Long Term Ionospheric VTEC Variation During Solar cycle 24 as Observed from Indian IGS GPS Station

— We use the IGS (International Geodesic Survey) dual-frequency GPS data of Indian low latitude IGS Station IISC, Bangalore (13.02 ᴼN, 77.57 ᴼE), to compute the Total Electron Content (TEC) to study the solar activity variation of the ionosphere. To study the TEC variation with solar activity, we choose a complete solar cycle 24 during the period 2007-2020. We study the variation of TEC with sunspot numbers which is the quantitative measure of the solar activity. We observe the variation of TEC with the solar activity parameter such as solar flux and the EUV flux. The estimated TEC gradually increases from a minimum to maximum during 2007 to 2014 and then again decreases during 2015-2020 to a minimum which follows the sunspot numbers variation over the complete solar cycle. We use the EOF decomposition model using GPS-TEC data for the entire solar cycle. The diurnal, seasonal, annual TEC variation and its corresponding trend with solar activity are observed using the EOF-TEC method. The EOF-TEC data is highly correlated to GPS-TEC data with a value of correlation coefficient of 0.9323. The performance of the model is also good with the RMSE value of 5.7891 and the NRMSE value is 16%. We also use the IRI-2016 TEC to study the diurnal, annual variation of TEC with solar activity and verify our observed and model data. The IRI-TEC is comparatively low but the solar activity dependence of TEC matches with the GPS-TEC and EOF-TEC values satisfactorily. the diurnal TEC attains a maximum value at the afternoon 13:00-17:00 IST for this low latitude station which is observed for all TEC throughout the solar cycle. We observe that for seasonal variation, the value of GPS-TEC is maximum for the equinox. The equinoctial GPS-TEC is maximum followed by winter and summer. A similar kind of outcome is found in EOF-TEC. We show the positive correlation between TEC, sunspot, solar flux (F10.7 cm), and EUV flux for the entire solar cycle.