Japan, the NLC imaging network has been updated since the first NLC detection in Hokkaido in 2015. Four more events were observed in 2020, one in 2021, and four in 2025, resulting in a total of 10 NLC cases at present. Except for one case in 20 LST, all events occurred in the early morning hours (2:00‒3:00 LST). Although the occurrence of NLCs after sunset and before sunrise is geometrically possible, the actual observations show an asymmetry in the morning and evening occurrence suggesting a strong local-time dependence. Previous studies have reported several instances of NLC occurrence showing a similar dependence on local time as our observations. Ground-based observations in North America (46°N-71°N) have reported higher detection rates in the evening (Fogle and Haurwitz, 1966 ; Fogle, 1968 ). Lidar observations in the European polar region (69°N, 16°E) attributed this dependence to diurnal and semi-diurnal atmospheric tides (Fiedler et al., 2011 ). To investigate the causes of this local-time dependence of NLCs in Hokkaido (43°N-45°N), we compared multiple ground-based NLC events with temperature and water vapor field data obtained from the Microwave Limb Sounder (MLS) onboard the Aura satellite. The analysis revealed that the background fields at the observation time consistently show lower temperature values compared with 8–12 h prior. A likely explanation is the effect of atmospheric solar tides. To confirm this hypothesis, we investigated the meteorological fields in the upper mesosphere around North Japan using the JAGUAR-DAS Whole neutral Atmosphere Reanalysis (JAWARA) data, which includes assimilation up to the lower thermosphere (z = 110 km). The results strongly suggest that tidal wave modulations are responsible for the local-time dependence of NLC occurrences in Japan. The remainder of this article is organized as follows. Section 2 describes the NLC events observed from Hokkaido from 2020 to 2025. Section 3 presents the estimation method for NLC occurrence latitude and outlines the procedures for background and advection analysis based on the Aura/MLS data. Section 4 presents our analysis results. Section 5 discusses the implications of the JAWARA data. Finally, Section 6 concludes this study. 2. Datasets 2.1 NLC Observation Data from Hokkaido In this study, NLC events were identified using image data captured by an observatory staff and an automatic camera system network. The presence of NLCs was confirmed by eye inspection based on differences in color, brightness, motion, and fine-scale structures compared to the surrounding tropospheric clouds. Details of the automatic camera system network were described by Suzuki et al. ( 2016 ); however, the camera at Moshiri had ceased operations since 2020. Figure 1 shows the location of the observation sites and the field of view in this study. This figure assumes an altitude of 84 km and no obstacles above the horizon for the field of view. Some updates to the observation network since 2015 include the transition of several cameras to a web-based remote-control system and the implementation of near real-time Himawari NLC observations (Tsuda et al., 2018 ; 2022 ) on the website ( http://ttt01.cei.uec.ac.jp/himawari/ ). This system enabled flexible adjustment of imaging intervals in response to NLC activities, allowing for more responsive observations. We also evaluated the clear-sky ratio at the observation sites. Here, “clear-sky ratio” is defined as the proportion of the NLC-observable period determined to be clear based on visual inspection of camera imagery. For example, in 2020, the mean clear-sky fraction across four observation sites in Hokkaido during the NLC season (1 June–30 July) was 23% at morning and 29% at evening, suggesting no substantial difference in observing conditions between morning and evening. Table 1 summarizes the NLC events analyzed in this study. A total of nine events were observed from 2020 to 2025. In June 2020, NLCs were observed for three successive nights from June 12 to 14, 2020. The event on June 14, 2020, which lasted for 72 min, was the longest recorded event in Japan. The event on July 18, 2020, represents the latest occurrence relative to the summer solstice among all NLC events observed in Japan. A single event was observed on June 22, 2021. No events were confirmed in Japan from 2022 to 2024. In 2025, one event was observed on June 25, 2025, followed by two consecutive events on July 7–8, 2025, and another on July 12, 2025. Figure 2 shows an example of the observation data: an NLC image captured from Monbetsu, Hokkaido (44.3°N, 143.3°E) at 02:30 LST on June 14, 2020. From the images, we determined the southernmost latitude and duration for each event using the method described in Section 3.1 . Table 1 Summary of NLC events observed over Hokkaido from 2020 to 2025. Site abbreviations are as follows: NYR, Nayoro (44.4°N, 142.5°E); RIK, Rikubetsu (43.5°N, 143.6°E); SAR, Sarobetsu (45.2°N, 141.8°E); MON, Monbetsu (44.3°N, 143.3°E); and SHU, Lake Shumarinai (44.3°N, 142.2°E) No. DATE YYYYMMDD SITE NAME LST period of NLC detection (LST = UTC + 9hr) 1 20200612 NYR,RIK,SAR 20:37 − 20:49 2 20200613 NYR,MON,SAR 02:27 − 02:30 3 20200614 RIK,MON 01:59 − 03:11 4 20200718 MON,SAR,SHU 02:47 − 02:57 5 20210622 RIK,MON 02:22 − 02:27 6 20250625 RIK,MON 02:04 − 02:55 7 20250707 MON 02:19 (single data) 8 20250708 MON 02:35 − 02:47 9 20250712 MON 02:42 − 02:57 2.2 Reanalysis and Satellite Data To investigate the mechanisms of NLC events, we analyzed the background atmospheric fields in the upper mesosphere using two datasets. The first dataset is Version 4 /Level 2 water vapor and temperature data from the MLS radiometer aboard the Aura satellite. The second dataset is JAWARA (Koshin et al., 2025 ; Sato and Koshin, 2025 ), which is a long-term meteorological reanalysis dataset that assimilates satellite observations into a global atmospheric model. A key feature of JAWARA is its extensive vertical range, covering altitudes from the surface to the lower thermosphere (up to 110 km). The dataset provides a temporal resolution of 1 h, a horizontal grid resolution of 2.8°, and a vertical resolution of less than 1 km. 3. Analysis and Method 3.1 Estimation of Southernmost Latitude from Images For each event, the southernmost latitude of NLCs was estimated using the image projection method described by Suzuki et al. ( 2015 ). In applying this method, the height of the NLC layer was assumed to be 84 km, following Suzuki et al. ( 2015 ). Table 2 summarizes the estimation results, indicating that the NLCs observed over Hokkaido advected southward to latitudes between 44.2°N and 48.8°N. Notably, the term “southernmost latitude” in this study refers to the visible extent of NLCs in the captured images. Figure 3 shows the analysis result for the event that reached the lowest latitude among the nine considered events. Figure 3 a shows the original image captured in Rikubetsu at 02:56:03 LST on June 14, 2020. Figure 3 b is the enhanced contrast version of Fig. 3 a. The enhancement was performed by subtracting the background count from the selected area of the original image. Figure 3 c shows the map projection result of the location of NLC; the NLC region is enclosed by the red line. This event had an estimated southernmost latitude of 44.6°N, as indicated by the red arrow in Fig. 3 c. As indicated by the red line in Fig. 3 c, Suzuki et al. ( 2016 ) reported the southernmost latitude of NLCs over Hokkaido as 47.5°N for the 2015 event. This 2020 event represents a southward extension of up to 2.9° compared with the 2015 event reported by Suzuki et al. ( 2016 ). It marks the first observation result of NLCs reaching the airspace over the main island of Hokkaido. Table 2 Estimated southernmost of NLC distribution latitude for each event No. Limit of South Latitude 1 48.8°N 2 47.7°N 3 44.6°N 4 48.3°N 5 48.7°N 6 45.5°N 7 48.4°N 8 46.5°N 9 47.1°N 3.2 Analysis of Background Fields and Advection History The formation of NLCs in the upper mesosphere at latitudes below 50° is difficult since extremely low temperatures (< 140 K) are required for ice particle nucleation. Therefore, it is widely hypothesized that most NLCs observed at midlatitudes do not form in situ but are instead advected from their formation regions at higher latitudes (Kaifler et al., 2018 ). We analyzed the background atmospheric fields possibly experienced by the air parcels associated with the observed NLCs using data from the Aura/MLS instrument. The 2025 events were not included in this analysis due to MLS data availability limitations. To evaluate the background field potential for NLC formation and conservation, we introduced a parameter, T s , defined as the difference between the atmospheric temperature retrieved from MLS ( T mls ) and the water vapor frost point ( T frost ). T frost was calculated from the saturated water vapor pressure based on the approximation given by Murphy and Koop ( 2005 ). T s was derived by substituting the water vapor pressure ( Pw mls [Pa]) and temperature ( T mls [K]) into Equations (1) and (2). $$\:{T}_{\text{f}\text{r}\text{o}\text{s}\text{t}}\:\approx\:\:\frac{1.814625\:\text{ln}\left({Pw}_{\text{m}\text{l}\text{s}}\right)+6190.134}{29.120\:-\:\text{ln}\left({Pw}_{\text{m}\text{l}\text{s}}\right)}\:\:\left({T}_{\text{f}\text{r}\text{o}\text{s}\text{t}}>115\text{K}\right)\:\:\left(1\right)$$ $$\:{T}_{\text{s}}={T}_{\text{m}\text{l}\text{s}}\:-\:{T}_{\text{f}\text{r}\text{o}\text{s}\text{t}}\:\:\:\:\left(2\right)$$ Regarding the relationship between NLCs and Ts , a larger T s indicates a field that is hostile to the formation and persistence of NLCs. On the other hand, a smaller T s indicates more favorable conditions. This is because T s represents the difference between the frost point and the atmospheric temperature for these ice-particle clouds. Figure 4 shows a time–latitude plot of the background field ( T s ) for the NLC event on June 14, 2020. The horizontal axis represents time in hours relative to the NLC detection time ( t = 0) for each event, whereas the vertical axis represents latitude. The color of each point corresponds to the T s , as indicated by the color bar. For visualization purposes, values exceeding 20 K or below − 20 K are saturated at these limits. The solid red, green, and blue lines represent typical velocities of a meridional advection of 5, 10, and 15 m/s, respectively (Kirkwood and Stebel, 2003 ). These velocity values are supposed to be constant during 30 hours before the NLC event. The trajectories’ origin is set to the southernmost latitude of the NLC event, as determined in Section 3.1 , and marked by the dashed black line. The T s values were derived from Aura/MLS observations at the 0.0046-hPa pressure level (height ~ 84 km), a longitude range of ± 30° from the observation site, and a 40–90°N latitude range. It should be noted that, because this analysis relies on the relatively sparse Aura/MLS dataset, a wide longitudinal window (± 30° from the observation site) was used when constructing the plot. As a result, the T s values shown in Fig. 4 do not necessarily represent conditions exactly at the same geographic location where the NLCs were observed, and thus cannot be interpreted as a direct indicator of NLC presence or absence. This limitation arises from the possibility that gravity waves with temperature amplitudes of approximately 10 K exist within the analysis region and may contribute to small-scale variability that is not resolved by Aura/MLS (Rapp, 2002). In addition to these spatial limitations, the Aura is a sun-synchronous polar orbit satellite with equatorial crossing times of 01:45 and 13:45 LT. Consequently, the data coverage is sparse, and caution is required as this analysis cannot fully capture diurnal variations. 