Abstract
Abstract The behaviour of electron density gradient scale length, L, around post-sunset hours during the magnetically disturbed days of the summer, winter and equinox seasons of solar maximum (2002) and minimum years (1995) has been studied, using ionosonde data of Trivandrum (8.5°N, 76.5°E, dip = 0.5°N) in the Indian longitude sector. The results indicate a clear seasonal and solar cycle variation in L. Seasonal variations of the maximum vertical drift of the F layer were also examined on these days. In particular, the seasonal variation of the Equatorial Spread F (ESF) during this period is examined in terms of the relative roles of L and the vertical drift of the F layer in the triggering of the collisional Rayleigh-Taylor instability. Our results on the clear-cut seasonal and solar cycle variation in L for disturbed days and its control of ESF occurrence are presented and discussed.
Highlights
Equatorial Spread F (ESF) is a spectacular phenomenon in which the equatorial region ionosphere is transformed after sunset
In the study reported here, we have examined the seasonal variations of electron density scale length, L, and the vertical drift of the F layer on magnetically disturbed days of solar minimum (1995–1996) and maximum (2002–2003) years
The seasonal mean vertical drift velocity is at a maximum in the equinox for both solar epochs, while it is at a minimum in the winter of the solar minimum and in the summer of the solar maximum
Summary
Equatorial Spread F (ESF) is a spectacular phenomenon in which the equatorial region ionosphere is transformed after sunset. Extensive experimental and theoretical research has been carried out on ESF during the last four decades to study equatorial Spread F, with the results showing that plasma instabilities play a major role in the generation of these irregularities. The primary process responsible for the generation of the irregularities is the Collisional Raleigh-Taylor (CRT) instability mechanism, which operates in the postsunset bottom side F region (Haerendel, 1973; Costa and Kelley, 1978) under certain favourable conditions, including the movement of the F-layer to very high altitudes in the post-sunset hours and/or formation of the steep bottom side F-layer electron density gradient.
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