[P-EM12] Coupling Processes in the Atmosphere-Ionosphere System

[PEM12-11] 2024年5月の巨大磁気嵐時におけるプラズマ圏・電離圏の電子密度の時間・空間変動について

*新堀 淳樹¹、北村 成寿¹、山本 和弘¹、熊本 篤志²、土屋 史紀²、松田 昇也³、笠原 禎也³、寺本 万里子⁴、松岡 彩子⁵、惣宇利 卓弥⁵、大塚 雄一¹、西岡 未知⁶、Perwitasari Septi⁶、三好 由純¹、篠原 育⁷ (1.名古屋大学宇宙地球環境研究所、2.東北大学、3.金沢大学、4.九州工業大学、5.京都大学、6.情報通信研究機構、7.JAXA宇宙科学研究所)

キーワード：巨大磁気嵐、プラズマ圏、電離圏、あらせ衛星、負相嵐、プラズマ圏再充填過程

Due to the arrival of southward interplanetary magnetic field (IMF) to the Earth’s magnetosphere, a magnetospheric convection is driven by the dayside magnetic reconnection between the IMF and Earth’s magnetic field. The magnetospheric convection electric field penetrates the plasmasphere and equatorial ionosphere together with the development of a ring current in the inner magnetosphere. The electric field changes a spatial distribution of the electron density in the plasmasphere and ionosphere. However, details of the temporal and spatial variations of the electron density have not fully been understood during a super geomagnetic storm such as the May 2024 geomagnetic storm with a SYM-H minimum value of -512 nT. In this study, we analyzed Global Navigation Satellite System - total electron content (TEC) and Arase satellite observation data to elucidate the characteristics of the electron density variation in the plasmasphere and ionosphere during the geomagnetic storm and the generation mechanism. To identify the electron density variation in the ionosphere, we calculated the ratio of the TEC difference (rTEC) defined as a difference from a 10 quiet-day average TEC normalized by the average value. Further, we derived the electron density in the plasmasphere and inner magnetosphere from the upper limit frequency of upper-hybrid resonance (UHR) waves observed by the Arase satellite. As a result, an L-t diagram of the electron density shows a shrink of the plasmasphere from L=7.2.0 to L=2.0 within 9 hours after the onset of the storm. This result suggests that the rapid erosion of the plasmasphere is mainly caused by the penetration of a strong convection electric field. Interestingly, the electron density in the inner plasmasphere of less than L=2.0 increased by a factor of 3-5 during the main and early recovery phases of the geomagnetic storm. This phenomenon is thought to be caused by the uplift of the lower altitude plasmaspheric and ionospheric plasmas due to the penetration of the eastward electric field or heating of the ionospheric plasmas by heat flux from the inner magnetosphere. After the onset of the recovery phase of the geomagnetic storm, the plasmapause moved slowly to the higher L-value and recovered the level of geomagnetically quiet times on a 4-day scale. The time constant of the plasmaspheric refilling is much longer than that of other coronal mass ejection (CME)-type geomagnetic storms. On the other hand, several polar maps of rTEC in the Northern Hemisphere in geomagnetic coordinates show that an enhancement of the rTEC value occurred around the cusp (12-13 h in magnetic local time (MLT)) approximately 1 hour after the onset of the sudden commencement. The enhanced rTEC region extended in the magnetic latitude (MLAT) and MLT directions as the geomagnetic storm developed. After that, a tongue of ionization (TOI) was formed in the polar cap due to the enhancement of the two-cell convection in the high-latitude ionosphere. During the recovery phase of the geomagnetic storm, a spatial distribution of rTEC showed a large depletion of the rTEC value in the entire region of the ionosphere from the high to low latitudes. This phenomenon persisted for more than 3 days. The depletion suggests the occurrence of a negative storm due to the neutral composition (O/N₂) change driven by energy input from the magnetosphere at the high-latitude thermosphere. Further, DMSP satellite observations showed a depletion of an oxygen ion in the mid- and high latitudes when a strong negative storm occurred during the recovery phase of the geomagnetic storm. From the above observational facts, it can be thought that the long duration of the negative storm in the ionosphere prevents the plasmaspheric refilling process and that the plasmasphere remained shrunk for a long time. Therefore, the contribution of a negative storm to the plasmaspheric refilling process should be included in the plasmasphere and inner magnetospheric models.