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Abstract:
To clarify the characteristics of temporal and spatial development of ionospheric total electron content (TEC) enhancements and depletions from high to low latitudes as a function of substorm intensity, we analyzed long-term Global Navigation Satellite System (GNSS)-TEC data together with solar wind, interplanetary magnetic field, and AE and SYM data. 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 ratio of the TEC difference (rTEC) [Shinbori et al., 2019] in pre-midnight (21–0 h MLT) and post-midnight (0–3 h MLT) at auroral latitudes (67o–74o MLAT) approximately 1 hour before the substorm expansions. 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]. For more than moderate substorm events, another rTEC enhancement appears in the afternoon sector (12–18 h MLT) from high to mid-latitudes (55o–75o MLAT), resembling an SED phenomenon as seen during geomagnetic storms.