4. Results Figure 5 shows the results of applying the analysis described in Section 3.2 for the six NLC events (Event numbers 1–6 in Table 1 ) observed from Hokkaido between 2015 and 2021. The reference time ( t = 0, NLC detection time) for each panel is as follows: (a) 02:30 LST, June 21, 2015; (b) 20:30 LST, June 12, 2020; (c) 02:30 LST, June 13, 2020; (d) 02:30 LST, June 14, 2020; (e) 02:47 LST, July 18, 2020; and (f) 02:27 LST, June 22, 2021. This analysis reveals a common feature among the events. Except for the evening event of June 12, 2020 (Fig. 5 b), the background fields at the observation time consistently show lower T s values compared with ~ 12 h prior. This feature suggests that the background fields at the observation time were favorable for NLC survival. Note that the T s values shown in Fig. 5 include cases where T s exceeds zero near the observation time. As discussed in Section 3 , our interpretation here focuses solely on whether the atmospheric conditions were relatively favorable or unfavorable for NLC occurrence at the observation time and 8–12 hours prior. Conversely, for all advection scenarios, the air parcels were in regions with T s values much larger than 0 approximately 8–12 h before the observation. This feature implies that the NLCs were advected through the subsequent cooling period for all six events. If the advection is valid, this suggests that colder conditions existed within the spatial-temporal gaps of the MLS data coverage. Furthermore, a similar trend was confirmed when evaluating the background field using only MLS temperature data ( T mls ) instead of T s (not shown). T mls were also higher approximately 8–12 hours prior to observation, indicating conditions that were hostile to the conservation and generation of NLCs. These characteristics point to the strong possibility that a background field with a large semi-diurnal variation and its local-time-fixed harmonic variation plays a key role in governing the frequency of NLC occurrences over Hokkaido. We have analyzed JAWARA data to verify the effect of atmospheric tidal waves, presented in the discussion. 5. Discussion In the previous section, we investigated the background fields for six NLC events over Hokkaido using Aura/MLS data. The analysis confirmed that air parcels had low T s values at the time of the observations, creating favorable conditions for NLC maintenance. Further, approximately 8–12 h before the observations, the environment was characterized by high T s values, which are unsuitable for NLC formation and conservation. However, Aura/MLS data are limited by their temporal resolution, providing observations only twice a day over the same longitude. This sparse coverage makes it difficult to fully characterize the diurnal temperature variations at a fixed location. To overcome this limitation, we employed the JAWARA dataset to investigate diurnal temperature variations at the upper mesosphere within the observed longitudinal sector (Yamazaki et al., 2025 ). This dataset provides a temporal resolution of 1 h, a horizontal grid resolution of 2.8°, and covers an extensive vertical range from the surface to the lower thermosphere with a vertical resolution of less than 1 km. This high temporal resolution makes it possible to analyze local-time-dependent temperature variations in the upper mesosphere region at a fixed longitude. For this analysis, we derived the temperature distribution at a 0.0049-hPa pressure level (height ~ 84 km) within a geographic region defined by 140–160°E longitude and 50–55°N latitude. This sector is the NLC-forming region in the longitudes of Hokkaido. The analysis period for each year from 2015 to 2024 was set to 40 days, centered on the summer solstice. Because NLCs typically begin to appear in early June in association with the solstitial season, we selected the 40-day interval from June 1 to July 10—corresponding to ± 20 days around the solstice—as the analysis window. It should be noted that, while the discussion in Section 4 was based on the parameter Ts calculated from Aura/MLS temperature and water vapor measurements, JAWARA does not provide water vapor data despite offering global atmospheric temperature fields with high temporal and spatial resolution. Therefore, the analysis in this section is conducted using temperature alone. Figure 6 shows the 40-day averaged temperature variation as a function of local time. Figure 6 a reveals that the temperature variation is dominated by a combination of semidiurnal and diurnal variations. The two local times (1:30 and 13:30) are indicated by the red line in Fig. 6 a. A consistent pattern is observed across all ten years, with minimum temperatures occurring around 0:00 and 10:00 LST, and maxima around 05:00 and 16:00 LT. Further, a comparison of temperatures around Aura/MLS observation times (01:30 and 13:30 LT) shows that the 01:30-LT temperature is 4.2 K lower than the 13:30-LT temperature, on average. Using the JAWARA dataset, it was also demonstrated that atmospheric conditions at 01:30 LST are more favorable for NLC occurrence than those at 13:30 LT. This result is consistent with the Aura/MLS analysis shown in Section 4 . For comparison, Figs. 6 b (60–80°E), 6c (0–20°E), and 6d (100–120°W) illustrate the results of the same analysis applied to different longitudinal sectors. Figure 6 b corresponds approximately to Central Asia, Fig. 6 c to Europe, and Fig. 6 d to North America. In the sector shown in Fig. 6 b, the diurnal and semidiurnal variations with a local-time-fixed pattern are similar to those over Hokkaido. In contrast, the corresponding variations in Figs. 6 c and 6 d differ markedly from those in the Hokkaido sector. These results indicate that the local-time dependence of temperature variability varies substantially with longitude, even at the same latitude. Using the JAWARA data, we compared temperatures during the typical evening (20:00–21:00 LT) and morning (02:00–03:00 LT) hours, which are suitable for NLC sighting by means of solar elevation. Figure 7 shows the longitudinal temperature dependence at the NLC altitude for these two time windows, analyzed under the same conditions as in Fig. 6 . Specifically, Figs. 7 a and 7 b show the mean temperature averaged over 50–55°N and calculated for every 10° of longitude for the morning (02:00 and 03:00 LT) and evening (20:00 and 21:00 LT) hours, respectively. Essentially, these figures visualize the temperature differences across longitudes at a fixed local time. Three key features are evident in Fig. 7 . First, both morning and evening results exhibited a longitudinally dependent pattern that is consistent across all years. Second, although the 40-day averaged temperature varies from year to year, this interannual variability is more pronounced in the morning than in the evening. Third, the specific structure of the longitudinal temperature dependence differs between the morning and the evening, attributable to longitudinal variations in the excitation and propagation of atmospheric tides originating from the lower atmosphere. The quasistationary nature of these patterns is thought to be influenced by factors such as topography and the land and sea distribution (Pancheva and Mukhtarov, 2011 ). Figure 8 shows the temperature difference between the morning and evening, calculated by subtracting the 7-year average evening temperatures (Fig. 7 b) from the morning temperatures (Fig. 7 a). The zero line is indicated by the red line in Fig. 8 . In this plot, a negative value indicates that the morning is colder than the evening, thereby representing conditions more favorable for NLC observation. The longitudinal sector corresponding to Hokkaido (139∼145°E; enclosed by two green dashed lines) shows a temperature difference of ~ − 2 K, suggesting a slight advantage for morning NLC sightings. However, a more important point is that the longitudinal sector with large negative values ( ~ − 6 K) is located in the eastern area of Hokkaido. Because the NLC is advected by westward zonal and southward meridional flows in the summer mesosphere, the temperature in the eastern part of the observation site is crucial. This finding is consistent with the observational fact that most NLC events in this study were observed during the 02:00–03:00 LT window. On a global scale, the analysis reveals that morning observations are favored across most longitudes. A particularly strong morning advantage is found in the region between 10 and 30°W, where the temperature difference reaches approximately − 9 K in the eastern area. Conversely, a prominent region where evening observations are favored is found between 20 and 30°E, with a positive difference of + 3 K in the eastern area. 6. Conclusion Following the initial detection of NLC in 2015, the total number of NLC events observed in Japan reached 10 as of July 2025. Therefore, it is now possible to initiate a detailed discussion of the property and the following mechanism of NLC occurrences in midlatitude regions. In this study, we investigated the occurrence characteristics of NLCs over Hokkaido and their relationship with the background atmospheric conditions. By analyzing the NLC formation environment and advection history using temperature and water vapor data from Aura/MLS and temperature data