Characterization of equatorial plasma bubbles over the East African Sector using the ROTI and sTEC depletion depth

This study investigates the prevalence of ionospheric irregularities and plasma bubbles over East Africa throughout 2015 using Global Navigation Satellite System (GNSS) data. Parameters including slant total electron content (sTEC), rate of change of total electron content index (ROTI), Kp, and Dst indices were employed to identify periods of relative quiet, defined as Dst$>-30$ nT and Kp $<3$. Equatorial plasma bubbles (EPBs) were detected at two stations: Addis Ababa (ADIS) and Roberts Camp Naivasha (RCMN). In geomagnetic coordinates, ADIS is at $5.35°$N, $112.51°$E, and RCMN at $4.42°$S, $109.06°$E. EPBs were identified by analyzing the detrended curves and sTEC depletion depths. Detecting detrended curves of sTEC and analyzing sTEC depletion depths holds significant importance in the investigation of ionospheric disturbances caused by EPBs, particularly when utilizing ground-based data from the GNSS in equatorial and low latitude regions. Notably, EPBs show an increased occurrence during the post-sunset hours 17:00 to 22:00 UT (20:00 to 1:00 LT), displaying distinct daily, monthly, and seasonal patterns. EPBs are most active during the March and September equinoxes, exhibiting higher occurrence rates compared to the solstices. This enhanced activity is primarily due to the alignment of the solar terminator with the geomagnetic meridian during equinoxes, which creates optimal conditions for EPB formation. Our observations indicate that EPB activity is more pronounced at RCMN than at ADIS. This is because RCMN is situated near the Equatorial Ionization Anomaly (EIA) crest, where plasma density is significantly higher due to the fountain effect. While EPBs originate at the magnetic equator via the Rayleigh–Taylor instability (RTI), their intensity and impact are often greater near the EIA region. Ultimately, our findings confirm that EPBs are stronger at RCMN due to a combination of factors, including enhanced instability growth, poleward expansion of magnetic field lines, steep plasma density gradients, post-sunset electrodynamics, and regional ionospheric variations.