Total electron content at an equatorial station during low solar activity: Geomagnetic activity effects emphasis

The initial results of Total Electron Content (TEC) data from September 2017 to November 2018 recorded over an equatorial station, Tarawa (geographic: 1.33(degrees)N, 173.01(degrees)E, geomagnetic: 2.68(degrees)S, 114.26(degrees)W) are presented which showed diurnal TEC maxima between 15:00-14:00 LT. The seasonal and semi-annual variations in the VTEC are clear, along with the less occurring winter anomalies and TEC bite-outs. The VTEC on magnetically disturbed days was higher than on quiet days. VTEC response to seven geomagnetic storms (5 moderate and 2 intense) showed positive ionospheric effect by all storms during their main phase. The VTEC response to two selected geomagnetic storms of 7-8 November 2017 (moderate strength, Dst =-72 nT) and 25-26 August 2018 (intense strength, Dst index =-174 nT) has been presented. During the November storm, TEC decreased by about 25.6% during the first step development of the main phase and increased by approximately 26.2% during the second step of main phase development. A long duration (>24 h) increase in VTEC of about 47.3% was found during the main phase of the August storm. VTEC changes during the main phase of both storms are due to the storm-time auroral prompt penetrating electric field, while the disturbance dynamo electric field produced negative ionospheric effects during their recovery phases as indicated by changes in equatorial electrojet values obtained from SWARM A, B, and C satellites and AE index, respectively. The global O/N-2 gas data from the Global Ultraviolet Imager (GUVI) onboard TIMED spacecraft show that O/N-2 changes, to a lesser extent, were also responsible for the geomagnetic storm effects observed at our station.