from JAWARA, we obtained the following key findings. (a) Asymmetry in the morning and evening occurrences of NLC A total of nine NLC events were observed from Hokkaido between 2020 and 2025, with the majority occurring in the morning (02:00–03:00 LST). The event on June 14, 2020, was particularly notable, reaching a southernmost latitude of 44.6°N, which constitutes the first observational evidence of NLCs extending over the main island of Hokkaido. (b) Background field characteristics around NLC events The Aura/MLS analysis revealed that air parcels were characterized by lower T s values at the time of observation, creating favorable conditions for NLC conservation. Conversely, the air parcels in Hokkaido longitudes were in a much warmer (high T s values), which is NLC-hostile environment approximately 8–12 h prior, suggesting that the NLCs formed during a subsequent cooling period. (c) Large contribution of atmospheric waves Analysis of the JAWARA data demonstrated that the temperature cycle in the NLC formation region near Hokkaido is dominated by semidiurnal and diurnal variations. This tidal activity creates a distinct observational advantage for morning NLC, which is consistent with the fact that most events in this study were observed in the morning. (d) Longitudinal dependence on evening and morning asymmetry The analysis revealed that the morning observational advantage is a global feature across most longitudes. However, certain regions, such as the 20–50°E sector, exhibit an evening advantage. This study provides fundamental insights into the mechanisms governing midlatitude NLCs. Further progress in understanding their formation and dynamics is expected through the combination of higher-resolution observations and advanced model simulations. Abbreviations NLCs: Noctilucent clouds, MLS: Microwave Limb Sounder, JAWARA: JAGUAR-DAS Whole neutral Atmosphere Reanalysis Declarations Ethics approval and consent to participate Not applicable Consent for publication Not applicable Availability of data and materials The Aura/MLS Version 4 /Level 2 water vapor and temperature data are available from the NASA GES DISK (https://disc.gsfc.nasa.gov/). The JAWARA temperature data is available from the JAWARA database site (https://jawara.nipr.ac.jp/home). The NLC images used in this study are included as additional file in this paper. Competing interests The authors declare that they have no competing interests. Funding This work was supported by MEXT/JSPS KAKENHI grants, 19H01956, 21H01144/23K20873 and 22H01289/23K22560. Authors’ contributions AE and HS designed this work. AE, YN, and MO conducted data analysis. AE wrote the first draft of the manuscript. AE, HS, YT, PD, and TTT contributed interpretations on results from data analysis. HS, NN, KS, KS, SI, and KI contributed operation of the imaging observation network in Hokkaido Japan and data archiving. All authors have contributed to revising and improving the manuscript. All authors have read and approved the final manuscript. Acknowledgments We thank the joint project of The University of Tokyo, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), and National Institute of Polar Research (NIPR) for providing JAWARA data and the Aura/MLS team for providing the Aura/MLS data. Authors’ information YN and OM had been belonged to Meiji University at the construction phase of this study and now moved another organization. References Dalin P, Suzuki H, Pertsev N, Perminov V, Shevchuk N, Tsimerinov E, Zalcik M, Brausch J, McEwan T, McEachran I, Connors M, Schofield I, Dubietis A, Černis K, Zadorozhny A, Solodovnik A, Lifatova D, Grønne J, Hansen O, Andersen H, Romejko V (2023) The strong activity of noctilucent clouds at middle latitudes in 2020. 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Suzuki H, Chino H, Sano Y, Kadokura A, Ejiri MK, Taguchi M (2015) Imaging-based observations of low-latitude auroras during 2001-2004 at Nayoro, Japan. Earth Planets Space 67:107. doi:10.1186/s40623-015-0278-z. Suzuki H, Sakanoi K, Nishitani N, Ogawa T, Ejiri MK, Kubota M, Kinoshita T, Murayama Y, Fujiyoshi Y (2016) First imaging and identification of a noctilucent cloud from multiple sites in Hokkaido (43.2-44.4°N), Japan. Earth Planets Space 68:182. doi:10.1186/s40623-016-0562-6. Tsuda TT, Hozumi Y, Kawaura K, Hosokawa K, Suzuki H, Nakamura T (2018) Initial report on polar mesospheric cloud observations by Himawari-8. Atmos Meas Tech 11:6163-6168. doi: 10.5194/ amt-11-6163-2018 Tsuda TT, Hozumi Y, Kawaura K, Tatsuzawa K, Ando Y, Hosokawa K, Suzuki H, Murata KT, Nakamura T, Yue J, Nielsen K (2022) Detection of polar mesospheric clouds utilizing Himawari-8/AHI full-disk images. Earth Space Sci 9:e2021EA002076. doi:10.1029/2021EA002076 Yamazaki Y, Sato K, Koshin D, Yasui R (2025) Symmetric and Antisymmetric Solar Migrating Semidiurnal Tides in the Mesosphere and Lower Thermosphere. Journal of Geophysical Research: Atmospheres, 130(3). doi:10.1029/2023JD040222 Supplementary Files ENDOetalSupplementaryMaterial.pdf renamed9e2ee.png Graphical Abstract Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Major Revision 12 Mar, 2026 Reviewers agreed at journal 28 Jan, 2026 Reviewers invited by journal 26 Jan, 2026 Editor assigned by journal 29 Dec, 2025 First submitted to journal 25 Dec, 2025 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-8452277","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":580627237,"identity":"7f3ebd2d-3dc5-4771-bab9-718e52cb9e13","order_by":0,"name":"Akiho Endo","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA7UlEQVRIiWNgGAWjYDACHsY2EMXYwN7AwJDAIwEWNCBOC88BorUwsEG0SCQQ6S7+nsNtDz5U3JHtl3yd+OCBjAUDf/sBhuICPFokzja2G84488x45uzczQYgh0mcSWAwnoHPmvOMbdK8bYcTN9zO3SYB9ssNBgZjHjw65MFa/gG13Dy7/QdIizwhLQZnG4FaGoBabvBuA4eYASEthmcOtknOOHbYeGZP7maQw3gMzyQ24PWL3Jn0ZxIfag7L9rOf3fjxZ0+dnNzxw8eM8YUYKmDsAcYTA2ObMdE6GBh+gEnmxyRoGQWjYBSMguEPAJd1Tj6aCCVkAAAAAElFTkSuQmCC","orcid":"https://orcid.org/0009-0007-8556-9931","institution":"Meiji University - Ikuta Campus: Meiji Daigaku - Ikuta Campus","correspondingAuthor":true,"prefix":"","firstName":"Akiho","middleName":"","lastName":"Endo","suffix":""},{"id":580627238,"identity":"51fb3092-7650-4c2a-884c-80e19c9cf1f3","order_by":1,"name":"Yoshihiro Tomikawa","email":"","orcid":"","institution":"National Institute of Polar Research (NIPR)/ The Graduate University for Advanced Studies (SOKENDAI)/ Polar Environment Data Science Center, Joint Support-Center for Data Science Research","correspondingAuthor":false,"prefix":"","firstName":"Yoshihiro","middleName":"","lastName":"Tomikawa","suffix":""},{"id":580627239,"identity":"32bc0fe4-9eae-4960-9175-5c44568579b7","order_by":2,"name":"Peter Dalin","email":"","orcid":"","institution":"Swedish Institute of Space Physics: Institutet for rymdfysik","correspondingAuthor":false,"prefix":"","firstName":"Peter","middleName":"","lastName":"Dalin","suffix":""},{"id":580627240,"identity":"897892db-203e-4514-8fdd-0354bc0df2b5","order_by":3,"name":"Takuo Tsuda","email":"","orcid":"","institution":"The University of Electro-Communications: Denki Tsushin Daigaku","correspondingAuthor":false,"prefix":"","firstName":"Takuo","middleName":"","lastName":"Tsuda","suffix":""},{"id":580627241,"identity":"56437c51-e704-446a-a31a-145605bfc6f1","order_by":4,"name":"Yuriko Nakamura","email":"","orcid":"","institution":"Meiji University - Ikuta Campus: Meiji Daigaku - Ikuta Campus","correspondingAuthor":false,"prefix":"","firstName":"Yuriko","middleName":"","lastName":"Nakamura","suffix":""},{"id":580627242,"identity":"0192bbcb-db36-4c97-ad48-2c9ef1748bf8","order_by":5,"name":"Masahiro Omote","email":"","orcid":"","institution":"Meiji University - Ikuta Campus: Meiji Daigaku - Ikuta Campus","correspondingAuthor":false,"prefix":"","firstName":"Masahiro","middleName":"","lastName":"Omote","suffix":""},{"id":580627243,"identity":"5d5d11fc-23ca-4a99-a6c6-af3a67c93cca","order_by":6,"name":"Nozomu Nishitani","email":"","orcid":"","institution":"Institute for Space-Earth Environment Research (ISEE) Nagoya University","correspondingAuthor":false,"prefix":"","firstName":"Nozomu","middleName":"","lastName":"Nishitani","suffix":""},{"id":580627244,"identity":"18dc01bc-3191-49eb-b00b-d2bf67cc53be","order_by":7,"name":"Kazuyo Sakanoi","email":"","orcid":"","institution":"Komazawa University: Komazawa Daigaku","correspondingAuthor":false,"prefix":"","firstName":"Kazuyo","middleName":"","lastName":"Sakanoi","suffix":""},{"id":580627245,"identity":"e7300343-8b0b-4d3e-b5ac-a41c98e2713b","order_by":8,"name":"Kaori Sakaguchi","email":"","orcid":"","institution":"Space Environment Laboratory, Radio Propagation Research Center, Radio Research Institute, National Institute of Information and Communications Technology","correspondingAuthor":false,"prefix":"","firstName":"Kaori","middleName":"","lastName":"Sakaguchi","suffix":""},{"id":580627246,"identity":"c16b952e-7da3-44ac-a755-df1985aa470e","order_by":9,"name":"Satoshi Ishii","email":"","orcid":"","institution":"Rikkyo University: Rikkyo Daigaku","correspondingAuthor":false,"prefix":"","firstName":"Satoshi","middleName":"","lastName":"Ishii","suffix":""},{"id":580627247,"identity":"d210326c-ee12-4ea5-8c23-79ca9d362783","order_by":10,"name":"Katsushi Iwamoto","email":"","orcid":"","institution":"Monbetsu City","correspondingAuthor":false,"prefix":"","firstName":"Katsushi","middleName":"","lastName":"Iwamoto","suffix":""},{"id":580627248,"identity":"428a0c23-8e94-415f-9c50-3ba0e076a7e3","order_by":11,"name":"Hidehiko Suzuki","email":"","orcid":"","institution":"Meiji University - Ikuta Campus: Meiji Daigaku - Ikuta Campus","correspondingAuthor":false,"prefix":"","firstName":"Hidehiko","middleName":"","lastName":"Suzuki","suffix":""}],"badges":[],"createdAt":"2025-12-26 05:24:25","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8452277/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8452277/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":101435943,"identity":"fdb6359d-82b3-415c-ab81-84c219b7defc","added_by":"auto","created_at":"2026-01-29 16:20:24","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":378822,"visible":true,"origin":"","legend":"\u003cp\u003eLocation of the observation sites and the field of view. This figure assumes an altitude of 84 km and no obstacles above the horizon for the field of view.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-8452277/v1/6a6e4813767dcc10fdb1e210.png"},{"id":101751592,"identity":"895773e6-d3f2-4c8d-9831-5a4c2c190844","added_by":"auto","created_at":"2026-02-03 10:21:32","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":1888000,"visible":true,"origin":"","legend":"\u003cp\u003eA photograph of the NLC event on June 14, 2020, captured at 02:30 LST from Monbetsu, Hokkaido, Japan.