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To gain deeper insights into radiation belt loss into the atmosphere, a statistical study of MeV electron precipitation during radiation belt dropout events is undertaken. During these events, electron intensities often drop by an order of magnitude or more within just a few hours. For this study, dropouts are defined as a decrease by at least a factor of five in less than 8 hours. Van Allen probe measurements are employed to identify dropouts across various parameters, complemented by precipitation data from the CALorimetric Electron Telescope instrument on the International Space Station. A temporal analysis unveils a notable increase in precipitation occurrence and intensity during dropout onset, correlating with the decline of SYM‐H, the north‐south component of the interplanetary magnetic field, and the peak of the solar wind dynamic pressure. Moreover, dropout occurrences show correlations with the solar cycle, exhibiting maxima at the spring and autumn equinoxes. This increase during equinoxes reflects the correlation between equinoxes and the SYM‐H index, which itself exhibits a correlation with precipitation during dropouts. Spatial analysis reveals that dropouts with precipitation penetrate into lower L‐star regions, mostly reaching L‐star <4, while most dropouts without precipitation don't penetrate deeper than L‐star 5. This is consistent with the larger average dimensions of dropouts associated with precipitation. During dropouts, precipitation is predominantly observed in the dusk‐midnight sector, coinciding with the most intense precipitation events. The results of this study provide insight into the contribution of precipitation to radiation belt dropouts by deciphering when and where precipitation occurred. Plain Language Summary: The outer radiation belt encircles Earth, trapping energetic electrons due to the planet's magnetic field. Under certain conditions, such as during geomagnetic storms, there can be a significant loss of these trapped electrons over a short period, known as a dropout. One reason for this can be particles reentering Earth's atmosphere, leading to their removal from the outer radiation belt. This is referred to as precipitation. This study investigates the role of precipitation for dropouts, focusing on when and where precipitation occurs during those events. To accomplish this, data from the Van Allen Probe spacecraft, measuring electron density, and the CALorimetric Electron Telescope instrument aboard the International Space Station, measuring precipitation, are utilized. The findings reveal a correlation between dropouts, whether aligned with precipitation or not, and the solar cycle and seasons. Seasonal variations are likely connected to the intensity of disturbances in the magnetosphere, consequently raising the probability of precipitation. Spatial investigations reveal that dropouts accompanied by precipitation penetrate deeper into lower regions of stably trapped particles. Furthermore, precipitation predominantly occurs on the dusk and midnight side of Earth, where the strongest precipitation measurements are also recorded. Key Points: MeV electron dropouts with and without associated precipitation show dependencies on solar cycle and seasonsAn increase in precipitation occurrence and intensity during MeV dropouts aligns with the decline in SYM‐H and Bz, and the peak in PdynDropouts with MeV electron precipitation, on average, spread over a wider radial extent, with occurrences related to SYM‐H [ABSTRACT FROM AUTHOR]

We report a citizen science‐motivated study on the cause of an unusually bright red aurora as witnessed from Hokkaido, Japan during a magnetic storm on 1 December 2023. The auroral brightness of 5 kR is unusual for the Dst index peak of only −107 nT. In spite of the moderate storm amplitude, the extremely high solar wind density of >50/cc and dynamic pressure of >25 nPa caused the aurora oval extension to 53 magnetic latitudes (L = 2.8). We discuss that the drift loss of the ring current particles across the small‐size magnetopause is important, and Hokkaido was at the right position to see the direct effect of the large particle injection of the storm‐time substorm. Plain Language Summary: Citizen scientists identified an unusually bright red aurora from Hokkaido, Japan during a not‐so‐unusual magnetic storm on 1 December 2023. The large dynamic pressure, driven by large density of >50/cc, contributed to a small magnetopause and the effects observed at such low latitude. The hypothesis of this study is that the loss of ring current particles across the small‐size magnetopause played an important role. Also, we discuss that Hokkaido was at the right position to see the direct effect of storm‐time substorm. Key Points: Unusually bright red aurora was witnessed by citizen scientists from Hokkaido, Japan on 1 December 2023The magnetic storm amplitude was not unusually large, but the solar wind density was high (50/cc)Dynamic pressure and asymmetric evolution of the ring current are important to understand the cause of red‐aurora magnetic storm events [ABSTRACT FROM AUTHOR]

