11:45 〜 12:00
[PEM10-10] A study of temporal and spatial variations of plasmaspheric total electron content during magnetic storms by using the worldwide GPS data
キーワード：全電子数、グローバル ポジショニング システム、バイアス、プラズマ圏
Total electron content (TEC), an integration of the electron density along a ray path from satellite to receiver, can be measured by dual-frequency Global Positioning System (GPS). However, the measured TEC contains inter-frequency biases inherent with satellites and receivers cannot be ignored in original TEC. Previous study (Otsuka et al., 2002) has developed a method to separate ionospheric TEC from the inter-frequency biases by using the least square fitting procedure. In this method, plasmaspheric TEC (PTEC) is not considered. However, it is known that the PTEC can be 10-50% of the ionospheric TEC. Considering the satellite zenith angle dependence on slant factor, which is a factor to convert the slant to vertical TEC, PTEC could be included in the estimated inter-frequency bias, because PTEC depends only weakly on the slant factor and inter-frequency bias do not depend on the slant factor. Therefore, we have analyzed the inter-frequency bias data obtained from approximately 9,000 receivers over the world to investigate PTEC with high spatial and temporal resolutions during geomagnetic storms. In this study, we have analyzed the inter-frequency bias data during three geomagnetic storms on March 2013, November 2017, and August 2018. The inter-frequency bias can be considered to be constant during several days, while PTEC will vary depending on storm progression. Reference of the bias value is defined as the averaged bias over a period of 1-7 days before each magnetic storm. Deviation of the bias from the reference is analyzed, in order to investigate day-to-day variation of the biases, which could indicate day-to-day variation of PTEC. The standard deviations of biases for each receiver are used as a proxy representing the stability of biases. Most of standard deviation ranges 0-2 TECU, indicating that most of biases are stable for a week. These three storm cases commonly show that the bias at middle and low latitudes decreases during main phase of the magnetic storms, and that the bias decreases are largest at nightside or dusk at the beginning of magnetic storms. This result indicates occurrence of erosion process in the plasmasphere during the main phase of magnetic storms. Longitudinal variations of PTEC during geomagnetic storms will be discussed with higher spatial resolutions in this presentation.