Abstract
Abstract A study has been carried out on the behaviour of electron density gradient scale length, L, vertical drift and layer height, around post sunset hours, during the magnetically quiet days of summer, winter and equinox seasons of solar maximum (2002) and minimum years (1995), 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 all the three parameters. Further, the seasonal variation of equatorial Spread F (ESF) during the above period is examined in terms of the relative roles of L, the vertical drift and layer height (of the F layer) in the triggering of the collisional Rayleigh-Taylor instability. The results, show for the first time, that L also plays an important role, in controlling the quiet time seasonal and solar cycle variability of ESF; whereas in earlier studies this parameter had been taken to be constant. The detailed results are presented and discussed.
Highlights
Equatorial Spread F (ESF) irregularities are plasma density irregularities of scale sizes ranging from centimetres to several hundred kilometres observed in the night time F region (Chandra and Rastogi, 1970; Farley et al, 1970; Woodman and LaHoz, 1976; Fejer and Kelly, 1980; Abdu et al, 1981; Kelley and McClure, 1981; Fejer et al, 1999; Hysell and Burcham, 2002)
This paper presents, the first time estimates of the seasonal variations of electron density scale length, L of the F layer for the magnetically quiet days of solar minimum (1995–1996) and maximum (2002–2003) years
The vertical drift increases by nearly 20% from winter to equinox and nearly 35% from summer to equinox, while in solar maximum it increases by ∼65% and by nearly 36% from winter to equinox
Summary
Equatorial Spread F (ESF) irregularities are plasma density irregularities of scale sizes ranging from centimetres to several hundred kilometres observed in the night time F region (Chandra and Rastogi, 1970; Farley et al, 1970; Woodman and LaHoz, 1976; Fejer and Kelly, 1980; Abdu et al, 1981; Kelley and McClure, 1981; Fejer et al, 1999; Hysell and Burcham, 2002). The primary process responsible for the generation of the irregularities is the Collisional RaleighTaylor (CRT) instability mechanism operating in the post sunset bottom side F region under certain favourable conditions (Haerendel, 1973). These conditions include the movement of the F-layer to very high altitudes in the postsunset hours, and/or formation of steep bottom side F-layer electron density gradient. The seasonal pattern of occurrence of ESF, during magnetically quiet period, had been examined (Subbarao and Krishna Murthy, 1994), but the implications of the seasonal variations of electron density gradient scale length L, had not been considered. The seasonal pattern of occurrence of ESF, during magnetically quiet period, had been examined (Subbarao and Krishna Murthy, 1994), but the implications of the seasonal variations of electron density gradient scale length L, had not been considered. Manju et al (2009) have examined the seasonal and solar activity dependence of L and its modulation of ESF
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