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 to these ESF events. 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 satellite observations during December solstice is explained by the slow vertical development of the ESF structures, which only reach the topside near midnight in most cases. In June solstice, however, JULIA observations show that ESF started predominately in the midnight to postmidnight sector. Finally, we also maintained, processed and analyzed measurements made by a 30 MHz coherent backscatter radar system located in the low-latitude site of São Luis, Brazil in the eastern American sector (∼45◦W). This radar system is referred to as Measurements of Equatorial and Low-latitude Ionospheric irregularities over São Luís, South America (MELISSA) and made observations between March 2014 and December 2018. We analyzed pre- and postmidnight F-region echoes with focus on the spectral features of postmidnight echoes and how they compare to spectra of echoes observed in the post-sunset sector. The radar observations indicate that post-midnight F-region irregularities were generated locally, and were not a result of “fossil” structures generated earlier in time (in other longitude sectors) and that drifted into the radar field of view. This also includes cases where the echoes are weak and that would be associated with decaying ESF structures. Collocated digisonde observations show modest but noticeable F-region apparent uplifts prior to post-midnight ESF events. We associate the equatorial uplifts with disturbed dynamo effects and with destabilizing F-region conditions leading to ESF development.