16:00 〜 16:15
[PEM13-09] Statistical behavior of large-scale ionospheric disturbances from high latitudes to mid-latitudes during geomagnetic storms using 20-year GNSS-TEC data:
Dependence on season and storm intensity
キーワード：磁気嵐、季節、Storm-enhanced density、Tongue of ionization、中緯度トラフ、負相嵐
The Earth’s ionosphere is formed by ionization of the upper atmosphere due to solar extreme ultraviolet (EUV) radiation and precipitation of energetic particles from the magnetosphere. The plasma density variations in the ionosphere are caused by the energy input from the lower atmosphere and magnetosphere. Especially, during a geomagnetic storm, the enhanced ionospheric electric field and neutral wind globally change the plasma density distribution in the ionosphere, which causes an increasing error of satellite positioning and navigation. Therefore, it is important to investigate the characteristics of spatial and temporal variations of ionospheric plasma density during geomagnetic storms and their generation mechanisms. In this study, we establish the statistical behavior of ionospheric TEC variations from high latitudes to mid-latitudes during the main and recovery phases of geomagnetic storms, we conducted a superposed epoch analysis of interplanetary magnetic field, solar wind, geomagnetic indices (AE and SYM-H), and global navigation satellite system (GNSS)-total electron content (TEC) data for 20 years (2000–2019). We also investigate seasonal and storm intensity dependence of the rTEC response to geomagnetic storms from high latitudes to mid-latitudes. In this study, we first identify 663 geomagnetic storm events with the minimum SYM-H value of less than -40 nT and investigate the characteristics of the TEC variations for the weak (-60 < SYM-Hmin < -40 nT), moderate (-100 < SYM-Hmin < -60 nT), and strong (-150 < SYM-Hmin < -100 nT) geomagnetic storms. For each group of geomagnetic storm events, we analyzed the ratio of the TEC difference (rTEC) which is defined as a difference between storm-time TEC and geomagnetically 10 quiet-day average TEC. The 10 quiet-day average TEC value was calculated by referring to a list of international quiet and disturbed days. The main results obtained from the present study are as follows: (1) The tongue of ionization (TOI) phenomenon in the polar cap is dominantly observed in winter during the main and recovery phases of geomagnetic storms. The magnitude of the TEC enhancement is the largest when the SYM-H index becomes minimum. (2) The significant delta-rTEC depletion occurs in the dawnside polar cap region (70–80 degrees in geomagnetic latitude (GMLAT)) during the main phase of geomagnetic storms and the magnitude tends to increase with depending on the intensity of geomagnetic storms. (3) The delta-rTEC enhancement at the auroral latitudes (55–70 degrees GMLAT) associated with auroral activities shows a strong seasonal dependence, indicating that the magnitude becomes the largest in winter. The center of the enhanced delta-rTEC region leans to the post-midnight sector (~2 h in geomagnetic local time (GMLT)) except for winter. (4) The location of the mid-latitude trough minimum moves equatorward from the afternoon to mid-night sectors, but the structure of the mid-latitude trough becomes unclear in the nighttime sector in winter. (5) The structure of the storm-enhanced density (SED) plume is observed on the dayside in winter during the main phase of geomagnetic storms. Further, the mid-latitude delta-rTEC enhancement as a source of the SED plume is more dominant in this season. (6) The delta-rTEC depletion starts to occur at auroral latitudes in the morning sector (8-10 h GMLT) during the main phase of geomagnetic storm, and the decrease region extends to the latitudinal and GMLT directions with time. This signature is more dominant in summer than in winter, which is agreement with the classical ionospheric storm scenario.