16:15 〜 16:30
[PEM12-19] Seasonal and magnetic local time variations of the high-latitude electron density irregularities during geomagnetic storms using long-term GNSS data
キーワード：磁気嵐、高緯度電離圏、rate of TEC index (ROTI)
Seasonal dependence of spatiotemporal variations of auroral oval during geomagnetically quiet and/or disturbed conditions has been studied, whereas a seasonal dependence of spatiotemporal variations of auroral oval during the main and recovery phases of geomagnetic storms has not been studied statistically, using long-term worldwide global navigation satellite system (GNSS) data with a high spatial resolution.
We performed a superposed epoch analysis of solar wind, interplanetary magnetic field, geomagnetic index, and the rate of total electron content (TEC) index (ROTI) derived from GNSS-TEC data during geomagnetic storms from 2000 to 2018 (653 events) to clarify the seasonal variation of auroral oval and tongue of ionization (TOI) and magnetic local time (MLT) variation of ROTI values associated with auroral oval. In this analysis, we defined the time of the SYM-H minimum as the zero epoch time. The total number of events used in this analysis was 150 for summer (May, June, and July: MJJ), 132 for winter (November, December, and January: NDJ), 187 for spring (February, March, and April), and 184 for fall (August, September, and October) in the northern hemisphere. As a result, during the main phase of geomagnetic storms, the high-latitude ROTI values associated with an auroral oval increased at all MLT for both MJJ and NDJ but the magnitude was higher for NDJ than for MJJ. During the recovery phase of geomagnetic storms, the magnitude of ROTI enhancement at the high latitudes decreased with time for both MJJ and NDJ. The ROTI enhancements in the polar cap associated with a TOI were seen during the main phase of geomagnetic storms for NDJ. Whereas, it cannot be clearly observed for MJJ even though the magnitude of the ROTI enhancement in the polar cap slightly increased. During the recovery phase of geomagnetic storms, the ROTI value in the polar cap decreased for both MJJ and NDJ. The low ROTI values at the high latitudes for summer are due to the ionizations by the solar EUV radiation. Plasma density irregularities are filled by the ionization effect. Furthermore, the ionospheric conductivity is also larger for summer than for winter because of the ionizations by the solar EUV radiation. Assuming a constant current, a small magnitude of the electric field is formed (or a low plasma drift velocity). This suggests that plasma instabilities do not contribute to causing plasma density irregularities much.
On the other hand, during the late main phase of geomagnetic storms, the high-latitude ROTI values showed a peak value at the dawnside (4–7 h MLT) of 65−70° in geomagnetic latitude for all seasons. The ROTI enhancement at dawn could represent the electron density irregularities caused by precipitation of the relatively low energy (less than several hundred electronvolt) electrons. The current result indicates that the low energy electrons precipitate into the high-latitude (65−70° in geomagnetic latitude) ionosphere at dawn in all seasons during geomagnetic storms.
We performed a superposed epoch analysis of solar wind, interplanetary magnetic field, geomagnetic index, and the rate of total electron content (TEC) index (ROTI) derived from GNSS-TEC data during geomagnetic storms from 2000 to 2018 (653 events) to clarify the seasonal variation of auroral oval and tongue of ionization (TOI) and magnetic local time (MLT) variation of ROTI values associated with auroral oval. In this analysis, we defined the time of the SYM-H minimum as the zero epoch time. The total number of events used in this analysis was 150 for summer (May, June, and July: MJJ), 132 for winter (November, December, and January: NDJ), 187 for spring (February, March, and April), and 184 for fall (August, September, and October) in the northern hemisphere. As a result, during the main phase of geomagnetic storms, the high-latitude ROTI values associated with an auroral oval increased at all MLT for both MJJ and NDJ but the magnitude was higher for NDJ than for MJJ. During the recovery phase of geomagnetic storms, the magnitude of ROTI enhancement at the high latitudes decreased with time for both MJJ and NDJ. The ROTI enhancements in the polar cap associated with a TOI were seen during the main phase of geomagnetic storms for NDJ. Whereas, it cannot be clearly observed for MJJ even though the magnitude of the ROTI enhancement in the polar cap slightly increased. During the recovery phase of geomagnetic storms, the ROTI value in the polar cap decreased for both MJJ and NDJ. The low ROTI values at the high latitudes for summer are due to the ionizations by the solar EUV radiation. Plasma density irregularities are filled by the ionization effect. Furthermore, the ionospheric conductivity is also larger for summer than for winter because of the ionizations by the solar EUV radiation. Assuming a constant current, a small magnitude of the electric field is formed (or a low plasma drift velocity). This suggests that plasma instabilities do not contribute to causing plasma density irregularities much.
On the other hand, during the late main phase of geomagnetic storms, the high-latitude ROTI values showed a peak value at the dawnside (4–7 h MLT) of 65−70° in geomagnetic latitude for all seasons. The ROTI enhancement at dawn could represent the electron density irregularities caused by precipitation of the relatively low energy (less than several hundred electronvolt) electrons. The current result indicates that the low energy electrons precipitate into the high-latitude (65−70° in geomagnetic latitude) ionosphere at dawn in all seasons during geomagnetic storms.