\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-8452277/v1/656fad3ce55773fc9b70c0b2.png"},{"id":101435946,"identity":"cb0e45ba-2bf7-4df0-aecd-baefd3384f8a","added_by":"auto","created_at":"2026-01-29 16:20:24","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":821749,"visible":true,"origin":"","legend":"\u003cp\u003eAnalysis results of the NLC event which reached the lowest latitude among the nine events in this study. Figure 3a shows the original image captured at 02:56 LST on June 14, 2020, in Rikubetsu. Figure 3b shows the enhanced contrast of Figure 3a. Figure 3c shows the projection result that the southernmost NLC. The NLC region is indicated by the red surrounding highlight area. This event had an estimated southernmost latitude of 44.6°N. This latitude is indicated by the red arrow in Figure 3c. In a previous study (Suzuki et al., 2016), the southernmost latitude of NLCs over Hokkaido was reported as 47.5°N for the 2015 event, indicated by the red line in Figure 3c.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-8452277/v1/5742daeb216509ea9f8e0fa7.png"},{"id":101435953,"identity":"4b52e7b5-86e2-4d20-a4c1-fe158837d84f","added_by":"auto","created_at":"2026-01-29 16:20:25","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":227124,"visible":true,"origin":"","legend":"\u003cp\u003eBackground field by Aura/MLS and a possible advection history for the NLC event on June 14, 2020. The time stamp in the upper left corner indicates the reference time (\u003cem\u003et \u003c/em\u003e= 0, NLC detection time) on the horizontal axis. Each colored point represents the \u003cem\u003eTs\u003c/em\u003ecalculated from MLS temperature. The solid blue, green, and red lines show typical southward advection trajectories, assuming constant meridional velocities of 15, 10, and 5 m/s, respectively. The origin of these trajectories corresponds to the estimated southernmost latitude of the event (See Section 3.1), which is marked by the black dashed line.\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-8452277/v1/24f68ef0f08e185053f1f4a4.png"},{"id":101751591,"identity":"10a25123-a73f-467b-a803-3e3717ed04c4","added_by":"auto","created_at":"2026-02-03 10:21:31","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":572621,"visible":true,"origin":"","legend":"\u003cp\u003eBackground field and possible advection history for all six NLC events observed over Hokkaido from 2015 to 2021. The reference time (t=0) for each panel corresponds to NLC detection time and is as follows: (a) 02:30 LST, 21 June 2015; (b) 20:30 LST, 12 June 2020; (c) 02:30 LST, 13 June 2020; (d) 02:30 LST, 14 June 2020; (e) 02:47 LST, 18 July 2020; and (f) 02:27 LST, 22 June 2021.\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-8452277/v1/ee1f1704d8039fe927233bba.png"},{"id":101435951,"identity":"b5a1a706-0194-47e4-b51a-6cd2ec53fcb4","added_by":"auto","created_at":"2026-01-29 16:20:25","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":439951,"visible":true,"origin":"","legend":"\u003cp\u003eTemperature variations derived from JAWARA reanalysis data. The data represents the temperature at the 0.00493 hPa pressure level, averaged over a 40-day period centered on the summer solstice for latitudes 50°–55°N. Each colored line corresponds to the average for a different year. The analysis is shown for four longitudinal sectors: (a) 140°–160°E (the Japan region), (b) 60°–80°E (the Central Asia region), (c) 0°–20°E (the Europe region) and (b) 100°–120°W (the North America region). The two local times (1:30 and 13:30) are indicated by the red line in Figure 5a. These results indicate that the local-time dependence of temperature variability varies substantially with longitude, even at the same latitude.\u003c/p\u003e","description":"","filename":"floatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-8452277/v1/ee2ce034946dc2a7638c5262.png"},{"id":101751574,"identity":"ac2bbab5-58cd-463b-aa01-e11d8defaaeb","added_by":"auto","created_at":"2026-02-03 10:21:24","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":369338,"visible":true,"origin":"","legend":"\u003cp\u003eLongitudinal dependence of temperature derived from JAWARA reanalysis. The data represents the temperature at the 0.00493 hPa pressure level, averaged over a 40-day period centered on the summer solstice for latitudes 50°–55°N. Each colored line corresponds to the average for a different year. The analysis is shown for two local time (LT) windows: (a) the morning (02:00–03:00) and (b) the evening (20:00–21:00).\u003c/p\u003e","description":"","filename":"floatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-8452277/v1/3a10ac70bd28dc75f146df8b.png"},{"id":101435949,"identity":"2a0df362-5b0b-41d0-97f5-6e0fd9b9489c","added_by":"auto","created_at":"2026-01-29 16:20:24","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":86244,"visible":true,"origin":"","legend":"\u003cp\u003eLongitudinal profile of the mean temperature difference between morning (02:00–03:00 LT) and evening (20:00–21:00 LT). The difference is calculated as the mean morning temperature minus the mean evening temperature. The green dashed line marks the longitude of Hokkaido (139~145°E). Negative values indicate that the morning is colder, thus favoring morning NLC observations, while positive values indicate an advantage for evening observations.\u003c/p\u003e","description":"","filename":"floatimage9.png","url":"https://assets-eu.researchsquare.com/files/rs-8452277/v1/400ab9ef2fa62544723774cb.png"},{"id":101755911,"identity":"a277a354-4fc3-488a-9c51-cf6dc44c5763","added_by":"auto","created_at":"2026-02-03 10:55:24","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":5113799,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8452277/v1/74f68b3d-33ca-4821-b2c9-4aead159dd0d.pdf"},{"id":101435944,"identity":"f3f8acfc-aba2-48cc-8958-dd1d078f4281","added_by":"auto","created_at":"2026-01-29 16:20:24","extension":"pdf","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":521412,"visible":true,"origin":"","legend":"","description":"","filename":"ENDOetalSupplementaryMaterial.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8452277/v1/9c161609af323fdac5a44abb.pdf"},{"id":101435950,"identity":"fcbe5c96-7cf5-44d2-aebe-f7c1a0eb9e3d","added_by":"auto","created_at":"2026-01-29 16:20:25","extension":"png","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":167848,"visible":true,"origin":"","legend":"\u003cp\u003eGraphical Abstract\u003c/p\u003e","description":"","filename":"renamed9e2ee.png","url":"https://assets-eu.researchsquare.com/files/rs-8452277/v1/21e5afabc90ee55dc66862f3.png"}],"financialInterests":"","formattedTitle":"Asymmetry in the Morning and Evening Occurrence of Noctilucent Clouds in Japan","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eNoctilucent clouds (NLCs) are the highest clouds in the Earth\u0026rsquo;s atmosphere, occurring at altitudes of approximately 82\u0026ndash;85 km in polar regions, and are typically observed during summer in high-latitude regions (50\u0026deg;\u0026ndash;60\u0026deg; in latitude). NLCs are composed of ice particles with mean radius of approximately 50 nm. Because NLCs become visible by scattering sunlight from below the horizon, their typical observation windows occur during the twilight periods after sunset and before sunrise, when the solar zenith angle ranges from approximately 96\u0026deg; and 106\u0026deg; (Fogle and Haurwitz, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e1966\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eNLC observations are effective for monitoring atmospheric dynamics in the mesopause region with high spatial and temporal resolution. In recent years, NLC occurrences have been reported to expand toward lower latitudes. Particularly, the number of NLC detections from midlatitudes (below 50\u0026deg;) is increasing, and these NLCs are generally interpreted as being advected from higher latitudes (Kaifler et al., \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). For example, 36 events were reported in North America between 2003 and 2011 (Russell III et al., \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). In Europe, more than five events per year were recorded from 2005 to 2021, with as many as 22 cases in 2009 (Dalin et al., \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). As an example of observations at extremely low latitudes, NLCs were observed at the Calar Alto Observatory, Spain (37\u0026deg;N, 3\u0026deg;W) in 2012, 2019, and 2021 (Russell III et al., \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Gerding et al., \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Lange et al., \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). In Asia, NLCs were observed in Hokkaido, Japan (42\u0026deg;N) in 2015 (Suzuki et al., \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2016\u003c/span\u003e) and in Beijing, China (40\u0026deg;N) in 2020 (Miao et al., \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). In the summer (June\u0026ndash;August) of 2020, numerous NLCs were observed at middle latitudes of the Northern Hemisphere. The number of NLCs in 2020 was unprecedented, indicating that 2020 was an NLC outbreak year, as noted in Dalin et al. (\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2023\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eIn Japan, the NLC imaging network has been updated since the first NLC detection in Hokkaido in 2015. Four more events were observed in 2020, one in 2021, and four in 2025, resulting in a total of 10 NLC cases at present. Except for one case in 20 LST, all events occurred in the early morning hours (2:00‒3:00 LST). Although the occurrence of NLCs after sunset and before sunrise is geometrically possible, the actual observations show an asymmetry in the morning and evening occurrence suggesting a strong local-time dependence. Previous studies have reported several instances of NLC occurrence showing a similar dependence on local time as our observations. Ground-based observations in North America (46\u0026deg;N-71\u0026deg;N) have reported higher detection rates in the evening (Fogle and Haurwitz, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e1966\u003c/span\u003e; Fogle, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e1968\u003c/span\u003e). Lidar observations in the European polar region (69\u0026deg;N, 16\u0026deg;E) attributed this dependence to diurnal and semi-diurnal atmospheric tides (Fiedler et al., \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). To investigate the causes of this local-time dependence of NLCs in Hokkaido (43\u0026deg;N-45\u0026deg;N), we compared multiple ground-based NLC events with temperature and water vapor field data obtained from the Microwave Limb Sounder (MLS) onboard the Aura satellite. The analysis revealed that the background fields at the observation time consistently show lower temperature values compared with 8\u0026ndash;12 h prior. A likely explanation is the effect of atmospheric solar tides. To confirm this hypothesis, we investigated the meteorological fields in the upper mesosphere around North Japan using the JAGUAR-DAS Whole neutral Atmosphere Reanalysis (JAWARA) data, which includes assimilation up to the lower thermosphere (z\u0026thinsp;=\u0026thinsp;110 km). The results strongly suggest that tidal wave modulations are responsible for the local-time dependence of NLC occurrences in Japan.