Wave structure and polarization electric field development in the bottomside F layer leading to postsunset equatorial spread F

In this paper we present the results of a study on the characteristics of large-scale wave structure in the equatorial ionospheric F region that serve as precursor to postsunset development of the spread F/plasma bubble irregularities. The study is based on analysis of Digisonde data from three equatorial sites in Brazil (Fortaleza, Sao Luis, and Cachimbo) for a period of about 2months at a medium solar activity phase. Small-amplitude oscillations in the F layer heights, extracted at a number of plasma frequencies, present characteristics as them being generated from upward propagating gravity waves. They represent wave structures in polarization electric field having zonal scale of a few hundred kilometers. Their amplitudes in the afternoon hours undergo amplification toward evening, leading to postsunset development of equatorial spread F/plasma bubble irregularities, on a statistical basis. On the days of their larger amplitudes they appear to occur in phase coherence on all days, and correspondingly, the evening prereversal vertical drift velocities are larger than on days of the smaller amplitudes of the wave structure that appear at random phase on the different days. The sustenance of these precursor wave structures is supported by the relatively large ratio (approaching unity) of the F region-to-total field line-integrated Pedersen conductivities as calculated using the Sheffield University Plasmasphere-Ionosphere Model simulation of the low-latitude ionosphere. The significant amplification in the wave structure toward sunset and the phase coherent nature of their occurrences on different days are explained tentatively on the basis of the spatial resonance mechanism.