Variability of VTEC Gradient and TEC Rate Index Over Kisumu, Kenya During Selected Quiet and Storm Days of 2013 and 2014

Abstract

Vertical Total electron content (VTEC) gradient or rate of change of TEC (ROT) and total electron content (TEC) rate index or rate of change of TEC index (ROTI) can directly be estimated from receiver independent exchange (RINEX) data and can be used to estimate presence and intensity of ionospheric irregularities. In this paper we present results of the variation of ROT and ROTI over Kisumu, Kenya (Geomagnetic coordinates: 9.64°S, 108.59°E; Geographic coordinates: 0.02°S, 34.6°E) for both selected quiet and disturbed conditions between 1^st January 2013 and 31^st December 2014 using data derived from NovAtel GSV4004B SCINDA-GPS receiver at Kisumu as a step in establishing ionospheric irregularities over Kisumu, Kenya during a high solar activity period of solar cycle 24. ROT was calculated from filtered average daily VTEC data within intervals of 120 seconds while ROTI was computed over 4 minutes window from ROT. ROT and corresponding ROTI for the selected quiet and storm days were plotted against universal time (UT). The presence and intensity of irregularities on the selected quiet and storm days of the years 2013 and 2014 were determined by checking ROT fluctuations and the corresponding ROTI values from the plots. The obtained results showed increased fluctuation of ROT corresponding with large ROTI values after sunset. The selected storm days of the years 2013 and 2014 had higher ROTI values than the selected quiet days of the years 2013 and 2014. The higher ROTI values during the selected storm days may be attributed to the impact of geomagnetic storms brought about by several competing dynamics including Prompt penetration electric field (PPEF), disturbance dynamo electric field (DDEF) and reduction in electron density due to increased recombination rates. The high ROTI values after sunset were a manifestation of post-sunset plasma irregularities. The obtained results also showed a diurnal trend for ROT and ROTI where ROT increased steadily from 0.00UT to around 12:00UT before falling and having an increased fluctuation between 18:00UT and 20:00UT. This corresponded well with low ROTI values between 0:00UT and 16:00UT followed by high ROTI values between 18:00UT and 20:00UT. The diurnal variability of ROT might have resulted from variation of TEC and which was attributed to changes in the intensity of incoming solar radiation.