Relativistic electron precipitation (REP) refers to the release of high‐energy electrons initially trapped in the outer radiation belt, which then precipitate into Earth's upper atmosphere, contributing significantly to the rapid depletion of radiation belt electron flux. This study presents a statistical analysis of REP observations collected by the Calorimetric Electron Telescope (CALET) experiment aboard the International Space Station from 2015 to the present day. Specifically, the analysis utilizes count rates acquired from the two top scintillators constituting the top charge detector, each sensitive to electrons with energies above 1.5 and 3.4 MeV, respectively. Analysis of CALET data reveals a previously unreported semi‐annual variation in the occurrence of REP events. REP periodicities resemble those observed for trapped electron fluxes in the outer belt. Furthermore, their amplitude follows the overall trend of solar wind high‐speed streams and the solar activity. Plain Language Summary: Relativistic electron precipitation (REP) refers to the release, toward the upper atmosphere, of high energy electrons initially trapped in a torus shaped region around Earth known as the outer Van Allen radiation belt. REP is relevant as it contributes to the fast depletion of the electrons from this region. This study presents a statistical analysis of the REP observations made by the Calorimetric Electron Telescope (CALET) experiment on board the International Space Station (2015–present). Data from CALET experiment reveals a previously unreported 6‐month periodicity similar to those observed for high energy electrons in the outer belt. Key Points: Semi‐annual variation of relativistic electron precipitation (REP) have been observed for the first timeReported periodicities have been compared with those characterizing the outer belt electron fluxesThe temporal variation in the REP and the trapped fluxes were found to be in strong correlation [ABSTRACT FROM AUTHOR]

The accidental reentry of 38 Starlink satellites occurred in early February 2022, associated with the occurrence of moderate magnetic storms. A poorly understood structure of Coronal Mass Ejections (CMEs) caused the magnetic storms at unexpected timing. Therefore, a better understanding of minor CME structures is necessary for the modern space weather forecast. During this event, the “up to 50%” enhancement of air drag force was observed at ~200 km altitude, preventing the satellites’ safety operations. Although the mass density enhancement predicted by the NRLMSIS2.0 empirical model is less than 25% under the present moderate magnetic storms, the real-time GAIA simulation showed a mass density enhancement of up to 50%. Further, the real-time GAIA simulation suggests that the actual thermospheric disturbances at 200 km altitude may occur with larger amplitude in a broader area than previously thought.

Solar modulation of galactic cosmic rays around the solar minimum in 2019–2020 looks different in the secondary neutrons and muons observed at the ground. To compare the solar modulation of primary cosmic rays in detail, we must remove the possible seasonal variations caused by the atmosphere and surrounding environment. As such surrounding environment effects, we evaluate the snow cover effect on neutron count rate and the atmospheric temperature effect on muon count rate, both simultaneously observed at Syowa Station in the Antarctic (69.01° S, 39.59° E). A machine learning technique, Echo State Network (ESN), is applied to estimate both effects hidden in the observed time series of the count rate. We show that the ESN with the input of GDAS data (temperature time series at 925, 850, 700, 600, 500, 400, 300, 250, 200, 150, 100, 70, 50, 30, and 20 hPa) at the local position can be useful for both the temperature correction for muons and snow cover correction for neutrons. The corrected muon count rate starts decreasing in late 2019, preceding the corrected neutron count rate which starts decreasing in early 2020, possibly indicating the rigidity-dependent solar modulation in the heliosphere.

Many studies have been conducted about the impact of energetic charged particles on the atmosphere during geomagnetically active times, while quiet time effects are poorly understood. We identified two energetic electron precipitation (EEP) events during the growth phase of moderate substorms and estimated the mesospheric ionization rate for an EEP event for which the most comprehensive dataset from ground-based and space-born instruments was available. The mesospheric ionization signature reached below 70 km altitude and continued for ~15 min until the substorm onset, as observed by the PANSY radar and imaging riometer at Syowa Station in the Antarctic region. We also used energetic electron flux observed by the Arase and POES 15 satellites as the input for the air-shower simulation code PHITS to quantitatively estimate the mesospheric ionization rate. The calculated ionization level due to the precipitating electrons is consistent with the observed value of cosmic noise absorption. The possible spatial extent of EEP is estimated to be ~8 h MLT in longitude and ~1.5° in latitude from a global magnetohydrodynamic simulation REPPU and the precipitating electron observations by the POES satellite, respectively. Such a significant duration and spatial extent of EEP events suggest a non-negligible contribution of the growth phase EEP to the mesospheric ionization. Combining the cutting-edge observations and simulations, we shed new light on the space weather impact of the EEP events during geomagnetically quiet times, which is important to understand the possible link between the space environment and climate.

