[P-EM11] Coupling Processes in the Atmosphere-Ionosphere System

[PEM11-05] 20年間のGNSS-TECデータを用いた磁気嵐時の大規模な電磁圏擾乱の統計的振る舞い

*新堀 淳樹¹、大塚 雄一¹、惣宇利 卓弥¹、津川 卓也²、西岡 未知² (1.名古屋大学宇宙地球環境研究所、2.情報通信研究機構)

キーワード：磁気嵐、電離圏、侵入電場、過遮蔽電場、電離圏擾乱ダイナモ、全電子数

To investigate the time history of spatial variation of global electron density in the ionosphere during geomagnetic storms, we conducted a super epoch analysis of interplanetary magnetic field (IMF), solar wind, geomagnetic indexes (AE and SYM-H), and global navigation satellite system (GNSS) – total electron content (TEC) data for 20 years (2000–2019). In this study, we analyzed the ratio of the TEC difference (rTEC) for 663 geomagnetic storms with the minimum SYM-H value of less than -40 nT. The rTEC is defined as a difference between the storm-time TEC and averaged quiet-day TEC normalized by the quiet-day one. During the main phase of geomagnetic storms associated with a negative excursion of the IMF Bz, the rTEC enhancement related to the mid-latitude broad storm-enhanced density (SED) appeared with a wide latitudinal range in the daytime (9–18 h, geomagnetic local time: GMLT) mid-latitude (more than 30^o in geomagnetic latitude: GMLAT) in the northern and southern hemispheres. The enhanced rTEC region expanded to the midnight with time. Around the noon, the rTEC enhancement related to the SED plume was observed at high latitudes (60–70^o, GMLAT). In the low-latitude and equatorial regions (less than 30^o, GMLAT), the rTEC enhancement appeared in the evening to morning sectors after -6 h epoch time. On the other hand, in the polar region, the rTEC enhancement and depression were observed in the duskside and dawnside polar cap in addition to the large rTEC enhancement in the pre-midnight to early morning sectors associated with auroral particle precipitation. The duskside rTEC enhancement had a stream-like structure along the anti-sunward convection plasma flow. This phenomenon has been named tongue of ionization (TOI). The depth and width of the dawnside rTEC depression increased as the geomagnetic storms developed. The region of rTEC depression corresponded to the downward region-1 field-aligned current (FAC) observed by the DMSP satellites. This result suggests that the rTEC depression is caused by the evacuation effect of the downward FAC. During the recovery phase, the magnitude of the rTEC enhancement from the polar cap to the mid-latitudes decreased rapidly within 4 hrs, and the rTEC depression around the morning at auroral latitudes expanded to the lower latitudes with a large MLT extent. After the epoch time of 8 h, the second weak rTEC enhancement appeared at the mid-latitudes in the afternoon – evening sectors although the geomagnetic activities as seen in the AE and SYM-H indexes remained low. Considering that the high-latitude convection electric field becomes weak, the second rTEC enhancement could be generated by the uplift of ionospheric plasmas due to the enhanced equatorward neutral wind in the thermosphere. On the other hand, the rTEC enhancement and depression were observed in the nighttime and morning equatorial (less than 10^o, GMLAT) regions, respectively. Further, the rTEC value decreased slightly in the off equatorial region around 15^o GMLAT after the epoch time of 2 h. The rTEC depression expanded to in the midnight within 4 hrs. Such the equatorial rTEC structures were maintained at least for 16 hrs. The equatorial rTEC signatures suggests that an over-shielding or disturbance dynamo field suppresses the equatorial fountain effect.