Day-to-day variability of the equatorial ionization anomaly and scintillations at dusk observed by GUVI and modeling by SAMI3



[1] The day-to-day variability in ionospheric irregularity generation giving rise to equatorial scintillation has remained an unresolved issue over many decades. We take a fresh look at the problem utilizing the global imagery provided by the Global Ultraviolet Imager (GUVI) instrument on NASA's Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics satellite. GUVI has been acquiring images of 135.6-nm emission in the Earth's ionosphere-thermosphere system since 2001. These GUVI disk images at dusk have been used to identify cases where the equatorial ionization anomaly (EIA) crests lie near the magnetic equator over a relatively narrow longitude range, so that the anomaly looks collapsed. A 16-month period of GUVI data collected during evening at solar maximum is used to study the morphology of these so-called collapses, since the EIA collapse is shown to be linked to the suppression of equatorial plasma bubbles and scintillations. In particular, we look at the June solstice, during which the Atlantic and Pacific show very different climatology and EIA collapses are most frequent in the GUVI data. On the other hand, EIA collapses are a relatively rare occurrence during the equinox period when scintillations are most prevalent globally. We obtained a few dramatic examples of day-to-day variability in EIA behavior and scintillations over India. The Sami3 is Also a Model of the Ionosphere (SAMI3) model was used to investigate the conditions during the evening collapse of the anomaly in the Indian longitude sector, where measurements of total electron content (TEC) and scintillations and estimates of the daytime vertical drifts and those at dusk were available. Results from SAMI3 show that the observed collapse of the anomaly at dusk can be simulated by a reversal of the upward vertical drift in midafternoon in agreement with the drift estimates from magnetometer observations. Such reversed vertical drifts at this time of the day are generally seen during counterelectrojet events. Introduction of neutral winds into SAMI3 better approximates the dusk behavior of TEC at low-latitude stations in India. This study reveals that the longitudinally confined EIA collapse may explain some of the differences in day-to-day variability of scintillations at different locations around the globe.