On the Genesis of Postmidnight Equatorial Spread F: Results for the American/Peruvian Sector

Previous studies of the Earth's low-latitude ionosphere using in situ measurements made by sensors on the Communication/Navigation Outage Forecasting System (C/NOFS) satellite showed an unexpected predominance of equatorial spread F (ESF) events in the postmidnight sector during June and December solstice months of the 2008-2009 deep solar minimum. It has been suggested that these events might have been driven by the unusual behavior of the equatorial plasma drifts, which showed an abnormal upward peak around midnight during the same period. We use coherent backscatter radar (Jicamarca Unattended Long-term Investigations of the Ionosphere and Atmosphere - JULIA) measurements made at the Jicamarca Radio Observatory (11.95 degrees S, 76.87 degrees W, similar to 1 degrees dip lat) in Peru to better understand the origin of the ESF irregularities observed by C/NOFS. The radar observations show that ESF events during December solstice start in the postsunset sector. These ESF events and the conditions for their development are shown to continue through midnight hours. The predominance of postmidnight irregularities on C/NOFS observations during December solstice is caused by the slow vertical development of the ESF structures, which only reach the topside near midnight in most cases. On June solstice, however, JULIA observations show that ESF started predominately in the midnight to postmidnight sector. Collocated digisonde observations provide additional insight on F-region conditions leading to these ESF events. Plain Language Summary Equatorial spread F (ESF) is the name given to a broad spectrum of irregularities in the low-latitude ionospheric electron density. ESF irregularities affect the propagation of radio signals used in various applications (remote sensing, navigation, communications, etc.). In this study, we analyzed radar observations of the evolution (in time and height) of ESF events made at the Jicamarca Radio Observatory, in Peru. The analysis was performed to better understand the morphology and the origin (in LT) of postmidnight ESF observed by satellites during the extreme solar minimum of 2008/2009. Our results show that, for the Peruvian sector, June solstice postmidnight ESF events started predominately in the postmidnight sector. December solstice ESF events and conditions for their evolution, however, started in the postsunset sector and continued to exist until postmidnight hours.