Radar Studies of Atypical Equatorial Spread F Events in the American Sector

Equatorial spread F (ESF) is the name given, for historical reasons, to the manifestation of a broad range of ionospheric F-region irregularities observed at low latitudes. Since the report of ESF by Booker and Wells (1938), a large number of efforts have been dedicated to studies of ESF events. These efforts led to a significant improvement of our understanding of typical, post-sunset ESF. Recently, however, renewed attention has been devoted to atypical ESF events. That is, to events that occurred in the post-midnight sector and/or June solstice conditions. The efforts of this dissertation contribute to a better understanding of atypical ESF events with focus on the American sector. We evaluated the hypothesis of weakening vertical plasma drifts creating favorable conditions for the generation of postmidnight ESF. We used numerical simulations aided by incoherent scatter radar measurements of a prototypical event observed at the Jicamarca Radio Observatory (JRO) to test this hypothesis during June solstice, low solar flux conditions in the American sector (∼75◦W). We found that the weakening drifts did produce unstable conditions with positive growth rates. The growth rates, however, were much slower than those obtained for typical, premidnight ESF events and those obtained for similar drift conditions in other longitude sectors. Further numerical analyses showed that the action of converging neutral winds, in combination with weakening drifts, could have produced favorable conditions for instability development. Next, we used long-term coherent backscatter radar (Jicamarca Unattended Long-term Investigations of the ionosphere and Atmosphere, JULIA) measurements made at the JRO to better understand the morphology and origin of the postmidnight ESF irregularities in the American sector. The study is motivated by high occurrence rates of postmidnight ESF observed by the C/NOFS satellite during the 2008/2009 solar minimum. Collocated digisonde observations provide additional insight on F-region conditions leading ...