Abstract
Abstract. The electrodynamics of the pre-sunrise equatorial F-region is investigated using HF Doppler radar and digital ionosonde. The observations are limited to those days for which the radar probing frequency is below the foF2 value. The ionosphere observation using HF Doppler radar exhibit interesting features during pre-sunrise period similar to the post sunset pre-reversal enhancement. The most striking feature observed during pre-sunrise period is the sudden downward excursion in the vertical drift around local sunrise followed by an upward turning. Pre-sunrise observations of vertical plasma drift and the sunrise downward excursion followed by an upward turning after the ground sunrise related to the zonal electric field at the equatorial F-region are the most significant results not reported earlier.
Highlights
Vertical plasma drift at the equatorial F-region during postsunset and pre-sunrise period is primarily driven by the zonal electric field formed as part of the polarization field developed due to thermospheric wind and the sharp conductivity gradients in the E-region
The enhancement in vertical drift during the post sunset period is explained in terms of the polarization fields produced by the large westward gradient of the ionospheric conductivity across the evening terminator
The apparent drift due to production is calculated for different altitudes around the region of the observation and it is found that the correction due to the production is less than 10 ms−1 implying that the large downward drift observed near sunrise is due to the E×B drift
Summary
Vertical plasma drift at the equatorial F-region during postsunset and pre-sunrise period is primarily driven by the zonal electric field formed as part of the polarization field developed due to thermospheric wind and the sharp conductivity gradients in the E-region. The enhancement in vertical drift during the post sunset period is explained in terms of the polarization fields produced by the large westward gradient of the ionospheric conductivity across the evening terminator. The F-region electric field builds first for a short duration and the E-region conductivity starts increasing resulting in the vertical velocity pattern due to the added effect of E- and F-regions. This type of horizontal gradient in the ionospheric conductivity should exist near the morning sunrise terminator leading to similar anomalous feature in vertical drift.
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