Observations of the Post-Sunrise Generation of Eastward Electric Fields in the Low Latitude Ionosphere and Their Possible Association with Deep Plasma Density Depletions Near Dawn

Deep depletions in the ionospheric plasma density near the equator have been observed just before sunrise using instruments on the Communications/Outage Forecast System (C/NOFS) satellite. Here we study six consecutive orbits using the planar plasma density probe and the VEFI electric field sensor. These data were obtained on June 17, 2008 during a period of extreme solar minimum. One of these depletions, on orbit 915, was accompanied by a very large eastward electric field in phase with the density depletion. The origin of this large eastward field, at a time when that component of the field is usually small and westward, is still unclear. We find on orbits 917, 918, and 919 that a distinctly different, second type of relationship between the electric field and plasma density near dawn occurred. Eastward electric field enhancements were also detected, one of them nearly 3 mV/m, but they were observed east of the pre-dawn density depletions, which were very modest in size and depth, rather than being collocated with the depletions themselves. The other orbits showed no post-sunrise enhancement. Theoretical implications will be discussed.