\u003c/p\u003e \u003cp\u003eThe remainder of this article is organized as follows. Section \u003cspan refid=\"Sec2\" class=\"InternalRef\"\u003e2\u003c/span\u003e describes the NLC events observed from Hokkaido from 2020 to 2025. Section \u003cspan refid=\"Sec5\" class=\"InternalRef\"\u003e3\u003c/span\u003e presents the estimation method for NLC occurrence latitude and outlines the procedures for background and advection analysis based on the Aura/MLS data. Section \u003cspan refid=\"Sec8\" class=\"InternalRef\"\u003e4\u003c/span\u003e presents our analysis results. Section \u003cspan refid=\"Sec9\" class=\"InternalRef\"\u003e5\u003c/span\u003e discusses the implications of the JAWARA data. Finally, Section \u003cspan refid=\"Sec10\" class=\"InternalRef\"\u003e6\u003c/span\u003e concludes this study.\u003c/p\u003e"},{"header":"2. Datasets","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 NLC Observation Data from Hokkaido\u003c/h2\u003e \u003cp\u003eIn this study, NLC events were identified using image data captured by an observatory staff and an automatic camera system network. The presence of NLCs was confirmed by eye inspection based on differences in color, brightness, motion, and fine-scale structures compared to the surrounding tropospheric clouds. Details of the automatic camera system network were described by Suzuki et al. (\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2016\u003c/span\u003e); however, the camera at Moshiri had ceased operations since 2020. Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e shows the location of the observation sites and the field of view in this study. This figure assumes an altitude of 84 km and no obstacles above the horizon for the field of view. Some updates to the observation network since 2015 include the transition of several cameras to a web-based remote-control system and the implementation of near real-time Himawari NLC observations (Tsuda et al., \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2022\u003c/span\u003e) on the website (\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttp://ttt01.cei.uec.ac.jp/himawari/\u003c/span\u003e\u003cspan address=\"http://ttt01.cei.uec.ac.jp/himawari/\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e). This system enabled flexible adjustment of imaging intervals in response to NLC activities, allowing for more responsive observations. We also evaluated the clear-sky ratio at the observation sites. Here, \u0026ldquo;clear-sky ratio\u0026rdquo; is defined as the proportion of the NLC-observable period determined to be clear based on visual inspection of camera imagery. For example, in 2020, the mean clear-sky fraction across four observation sites in Hokkaido during the NLC season (1 June\u0026ndash;30 July) was 23% at morning and 29% at evening, suggesting no substantial difference in observing conditions between morning and evening.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTable\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e summarizes the NLC events analyzed in this study. A total of nine events were observed from 2020 to 2025. In June 2020, NLCs were observed for three successive nights from June 12 to 14, 2020. The event on June 14, 2020, which lasted for 72 min, was the longest recorded event in Japan. The event on July 18, 2020, represents the latest occurrence relative to the summer solstice among all NLC events observed in Japan. A single event was observed on June 22, 2021. No events were confirmed in Japan from 2022 to 2024. In 2025, one event was observed on June 25, 2025, followed by two consecutive events on July 7\u0026ndash;8, 2025, and another on July 12, 2025. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e shows an example of the observation data: an NLC image captured from Monbetsu, Hokkaido (44.3\u0026deg;N, 143.3\u0026deg;E) at 02:30 LST on June 14, 2020. From the images, we determined the southernmost latitude and duration for each event using the method described in Section \u003cspan refid=\"Sec6\" class=\"InternalRef\"\u003e3.1\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eSummary of NLC events observed over Hokkaido from 2020 to 2025. Site abbreviations are as follows: NYR, Nayoro (44.4\u0026deg;N, 142.5\u0026deg;E); RIK, Rikubetsu (43.5\u0026deg;N, 143.6\u0026deg;E); SAR, Sarobetsu (45.2\u0026deg;N, 141.8\u0026deg;E); MON, Monbetsu (44.3\u0026deg;N, 143.3\u0026deg;E); and SHU, Lake Shumarinai (44.3\u0026deg;N, 142.2\u0026deg;E)\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNo.\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eDATE\u003c/p\u003e \u003cp\u003eYYYYMMDD\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eSITE NAME\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eLST period of NLC detection (LST\u0026thinsp;=\u0026thinsp;UTC\u0026thinsp;+\u0026thinsp;9hr)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e20200612\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eNYR,RIK,SAR\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e20:37\u0026thinsp;\u0026minus;\u0026thinsp;20:49\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e20200613\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eNYR,MON,SAR\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e02:27\u0026thinsp;\u0026minus;\u0026thinsp;02:30\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e20200614\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eRIK,MON\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e01:59\u0026thinsp;\u0026minus;\u0026thinsp;03:11\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e20200718\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eMON,SAR,SHU\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e02:47\u0026thinsp;\u0026minus;\u0026thinsp;02:57\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e20210622\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eRIK,MON\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e02:22\u0026thinsp;\u0026minus;\u0026thinsp;02:27\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e20250625\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eRIK,MON\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e02:04\u0026thinsp;\u0026minus;\u0026thinsp;02:55\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e20250707\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eMON\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e02:19 (single data)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e20250708\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eMON\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e02:35\u0026thinsp;\u0026minus;\u0026thinsp;02:47\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e20250712\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eMON\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e02:42\u0026thinsp;\u0026minus;\u0026thinsp;02:57\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Reanalysis and Satellite Data\u003c/h2\u003e \u003cp\u003eTo investigate the mechanisms of NLC events, we analyzed the background atmospheric fields in the upper mesosphere using two datasets. The first dataset is Version 4 /Level 2 water vapor and temperature data from the MLS radiometer aboard the Aura satellite. The second dataset is JAWARA (Koshin et al., \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2025\u003c/span\u003e; Sato and Koshin, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2025\u003c/span\u003e), which is a long-term meteorological reanalysis dataset that assimilates satellite observations into a global atmospheric model. A key feature of JAWARA is its extensive vertical range, covering altitudes from the surface to the lower thermosphere (up to 110 km). The dataset provides a temporal resolution of 1 h, a horizontal grid resolution of 2.8\u0026deg;, and a vertical resolution of less than 1 km.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Analysis and Method","content":"\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\n \u003ch2\u003e\u003cstrong\u003e3.1 Estimation of Southernmost Latitude from Images\u003c/strong\u003e\u003c/h2\u003e\n \u003cp\u003eFor each event, the southernmost latitude of NLCs was estimated using the image projection method described by Suzuki et al. (\u003cspan class=\"CitationRef\"\u003e2015\u003c/span\u003e). In applying this method, the height of the NLC layer was assumed to be 84 km, following Suzuki et al. (\u003cspan class=\"CitationRef\"\u003e2015\u003c/span\u003e). Table \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e summarizes the estimation results, indicating that the NLCs observed over Hokkaido advected southward to latitudes between 44.2\u0026deg;N and 48.8\u0026deg;N. Notably, the term \u0026ldquo;southernmost latitude\u0026rdquo; in this study refers to the visible extent of NLCs in the captured images. Figure \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e shows the analysis result for the event that reached the lowest latitude among the nine considered events. Figure \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ea shows the original image captured in Rikubetsu at 02:56:03 LST on June 14, 2020. Figure \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eb is the enhanced contrast version of Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ea. The enhancement was performed by subtracting the background count from the selected area of the original image. Figure \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ec shows the map projection result of the location of NLC; the NLC region is enclosed by the red line. This event had an estimated southernmost latitude of 44.6\u0026deg;N, as indicated by the red arrow in Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ec. As indicated by the red line in Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ec, Suzuki et al. (\u003cspan class=\"CitationRef\"\u003e2016\u003c/span\u003e) reported the southernmost latitude of NLCs over Hokkaido as 47.5\u0026deg;N for the 2015 event. This 2020 event represents a southward extension of up to 2.9\u0026deg; compared with the 2015 event reported by Suzuki et al. (\u003cspan class=\"CitationRef\"\u003e2016\u003c/span\u003e). It marks the first observation result of NLCs reaching the airspace over the main island of Hokkaido.