10:45 〜 12:15
[PEM12-P03] 孤立サブストーム時における電離圏全電子数の時間・空間変動について
キーワード：サブストーム、電離圏、沿磁力線電流、全電子数、電場
The signatures of substorm onset are identified as a sudden auroral brightening, mid-latitude positive bay, ionospheric total electron content (TEC) variation, and a sharp increase in particle precipitation at the auroral zone. From a superposed epoch analysis result of 239 substorm events with GPS-TEC and satellite observation data, Weygand and Wing [2020] found that a significant enhancement of TEC occurred within the nightside region 1 field-aligned currents (FACs) system during the substorm expansion phase. However, they could not clarify a dependence of TEC variations on season and substorm intensity due to the shortage of analyzed substorm events. In this study, we clarify the characteristics of temporal and spatial evolution of TEC enhancements and depletions from high to low latitudes as a function of season and substorm intensity using long-term Global Navigation Satellite System (GNSS)-TEC data with high time and spatial resolution. Further, we discuss a main cause of the TEC variations during substorms. To monitor the magnetosphere and ionosphere conditions, we also used solar wind, interplanetary magnetic field (IMF), and geomagnetic indices (AE and SYM) data. Here, we analyzed the ratio of the TEC difference (rTEC) defined by Shinbori et al. [2020] to investigate the rTEC enhancements and depletions. For identification of substorm events for a period of 2000-2019, we used a list of substorm events derived from the SML index [Ohtani and Gjerloev, 2020]. Further, we calculated the maximum value of the AE index 2 hours after the onsets of substorms to determine the substorm intensity. According to the maximum AE value, we classified five categories: very weak (< 200 nT), weak (< 400 nT), moderate (< 600 nT), strong (< 1000 nT), and very strong (>= 1000 nT) events. The statistical analysis clearly shows an increase and decrease of the rTEC value in the pre-midnight (21-0 h magnetic local time (MLT)) and post-midnight (0-3 h MLT) sectors at auroral latitudes (67o-74o geomagnetic latitude (MLAT)) approximately 1 hour before the substorm expansions in spring and fall equinoxes. In winter, the location of the onset of rTEC enhancement was near the dusk sector (18-21 h MLT). In summer, the magnitude of the rTEC variations is much smaller, compared with that in other seasons. The post-midnight rTEC depletion corresponds to the high-latitude trough [Rodger et al., 1992] and is mainly generated by the evacuation of downward FACs due to the enhancement of the nightside substorm current wedge [Zou et al., 2013]. Further, another rTEC enhancement occurs in the polar cap (more than 80o MLAT). This enhancement corresponds to a tongue of ionization (TOI) phenomenon caused by a significant increase of two-cell convection at high latitudes. The amplitude of the rTEC enhancement depends on the substorm intensity and was the largest in winter. This means that the convection electric field is enhanced more significantly for the large substorms. After the substorm expansion, the enhanced rTEC region in the post-midnight sector shows a rapid eastward extension to the post-midnight sector (approximately 6 h MLT) within 1 hour. The expansion speed was 1.38 km/sec, corresponding to the drift velocity of the ring current electrons in the inner magnetosphere. At this time, the depth of mid-latitude trough below the enhanced rTEC region tends to increase significantly in the dusk sector (17-22 h MLT). For more than moderate substorm events (maximum AE >= 600 nT), another rTEC enhancement appears in the afternoon sector (12-18 h MLT) from high to mid-latitudes (55o-75o MLAT). The amplitude tends to increase with an increasing intensity of substorms. The enhanced rTEC region extends to the low latitudes and midnight with time. This behavior is almost similar to that of a storm-enhanced density (SED) phenomenon as seen during geomagnetic storms.