Abstract
In the present study, slant total electron content (STEC) data computed from ground based GPS measurements over Hyderabad (Geog. Lat. 17.41° N, geog. long. 78.55° E, mag. lat. 08.81° N) and two close stations at Bangalore (Geog. Lat. 13.02°/13.03° N, geog. long. 77.57°/77.51° E, mag. lat. 04.53°/04.55° N) in Indian region during 2007–2012, have been used to study the occurrences and characteristics of equatorial plasma bubbles (EPBs). The analysis found maximum EPB occurrences during the equinoctial months and minimum during the December solstice throughout 2007–2012 except during the solar minimum years in 2007–2009. During 2007–2009, the maximum EPB occurrences were observed in June solstice which could not be predicted by the model proposed by Tsunoda (J. Geophys. Res., 90:447–456, 1985). The equinox maximum in EPB occurrences for high solar activity years could be caused by the vertical F-layer drift due to pre-reversal electric field (PRE), and expected to be maximum when day-night terminator aligns with the magnetic meridian i.e. during the equinox months whereas maximum occurrences during the solstice months of solar minimum could be caused by the seed perturbation in plasma density induced by gravity waves from tropospheric origins. Generally EPB occurrences are found to be more prominent during nighttime hours (2000–2400 hours) than the daytime hours. Peak in EPB occurrences is in early night for high solar activity years whereas same is late night for low solar activity. The day and nighttime EPB occurrences have been analyzed and found to vary in accordance with solar activity with an annual correlation coefficient ( $R$ ) of ∼0.99 with $\mathrm{F}_{10.7}~\mbox{cm}$ solar Flux. Additionally, solar activity influence on EPB occurrences is seasonal dependent with a maximum influence during the equinox season ( $R=0.88$ ) and a minimum during winter season ( $R =0.73$ ). The solar activity influences on EPB occurrences are found in agreement with the previous works reported in the Brazilian, African-Asian and Pacific longitudes sector but different than that in Atlantic sector.
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