\u003c/p\u003e\n \u003cp\u003e\u003c/p\u003e\u0026nbsp;\u003ctable id=\"Tab2\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eEstimated southernmost of NLC distribution latitude for each event\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eNo.\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eLimit of South Latitude\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e48.8\u0026deg;N\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e47.7\u0026deg;N\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e44.6\u0026deg;N\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e48.3\u0026deg;N\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e48.7\u0026deg;N\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e45.5\u0026deg;N\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e48.4\u0026deg;N\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e46.5\u0026deg;N\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e47.1\u0026deg;N\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003cp\u003e\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\n \u003ch2\u003e3.2 Analysis of Background Fields and Advection History\u003c/h2\u003e\n \u003cp\u003eThe formation of NLCs in the upper mesosphere at latitudes below 50\u0026deg; is difficult since extremely low temperatures (\u0026lt;\u0026thinsp;140 K) are required for ice particle nucleation. Therefore, it is widely hypothesized that most NLCs observed at midlatitudes do not form \u003cem\u003ein situ\u003c/em\u003e but are instead advected from their formation regions at higher latitudes (Kaifler et al., \u003cspan class=\"CitationRef\"\u003e2018\u003c/span\u003e). We analyzed the background atmospheric fields possibly experienced by the air parcels associated with the observed NLCs using data from the Aura/MLS instrument. The 2025 events were not included in this analysis due to MLS data availability limitations. To evaluate the background field potential for NLC formation and conservation, we introduced a parameter, \u003cem\u003eT\u003c/em\u003e\u003csub\u003es\u003c/sub\u003e, defined as the difference between the atmospheric temperature retrieved from MLS (\u003cem\u003eT\u003c/em\u003e\u003csub\u003emls\u003c/sub\u003e) and the water vapor frost point (\u003cem\u003eT\u003c/em\u003e\u003csub\u003efrost\u003c/sub\u003e). \u003cem\u003eT\u003c/em\u003e\u003csub\u003efrost\u003c/sub\u003e was calculated from the saturated water vapor pressure based on the approximation given by Murphy and Koop (\u003cspan class=\"CitationRef\"\u003e2005\u003c/span\u003e). \u003cem\u003eT\u003c/em\u003e\u003csub\u003es\u003c/sub\u003e was derived by substituting the water vapor pressure (\u003cem\u003ePw\u003c/em\u003e\u003csub\u003emls\u003c/sub\u003e [Pa]) and temperature (\u003cem\u003eT\u003c/em\u003e\u003csub\u003emls\u003c/sub\u003e [K]) into Equations (1) and (2).\u003c/p\u003e\n \u003cdiv id=\"Equa\" class=\"Equation\"\u003e\n \u003cdiv class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e$$\\:{T}_{\\text{f}\\text{r}\\text{o}\\text{s}\\text{t}}\\:\\approx\\:\\:\\frac{1.814625\\:\\text{ln}\\left({Pw}_{\\text{m}\\text{l}\\text{s}}\\right)+6190.134}{29.120\\:-\\:\\text{ln}\\left({Pw}_{\\text{m}\\text{l}\\text{s}}\\right)}\\:\\:\\left({T}_{\\text{f}\\text{r}\\text{o}\\text{s}\\text{t}}\u0026gt;115\\text{K}\\right)\\:\\:\\left(1\\right)$$\u003c/div\u003e\n \u003c/div\u003e\n \u003cdiv id=\"Equb\" class=\"Equation\"\u003e\n \u003cdiv class=\"mathdisplay\" id=\"FileID_Equb\" name=\"EquationSource\"\u003e$$\\:{T}_{\\text{s}}={T}_{\\text{m}\\text{l}\\text{s}}\\:-\\:{T}_{\\text{f}\\text{r}\\text{o}\\text{s}\\text{t}}\\:\\:\\:\\:\\left(2\\right)$$\u003c/div\u003e\n \u003c/div\u003e\n \u003cp\u003eRegarding the relationship between NLCs and \u003cem\u003eTs\u003c/em\u003e, a larger \u003cem\u003eT\u003c/em\u003e\u003csub\u003es\u003c/sub\u003e indicates a field that is hostile to the formation and persistence of NLCs. On the other hand, a smaller \u003cem\u003eT\u003c/em\u003e\u003csub\u003es\u003c/sub\u003e indicates more favorable conditions. This is because \u003cem\u003eT\u003c/em\u003e\u003csub\u003es\u003c/sub\u003e represents the difference between the frost point and the atmospheric temperature for these ice-particle clouds. Figure \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e shows a time\u0026ndash;latitude plot of the background field (\u003cem\u003eT\u003c/em\u003e\u003csub\u003es\u003c/sub\u003e) for the NLC event on June 14, 2020. The horizontal axis represents time in hours relative to the NLC detection time (\u003cem\u003et\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0) for each event, whereas the vertical axis represents latitude. The color of each point corresponds to the \u003cem\u003eT\u003c/em\u003e\u003csub\u003es\u003c/sub\u003e, as indicated by the color bar. For visualization purposes, values exceeding 20 K or below \u0026minus;\u0026thinsp;20 K are saturated at these limits. The solid red, green, and blue lines represent typical velocities of a meridional advection of 5, 10, and 15 m/s, respectively (Kirkwood and Stebel, \u003cspan class=\"CitationRef\"\u003e2003\u003c/span\u003e). These velocity values are supposed to be constant during 30 hours before the NLC event. The trajectories\u0026rsquo; origin is set to the southernmost latitude of the NLC event, as determined in Section \u003cspan class=\"InternalRef\"\u003e3.1\u003c/span\u003e, and marked by the dashed black line. The \u003cem\u003eT\u003c/em\u003e\u003csub\u003es\u003c/sub\u003e values were derived from Aura/MLS observations at the 0.0046-hPa pressure level (height\u0026thinsp;~\u0026thinsp;84 km), a longitude range of \u0026plusmn;\u0026thinsp;30\u0026deg; from the observation site, and a 40\u0026ndash;90\u0026deg;N latitude range. It should be noted that, because this analysis relies on the relatively sparse Aura/MLS dataset, a wide longitudinal window (\u0026plusmn;\u0026thinsp;30\u0026deg; from the observation site) was used when constructing the plot. As a result, the \u003cem\u003eT\u003c/em\u003es values shown in Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e do not necessarily represent conditions exactly at the same geographic location where the NLCs were observed, and thus cannot be interpreted as a direct indicator of NLC presence or absence. This limitation arises from the possibility that gravity waves with temperature amplitudes of approximately 10 K exist within the analysis region and may contribute to small-scale variability that is not resolved by Aura/MLS (Rapp, 2002). In addition to these spatial limitations, the Aura is a sun-synchronous polar orbit satellite with equatorial crossing times of 01:45 and 13:45 LT. Consequently, the data coverage is sparse, and caution is required as this analysis cannot fully capture diurnal variations.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"4. Results","content":"\u003cp\u003eFigure \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e shows the results of applying the analysis described in Section \u003cspan refid=\"Sec7\" class=\"InternalRef\"\u003e3.2\u003c/span\u003e for the six NLC events (Event numbers 1\u0026ndash;6 in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) observed from Hokkaido between 2015 and 2021. The reference time (\u003cem\u003et\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0, NLC detection time) for each panel is as follows: (a) 02:30 LST, June 21, 2015; (b) 20:30 LST, June 12, 2020; (c) 02:30 LST, June 13, 2020; (d) 02:30 LST, June 14, 2020; (e) 02:47 LST, July 18, 2020; and (f) 02:27 LST, June 22, 2021. This analysis reveals a common feature among the events. Except for the evening event of June 12, 2020 (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb), the background fields at the observation time consistently show lower \u003cem\u003eT\u003c/em\u003e\u003csub\u003es\u003c/sub\u003e values compared with ~\u0026thinsp;12 h prior. This feature suggests that the background fields at the observation time were favorable for NLC survival. Note that the \u003cem\u003eT\u003c/em\u003e\u003csub\u003es\u003c/sub\u003e values shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e include cases where \u003cem\u003eT\u003c/em\u003e\u003csub\u003es\u003c/sub\u003e exceeds zero near the observation time. As discussed in Section \u003cspan refid=\"Sec5\" class=\"InternalRef\"\u003e3\u003c/span\u003e, our interpretation here focuses solely on whether the atmospheric conditions were relatively favorable or unfavorable for NLC occurrence at the observation time and 8\u0026ndash;12 hours prior. Conversely, for all advection scenarios, the air parcels were in regions with \u003cem\u003eT\u003c/em\u003e\u003csub\u003es\u003c/sub\u003e values much larger than 0 approximately 8\u0026ndash;12 h before the observation. This feature implies that the NLCs were advected through the subsequent cooling period for all six events. If the advection is valid, this suggests that colder conditions existed within the spatial-temporal gaps of the MLS data coverage. Furthermore, a similar trend was confirmed when evaluating the background field using only MLS temperature data (\u003cem\u003eT\u003c/em\u003e\u003csub\u003e\u003cem\u003emls\u003c/em\u003e\u003c/sub\u003e) instead of \u003cem\u003eT\u003c/em\u003e\u003csub\u003es\u003c/sub\u003e (not shown). \u003cem\u003eT\u003c/em\u003e\u003csub\u003e\u003cem\u003emls\u003c/em\u003e\u003c/sub\u003e were also higher approximately 8\u0026ndash;12 hours prior to observation, indicating conditions that were hostile to the conservation and generation of NLCs. These characteristics point to the strong possibility that a background field with a large semi-diurnal variation and its local-time-fixed harmonic variation plays a key role in governing the frequency of NLC occurrences over Hokkaido. We have analyzed JAWARA data to verify the effect of atmospheric tidal waves, presented in the discussion.