Suppression of equatorial spread F by sporadic E

Abstract

[1] We have examined quantitatively the influence a low-latitude, premidnight sporadic E layer might have on the daily and hourly development of equatorial spread F (ESF). In particular, we calculated changes in the flux tube -integrated Pedersen conductivity as it affects the growth rate of the Rayleigh-Taylor instability, which governs the initial development of ESF. We find that the growth rate is lowered by an order of magnitude with a density of 1 Â 10 6 cm À3 in a slab from 115 to 120 km. Since sporadic E layers observed after dusk do not regularly reach these values, they are not a likely source of the daily variability in ESF. However, even a mild enhancement in the postsunset E region could lead to a significant suppression of ESF if it also inhibits the upward plasma drift of the prereversal enhancement, a key variable in the growth rate of the equatorial spread F instability. Thus, consistent with the nature of an instability, the second-order effect (suppressed upward drift) is more important than the first-order cause (reduced F region to E region conductivity) of inhibited ESF onset.

On the nature of low‐latitude Es influencing the genesis of equatorial plasma bubble

“…We also found that the layer height and the shape of the electron density profile could considerably alter the integrated Pedersen conductivity and hence the RT instability growth efficiency. In an earlier study, Stephan et al [2002] examined the flux tube integrated conductivity ratio (same as the growth efficiency here) for different electron density, ion mass, altitude and thickness of E s layer and came to quite similar conclusions, but the E s parameters used by them and those used here are somewhat different. Because they used E s layer peak electron density of 1Â10 5 /10 6 electrons/cm 3 , which is 10/100 times larger, layer height at a higher altitude and smaller layer thickness, than those used in the present study, the growth efficiency in their study were 10-20% lower than those estimated here.…”

“…The sporadic E layer was assumed to be composed mainly of metallic ions and the Pedersen conductivity was calculated by considering the mass of the Fe + ions. Ionic mass, however, has only a marginal role to play [Stephan et al, 2002], but the E region peak electron density can play a significant role in influencing the growth efficiency. F region field line integrated Pedersen conductivity has been calculated considering that the equatorial and low-latitude F region is represented by F region plasma density profile shown in Figures 7a-7c.…”

“…However, the F layer altitude at the magnetic equator decides the quantum of restriction imposed by the low-latitude E region conductivity on the growth of EPB [Bhattacharyya and Burke, 2000]. Simulation studies revealed that the postsunset sporadic E layers are capable of influencing the growth of EPB and its evolution in the postsunset period [Stephan et al, 2002].…”

“…Also, the sporadic E layer observations were made from latitude in the close vicinity of the magnetic equator [Stephan et al, 2002], which could be drastically different from the sporadic E at conjugate low latitudes due to difference in the mechanism of their formation. In this paper, we present the relationship of the low-latitude sporadic E layers and the genesis of the EPB with due importance given to the observational data.…”

The effects of the pre-reversal ExB drift, the EIA asymmetry, and magnetic activity on the equatorial spread F during solar maximum

“…The ESF studies using ionosonde data (Sales et al, 1996;Stephan et al, 2002;Whalen, 2002), the Global Positioning System (GPS) (Aarons et al, 1996;Mendillo et al, 2000Mendillo et al, , 2001, radars (Woodman and LaHoz, 1976;Hysell and Burcham, 1998), satellite (Huang et al, 2001;Su et al, 2001), and numerical modeling (Maruyama, 1988;Sultan, 1996) have demonstrated the general morphology of the ESF phenomenon. Now, it is known that there are some candidate mechanisms helping the ESF development.…”