Observations of the Day to Night transition of Equatorial Ionization Anomaly

The Equatorial Fountain creates global scale ionospheric density enhancements on both sides of the magnetic equator in the Earth's sub-tropical regions which is the well known Equatorial Ionization Anomaly or EIA. It is understood that this phenomena result from thermospheric winds interacting with ionospheric plasma. We examine aspects of this process by comparing the equatorial upward plasma drifts of the fountain to the observed plasma distribution in the Equatorial Anomaly. We focus on the development of the density structures from the day period into the nighttime with particular focus on the pre-reversal enhancement of the vertical plasma drifts on the EIA. We examine if this relatively brief enhancement in vertical drift velocity can be traced to significant plasma transport into the night time Equatorial Anomaly and examine the important seasonal and longitudinal differences. The density observations of the Equatorial Anomaly are deduced from the GUVI instrument on NASA's TIMED spacecraft. This instrument records radiance data at 135.6nm that results from recombination of Oxygen ions and can be used as a proxy for plasma density. GUVI's high altitude limb scans are used to study both the dayside and night side Equatorial Anomaly as a function of local time, season and longitude. Plasma drifts from ROCSAT-1 observations as presented by Fejer etal, JGR 2008 are used for the vertical drifts. Further comparisons of the Equatorial Anomaly density structure as a function of season are made with the NRL Horizontal Neutral Wind model to discuss the driving sources from the EIA. This paper presents the data analysis techniques to produce observations of the EIA at all local times from GUVI as well as the comparisons of GUVI data with plasma drifts and neutral wind models.