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Dependency of the Ionospheric Storm on the Local Time of Storm Onset in the Southeast Asia Sector

The characteristics of ionospheric storms in the Southeast Asia region were investigated, using the ionospheric total electron content (TEC) derived from CUSV in Thailand (13.74°N, 100.53°E, MLAT 4.03°N) and BAKO in Indonesia (6.49°S, 106.85°E, MLAT 15.59°S) during 2011–2016. These two stations are located on the opposite sides of the magnetic equator, where BAKO is near the crest of the equatorial ionization anomaly (EIA) while CUSV is far from it. The storm-induced TEC were derived by subtracting the TEC during and after the storm day from the quiet-time TEC, which was the TEC averaged over three quiet days before the storm day. Positive and negative storms were identified where the amount of TEC increase and decrease was larger than the maximum standard deviation of the three quiet-day TEC. The characteristics of ionospheric storms were examined in terms of four local time periods of storm onsets, i.e., (I) 0000–0600 LT, (II) 0600–1200 LT, (III) 1200–1800 LT, and (IV) 1800–2400 LT. At both CUSV and BAKO, positive storms were mostly observed during 1200–1800 LT on the next two days for time period I, while negative storms were mostly seen during 1200–2400 LT on the next day for time period III. For time periods II and IV, common features were observed at both CUSV and BAKO but their storm signatures were different. It is concluded that the overall ionospheric storms show a dependency on the local time of storm onset, albeit with some exceptional storm events. These exceptional events were found to have a different Kp behavior during the recovery phase of the storm as compared to the typical ones for the same time period of storm onset.