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"5. Discussion","content":"\u003cp\u003eIn the previous section, we investigated the background fields for six NLC events over Hokkaido using Aura/MLS data. The analysis confirmed that air parcels had low \u003cem\u003eT\u003c/em\u003e\u003csub\u003es\u003c/sub\u003e values at the time of the observations, creating favorable conditions for NLC maintenance. Further, approximately 8\u0026ndash;12 h before the observations, the environment was characterized by high \u003cem\u003eT\u003c/em\u003e\u003csub\u003es\u003c/sub\u003e values, which are unsuitable for NLC formation and conservation. However, Aura/MLS data are limited by their temporal resolution, providing observations only twice a day over the same longitude. This sparse coverage makes it difficult to fully characterize the diurnal temperature variations at a fixed location. To overcome this limitation, we employed the JAWARA dataset to investigate diurnal temperature variations at the upper mesosphere within the observed longitudinal sector (Yamazaki et al., \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). This dataset provides a temporal resolution of 1 h, a horizontal grid resolution of 2.8\u0026deg;, and covers an extensive vertical range from the surface to the lower thermosphere with a vertical resolution of less than 1 km. This high temporal resolution makes it possible to analyze local-time-dependent temperature variations in the upper mesosphere region at a fixed longitude. For this analysis, we derived the temperature distribution at a 0.0049-hPa pressure level (height\u0026thinsp;~\u0026thinsp;84 km) within a geographic region defined by 140\u0026ndash;160\u0026deg;E longitude and 50\u0026ndash;55\u0026deg;N latitude. This sector is the NLC-forming region in the longitudes of Hokkaido. The analysis period for each year from 2015 to 2024 was set to 40 days, centered on the summer solstice. Because NLCs typically begin to appear in early June in association with the solstitial season, we selected the 40-day interval from June 1 to July 10\u0026mdash;corresponding to \u0026plusmn;\u0026thinsp;20 days around the solstice\u0026mdash;as the analysis window. It should be noted that, while the discussion in Section \u003cspan refid=\"Sec8\" class=\"InternalRef\"\u003e4\u003c/span\u003e was based on the parameter \u003cem\u003eTs\u003c/em\u003e calculated from Aura/MLS temperature and water vapor measurements, JAWARA does not provide water vapor data despite offering global atmospheric temperature fields with high temporal and spatial resolution. Therefore, the analysis in this section is conducted using temperature alone.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e shows the 40-day averaged temperature variation as a function of local time. Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea reveals that the temperature variation is dominated by a combination of semidiurnal and diurnal variations. The two local times (1:30 and 13:30) are indicated by the red line in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea. A consistent pattern is observed across all ten years, with minimum temperatures occurring around 0:00 and 10:00 LST, and maxima around 05:00 and 16:00 LT. Further, a comparison of temperatures around Aura/MLS observation times (01:30 and 13:30 LT) shows that the 01:30-LT temperature is 4.2 K lower than the 13:30-LT temperature, on average. Using the JAWARA dataset, it was also demonstrated that atmospheric conditions at 01:30 LST are more favorable for NLC occurrence than those at 13:30 LT. This result is consistent with the Aura/MLS analysis shown in Section \u003cspan refid=\"Sec8\" class=\"InternalRef\"\u003e4\u003c/span\u003e. For comparison, Figs.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eb (60\u0026ndash;80\u0026deg;E), 6c (0\u0026ndash;20\u0026deg;E), and 6d (100\u0026ndash;120\u0026deg;W) illustrate the results of the same analysis applied to different longitudinal sectors. Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eb corresponds approximately to Central Asia, Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ec to Europe, and Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ed to North America. In the sector shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eb, the diurnal and semidiurnal variations with a local-time-fixed pattern are similar to those over Hokkaido. In contrast, the corresponding variations in Figs.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ec and \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ed differ markedly from those in the Hokkaido sector. These results indicate that the local-time dependence of temperature variability varies substantially with longitude, even at the same latitude.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eUsing the JAWARA data, we compared temperatures during the typical evening (20:00\u0026ndash;21:00 LT) and morning (02:00\u0026ndash;03:00 LT) hours, which are suitable for NLC sighting by means of solar elevation. Figure\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e shows the longitudinal temperature dependence at the NLC altitude for these two time windows, analyzed under the same conditions as in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e. Specifically, Figs.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ea and \u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eb show the mean temperature averaged over 50\u0026ndash;55\u0026deg;N and calculated for every 10\u0026deg; of longitude for the morning (02:00 and 03:00 LT) and evening (20:00 and 21:00 LT) hours, respectively. Essentially, these figures visualize the temperature differences across longitudes at a fixed local time. Three key features are evident in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e. First, both morning and evening results exhibited a longitudinally dependent pattern that is consistent across all years. Second, although the 40-day averaged temperature varies from year to year, this interannual variability is more pronounced in the morning than in the evening. Third, the specific structure of the longitudinal temperature dependence differs between the morning and the evening, attributable to longitudinal variations in the excitation and propagation of atmospheric tides originating from the lower atmosphere. The quasistationary nature of these patterns is thought to be influenced by factors such as topography and the land and sea distribution (Pancheva and Mukhtarov, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). Figure\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e shows the temperature difference between the morning and evening, calculated by subtracting the 7-year average evening temperatures (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eb) from the morning temperatures (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ea). The zero line is indicated by the red line in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e. In this plot, a negative value indicates that the morning is colder than the evening, thereby representing conditions more favorable for NLC observation. The longitudinal sector corresponding to Hokkaido (139\u0026sim;145\u0026deg;E; enclosed by two green dashed lines) shows a temperature difference of ~\u0026thinsp;\u0026minus;\u0026thinsp;2 K, suggesting a slight advantage for morning NLC sightings. However, a more important point is that the longitudinal sector with large negative values (\u0026thinsp;~\u0026thinsp;\u0026minus;\u0026thinsp;6 K) is located in the eastern area of Hokkaido. Because the NLC is advected by westward zonal and southward meridional flows in the summer mesosphere, the temperature in the eastern part of the observation site is crucial. This finding is consistent with the observational fact that most NLC events in this study were observed during the 02:00\u0026ndash;03:00 LT window. On a global scale, the analysis reveals that morning observations are favored across most longitudes. A particularly strong morning advantage is found in the region between 10 and 30\u0026deg;W, where the temperature difference reaches approximately \u0026minus;\u0026thinsp;9 K in the eastern area. Conversely, a prominent region where evening observations are favored is found between 20 and 30\u0026deg;E, with a positive difference of +\u0026thinsp;3 K in the eastern area.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"6. Conclusion","content":"\u003cp\u003eFollowing the initial detection of NLC in 2015, the total number of NLC events observed in Japan reached 10 as of July 2025. Therefore, it is now possible to initiate a detailed discussion of the property and the following mechanism of NLC occurrences in midlatitude regions. In this study, we investigated the occurrence characteristics of NLCs over Hokkaido and their relationship with the background atmospheric conditions. By analyzing the NLC formation environment and advection history using temperature and water vapor data from Aura/MLS and temperature data from JAWARA, we obtained the following key findings.\u003c/p\u003e \u003cp\u003e(a) Asymmetry in the morning and evening occurrences of NLC\u003c/p\u003e \u003cp\u003eA total of nine NLC events were observed from Hokkaido between 2020 and 2025, with the majority occurring in the morning (02:00\u0026ndash;03:00 LST). The event on June 14, 2020, was particularly notable, reaching a southernmost latitude of 44.6\u0026deg;N, which constitutes the first observational evidence of NLCs extending over the main island of Hokkaido.\u003c/p\u003e \u003cp\u003e(b) Background field characteristics around NLC events\u003c/p\u003e \u003cp\u003eThe Aura/MLS analysis revealed that air parcels were characterized by lower \u003cem\u003eT\u003c/em\u003e\u003csub\u003es\u003c/sub\u003e values at the time of observation, creating favorable conditions for NLC conservation. Conversely, the air parcels in Hokkaido longitudes were in a much warmer (high \u003cem\u003eT\u003c/em\u003e\u003csub\u003es\u003c/sub\u003e values), which is NLC-hostile environment approximately 8\u0026ndash;12 h prior, suggesting that the NLCs formed during a subsequent cooling period.