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

Abstract

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 2 months 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.

Medium‐Scale Traveling Ionospheric Disturbances Observed by Detrended Total Electron Content Maps Over Brazil

“…Abdu et al () analyzed the digisonde data from October to November 2001, at three sites in the equatorial region of Brazil, during the period of plasma bubbles. They found that the F ‐layer oscillations between 12:00 LT and dusk period may be the precursor of plasma bubbles.…”

Shrinking equatorial plasma bubbles

“…Equatorial plasma bubbles (EPBs) are large‐scale plasma depletions that originate over the dip equator in the nighttime F region ionosphere due to the generalized Rayleigh‐Taylor instability process. The instability is triggered by perturbations in the bottomside F region [ Kelley et al , ; McClure et al , ; Sekar et al , ; Abdu et al , , ; Narayanan et al , ] when the background ionosphere in the post sunset hours is favorable for growth of the instability [ Abdu et al , ; Jayachandran et al , ; Fejer et al , ]. They often grow to the topside ionospheric altitudes within few tens of minutes [ Sultan , ; Sidorova and Filippov , ; Patra et al , ].…”

Electrodynamics of ionospheric weather over low latitudes

“…The F layer vertical drifts averaged for the two groups of days, the red and blue curves corresponding to the first group and second group, respectively, (bottom) the smaller amplitude (random) oscillations. Based on the well-known role of the E layer tidal winds in shaping the longitudinal/local time gradient in the E layer conductivity at sunset (Abdu et al 2003), the above results suggested possible tidal modes-gravity wave coupling process impacting on the PRE-ESF relationship (for further details, see Abdu et al 2015b). This appears to be an important connecting link in determining the precursor conditions for the plasma bubble/ESF irregularity development.…”

“…8 wherein we note height oscillations of significant amplitude during the daytime, getting amplified toward evening hours and further increasing in amplitude as a result of the post-sunset spread F/bubble irregularities that they initiated, as indicated by the gray segment of each curve (with some exceptions that is determined by the PRE vertical drift amplitude, not shown here). It was shown by Abdu et al 2015b that these height oscillations were driven by polarization electric field induced by upward propagating gravity waves, which therefore represented the precursor seed required for ESF instability growth. An intriguing aspect of the results in Fig.…”

“…These oscillations were present several hours prior to sunset (that is, in the afternoon hours), and their continuation into the post-sunset hours contributed to R-T instability growth resulting in plasma bubble irregularity development. From their estimated zonal scale sizes (of a few to several hundreds of kms), these oscillations were identified as signatures of the LSWS (Abdu et al , 2015b. Takahashi et al 2010 observed bottom-side sinusoidal intensity depletions in the all-sky images over Cariri and Brasilia, with simultaneous occurrence of bottom-side irregularities as seen by a VHF radar over Sao Luis and bottom-side spread F and height oscillations as seen by a Digisonde over Fortaleza, that were subsequently followed by well-developed plasma bubbles.…”