A 61 years old watercolor painting of red aurora was recently provided from a Japanese citizen, and it contributed to understand the detailed time evolution around the peak time of the large magnetic storm on 11 February 1958. The painting gives information of the elevation angle of the red aurora seen from low latitude (27.4° magnetic latitude) at 1205–1225 UT during the beginning of the recovery phase of the magnetic storm. Combined with the hand-made sketch of the same red aurora seen from the Abashiri Local Meteorological Office (located at 34° magnetic latitude) at 1215 UT, the position of the red aurora is determined via triangulation. It is found that the red aurora reached up to 400 km at 41° magnetic latitude, which is 1.0° higher in magnetic latitude than the red aurora which appeared just before the peak time of the magnetic storm.

During a great magnetic storm on Feb 11, 1958, a fan-shaped aurora was photographed at Memambetsu, Hokkaido, Japan – the first and oldest photograph record of auroras observed in Japan, accompanied by many hand-made drawings, thus, portraying a rare opportunity of coexistence between photograph images and hand-made drawings. In fact, the same portrayal reminds us of the great red aurora with fan-shaped white pillars observed during the 1872 and 1770 great magnetic storms. The hand-made sketches, photographs, and the spectral data revealed that the white pillars and red glow of the fan-shaped aurora were dominated by auroral green and red lines, respectively. From the analysis of newly digitized microfilm data and hand-made drawings, we found that the fan-shaped aurora appeared during the peak activity of magnetic storm and moved westward at 0.4 km/s at 400-km altitude at 38°–40° magnetic latitudes, which is consistent with the enhanced convection pattern in the middle latitude at storm time. Such a fan-shaped aurora can fundamentally characterize the middle-latitude evening-to-midnight auroras during great magnetic storms, which show the most destabilized transient appearance of the inner magnetosphere.

Real-time estimation of cosmic-ray fluxes on satellite orbits is one of the greatest challenges in space weather research. Therefore, we develop a system for nowcasting and forecasting the galactic cosmic ray (GCR) and solar energetic particle (SEP) fluxes at any location in the magnetosphere and ionosphere during ground-level enhancement (GLE) events. It is an extended version of the WArning System for AVIation Exposure to SEP (WASAVIES), which can determine event profiles by using real-time data of the count rates of several neutron monitors (NMs) at the ground level and high-energy proton fluxes observed by Geostationary Operational Environmental Satellites (GOES) satellites. The extended version, called WASAVIES-EO, can calculate the GCR and SEP fluxes outside a satellite based on its two-line element (TLE) data. Moreover, organ absorbed-dose and dose-equivalent rates of astronauts in the International Space Station (ISS) can be estimated using the system, considering its shielding effect. The accuracy of WASAVIES-EO was validated based on the dose rates measured in ISS, as well as based on high-energy proton fluxes observed by POES satellites during large GLEs that have occurred in the 21st century. Agreement between the nowcast and forecast dose rates in ISS, especially in terms of their temporal structures, indicates the usefulness of the developed system for future mission operations.

Muon detectors and neutron monitors were recently installed at Syowa Station, in the Antarctic, to observe different types of secondary particles resulting from cosmic ray interactions simultaneously from the same location. Continuing observations will give new insight into the response of muon detectors to atmospheric and geomagnetic effects. Operation began in February, 2018 and the system has been stable with a duty-cycle exceeding 94%. Muon data shows a clear seasonal variation, which is expected from the atmospheric temperature effect. We verified successful operation by showing that the muon and neutron data are consistent with those from other locations by comparing intensity variations during a space weather event. We have established a web page to make real time data available with interactive graphics (http://polaris.nipr.ac.jp/cosmicrays/).