\u003c/p\u003e \u003cp\u003e(c) Large contribution of atmospheric waves\u003c/p\u003e \u003cp\u003eAnalysis of the JAWARA data demonstrated that the temperature cycle in the NLC formation region near Hokkaido is dominated by semidiurnal and diurnal variations. This tidal activity creates a distinct observational advantage for morning NLC, which is consistent with the fact that most events in this study were observed in the morning.\u003c/p\u003e \u003cp\u003e(d) Longitudinal dependence on evening and morning asymmetry\u003c/p\u003e \u003cp\u003eThe analysis revealed that the morning observational advantage is a global feature across most longitudes. However, certain regions, such as the 20\u0026ndash;50\u0026deg;E sector, exhibit an evening advantage.\u003c/p\u003e \u003cp\u003eThis study provides fundamental insights into the mechanisms governing midlatitude NLCs. Further progress in understanding their formation and dynamics is expected through the combination of higher-resolution observations and advanced model simulations.\u003c/p\u003e"},{"header":"Abbreviations","content":"\u003cp\u003eNLCs: Noctilucent clouds, MLS: Microwave Limb Sounder, JAWARA: JAGUAR-DAS Whole neutral Atmosphere Reanalysis\u003c/p\u003e\n\u003cp\u003e\u003cbr\u003e\u003c/p\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 Aura/MLS Version 4 /Level 2 water vapor and temperature data are available from the NASA GES DISK (https://disc.gsfc.nasa.gov/). The JAWARA temperature data is available from the JAWARA database site (https://jawara.nipr.ac.jp/home). The NLC images used in this study are included as additional file in this paper.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u0026nbsp;\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\u003eThis work was supported by MEXT/JSPS KAKENHI grants, 19H01956, 21H01144/23K20873 and 22H01289/23K22560.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthors\u0026rsquo; contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAE and HS designed this work. AE, YN, and MO conducted data analysis. AE wrote the first draft of the manuscript. AE, HS, YT, PD, and TTT contributed interpretations on results from data analysis. HS, NN, KS, KS, SI, and KI contributed operation of the imaging observation network in Hokkaido Japan and data archiving. All authors have contributed to revising and improving the manuscript. All authors have read and approved the final manuscript.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgments\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe thank the joint project of The University of Tokyo, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), and National Institute of Polar Research (NIPR) for providing JAWARA data and the Aura/MLS team for providing the Aura/MLS data.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthors\u0026rsquo; information\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eYN and OM had been belonged to Meiji University at the construction phase of this study and now moved another organization.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eDalin P, Suzuki H, Pertsev N, Perminov V, Shevchuk N, Tsimerinov E, Zalcik M, Brausch J, McEwan T, McEachran I, Connors M, Schofield I, Dubietis A, Černis K, Zadorozhny A, Solodovnik A, Lifatova D, Gr\u0026oslash;nne J, Hansen O, Andersen H, Romejko V (2023) The strong activity of noctilucent clouds at middle latitudes in 2020. Polar Sci 35:100920. doi:10.1016/j.polar.2022.100920.\u003c/li\u003e\n \u003cli\u003eFiedler J, Baumgarten G, Berger U, Hoffmann P, Kaifler N, L\u0026uuml;bken FJ (2011) NLC and the background atmosphere above ALOMAR. Atmos Chem Phys 11:5701-5717. doi:10.5194/acp-11-5701-2011.\u003c/li\u003e\n \u003cli\u003eFogle B, Haurwitz B, (1966) Noctilucent clouds. Space Sci Rev 6: 279\u0026ndash;340. doi:10.1007/BF00173768\u003c/li\u003e\n \u003cli\u003eFogle, B. (1968). The Climatology of Noctilucent Clouds According to observations made from North America during 1964-66. The Meteorological Magazine, \u003cem\u003e97\u003c/em\u003e, 193\u0026ndash;204.\u003c/li\u003e\n \u003cli\u003eGerding M, Baumgarten G, Zecha M, L\u0026uuml;bken FJ, Baumgarten K, Latteck R (2021) On the unusually bright and frequent noctilucent clouds in summer 2019 above Northern Germany. J Atmos Sol-Terr Phys 217:105577. doi:10.1016/j.jastp.2021.105577.\u003c/li\u003e\n \u003cli\u003eKaifler N, Kaifler B, Wilms H, Rapp M, Stober G, Jacobi C (2018) Mesospheric temperature during the extreme midlatitude noctilucent cloud event on 18/19 July 2016. J Geophys Res: Atmos 123:13,775-13,789. doi:10.1029/2018JD029717.\u003c/li\u003e\n \u003cli\u003eKirkwood S, Stebel K (2003) Influence of planetary waves on noctilucent cloud occurrence over NW Europe. J Geophys Res: Atmos 108:8440. doi:10.1029/2002jd002356.\u003c/li\u003e\n \u003cli\u003eKoshin D, Sato K, Watanabe S, Miyazaki K (2025) The JAGUAR-DAS whole neutral atmosphere reanalysis: JAWARA. Progs Earth Planet Sci 12:1. doi:10.1186/s40645-024-00674-3.\u003c/li\u003e\n \u003cli\u003eLange A, Baumgarten G, Rozanov A, von Savigny C (2022) On the colour of noctilucent clouds. 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Springer Netherlands, Dordrecht, pp 19-56. doi:10.1007/978-94-007-0326-1_2.\u003c/li\u003e\n \u003cli\u003eRapp, M., L\u0026uuml;bken, F. J., M\u0026uuml;llemann, A., Thomas, G. E., \u0026amp; Jensen, E. J (2002) Small-scale temperature variations in the vicinity of NLC: Experimental and model results. Journal of Geophysical Research Atmospheres 107(D19), 4392. \u0026nbsp;doi:10.1029/2001JD001241\u003c/li\u003e\n \u003cli\u003eRussell III JM, Rong P, Hervig ME, Siskind DE, Stevens MH, Bailey SM, Gumbel J (2014) Analysis of northern midlatitude noctilucent cloud occurrences using satellite data and modeling. J Geophys Res 119:3238-3250. doi:10.1002/2013JD021017.\u003c/li\u003e\n \u003cli\u003eSato K, Koshin D (2025) JAGUAR data assimilation system whole neutral atmosphere reanalysis (JAWARA) [Data set]. National Institute of Polar Research. doi:10.17592/002.2025010407.\u003c/li\u003e\n \u003cli\u003eSuzuki H, Chino H, Sano Y, Kadokura A, Ejiri MK, Taguchi M (2015) Imaging-based observations of low-latitude auroras during 2001-2004 at Nayoro, Japan. Earth Planets Space 67:107. doi:10.1186/s40623-015-0278-z.\u003c/li\u003e\n \u003cli\u003eSuzuki H, Sakanoi K, Nishitani N, Ogawa T, Ejiri MK, Kubota M, Kinoshita T, Murayama Y, Fujiyoshi Y (2016) First imaging and identification of a noctilucent cloud from multiple sites in Hokkaido (43.2-44.4\u0026deg;N), Japan. Earth Planets Space 68:182. doi:10.1186/s40623-016-0562-6.\u003c/li\u003e\n \u003cli\u003eTsuda TT, Hozumi Y, Kawaura K, Hosokawa K, Suzuki H, Nakamura T (2018) Initial report on polar mesospheric cloud observations by Himawari-8. Atmos Meas Tech 11:6163-6168. doi: 10.5194/ amt-11-6163-2018\u003c/li\u003e\n \u003cli\u003eTsuda TT, Hozumi Y, Kawaura K, Tatsuzawa K, Ando Y, Hosokawa K, Suzuki H, Murata KT, Nakamura T, Yue J, Nielsen K (2022) Detection of polar mesospheric clouds utilizing Himawari-8/AHI full-disk images. Earth Space Sci 9:e2021EA002076. doi:10.1029/2021EA002076\u003c/li\u003e\n \u003cli\u003eYamazaki Y, Sato K, Koshin D, Yasui R (2025) Symmetric and Antisymmetric Solar Migrating Semidiurnal Tides in the Mesosphere and Lower Thermosphere. Journal of Geophysical Research: Atmospheres, 130(3). doi:10.1029/2023JD040222\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":"Noctilucent clouds (NLCs) , Mesopause region , Upper mesosphere , Middle latitudes NLCs , Atmospheric tides , JAWARA , Aura/MLS , Hokkaido Japan","lastPublishedDoi":"10.21203/rs.3.rs-8452277/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8452277/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eNoctilucent clouds (NLCs) are the highest clouds in the Earth’s atmosphere, occurring at altitudes approximately 82–85 km in polar regions, and are typically observed during summer in high-latitude regions (50°–60° in latitude). In recent years, NLC occurrences have been reported to expand toward lower latitudes. Particularly, the number of NLC detections from midlatitudes (below 50°) is increasing. In Japan, the NLC imaging network has been updated since the first NLC detection from Hokkaido in 2015. Four more events were observed in 2020, one in 2021, and four in 2025, resulting in a total of 10 NLC observations at present. The event on June 14, 2020, was particularly notable, reaching a southernmost latitude of 44.6°N, which is the first observation of NLCs extending over the main island of Hokkaido. Except for one case, all events in Hokkaido occurred in the early morning hours (2:00–3:00 LST), indicating a strong local-time dependence on the occurrence. In this study, we investigated the occurrence characteristics of NLCs from Hokkaido and their relationship with the background atmospheric conditions. By analyzing the NLC formation environment and advection history using temperature and water vapor data from Aura/Microwave Limb Sounder (MLS) and temperature data from the JAGUAR-DAS Whole neutral Atmosphere Reanalysis (JAWARA), we obtained the following key findings. In the longitudinal sector including Hokkaido, a low-temperature condition favorable for NLC conservation is formed by atmospheric tidal waves prior to the morning hours. This creates a favorable environment for NLC survival during its advection from high latitudes. This tidal activity creates a distinct observational advantage for morning NLC observations, consistent with the fact that most events in this study were observed in the morning.\u003c/p\u003e","manuscriptTitle":"Asymmetry in the Morning and Evening Occurrence of Noctilucent Clouds in Japan","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-01-29 16:20:19","doi":"10.21203/rs.3.rs-8452277/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Major Revision","date":"2026-03-13T00:07:15+00:00","index":"","fulltext":""},{"type":"reviewerAgreed","content":"","date":"2026-01-28T08:25:08+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-01-26T15:48:04+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-12-29T10:30:13+00:00","index":"","fulltext":""},{"type":"submitted","content":"Earth, Planets and Space","date":"2025-12-26T00:24:04+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":"a01a3e10-6f69-4fa3-91a2-50e262edaf2e","owner":[],"postedDate":"January 29th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-05-14T16:26:56+00:00","versionOfRecord":[],"versionCreatedAt":"2026-01-29 16:20:19","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8452277","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8452277","identity":"rs-8452277","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}