Diurnal and seasonal variability of total electron content observed at Kampala in the Equatorial Region

Abstract

Total electron content (TEC) is a vital and most dominant ionospheric parameter that can cause GPS signal delays, signal degradation and in extreme cases loss of lock. This results into inefficient operations of ground and space based Global Navigation Satellite System (GNSS) applications. Therefore the study of TEC variability is useful for GNSS users for space weather predictions in order to minimize range-rate errors for users when high level accuracy in measurements is required. This research comprehensively explored the temporal, diurnal (local time), seasonal and solar activity dependence of TEC and S4 index using data obtained from the SCIntillation Network Decision Aid Global Positioning System (SCINDA GPS) station in Kampala to understand the TEC variation trend over this station. Local season dependence of TEC was also studied. Data for the two years (2010 and 2011), recorded using the SCINDA GPS NovAtel GSV 4004B model equipment located at geographic latitude 0.3o N, longitude 32.6o E and dip -20.6o at Makerere University were used for this study. The GPS-TEC analysis programme, developed at Boston College (Seemala, 2011) and a locally written and automated FORTRAN codes were employed to process the carrier phase and pseudo-range TEC to absolute TEC and S4 index was computed at a sampling rate of 60s. Results obtained suggest that the diurnal TEC is lowest at 3 hours UT (6.00hrs LT) registering a steady increase to peak from 10 hours UT to about 15 hours UT with the maximum value for the year 2011 being about 65 TECU and about 40 TECU for the year 2010. Peak TEC lasts for about 5 hours and it gradually reduces after sunset. Scintillations a consequence of TEC fluctuations were observed from about 18 hours UT (21.00hrs LT) to midnight with the S4 index above 0.4. The equinox months recorded higher occurrence than the other seasons. Scintillation thus remains a night time event as previously reported. TEC depletions and enhancement were observed to commence at about 17 hours UT and depletion depth of about 5-10 TECU caused high S4 index of > 0.4. The occurrence of such large scale equatorial depletions is due to the post-sunset plasma instabilities via the Rayleigh–Taylor instability near the magnetic equator. However, TEC enhancement did not result into high S4 index. There is also a fair correlation between TEC variability with tropospheric dry and rainy periods especially in the months of March, June and October. This however requires further investigation.