A Study of Post-Sunset Spread-F Initiation During the 2013 EVEX Campaign

Equatorial Vortex Experiment (EVEX), the EVEX mission of May 2013, carried out from Roi-Namur, Kwajalein Atoll, explored the initiation physics of equatorial spread-F with a pair of rockets launched simultaneously during local sunset to apogees of 182 and 331 km, respectively. EVEX measurements presented in this paper establish the existence of a differential zonal flow between plasma and neutrals at bottom-side F-region heights during the sunset time in support of a post-sunset spread-F initiation mechanism described in Kudeki et al. (2007) that is characterized by large growth rates for oblique-propagating 10's-of-km-scale electron density perturbations. F-region plasma drifts were measured in situ using an electric field double-probe instrument while neutral winds were measured by triangulating drifting vapor clouds. The state of the ionosphere and spread-F irregularities were monitored by ground based ARPA Long-Range Tracking and Instrumentation Radar and Illinois Radar Interferometer System radars using incoherent and coherent scatter techniques, respectively, as well as an ionosonde.