On the role of vertical electron density gradients in the generation of type II irregularities associated with blanketing E_S (E_Sb) during counter equatorial electrojet events: A case study

Abstract

[1] The characteristics of different types of Sporadic E (E S ) layers and the associated plasma density irregularities over the magnetic equator have been studied in a campaign mode using VHF backscatter radar, digital ionosonde, and ground magnetometer data from Trivandrum (dip latitude 0.5°N, geographic latitude 8.5°N, geographic longitude 77°E), India. The presence of blanketing type E S (E Sb ) in the ionograms with varying intensity and duration were observed in association with afternoon Counter Equatorial Electrojet (CEEJ) events. E Sb was associated with intense backscatter returns and with either very low zonal electric field and/or with distortions present in the altitude profile of the drift velocity of the type II irregularities. The results of the coordinated study indicate the possible role of vertical electron density gradients in E Sb layers in addition to providing evidence for the local winds to be responsible for the vertical gradients themselves.

Solar cycle dependent characteristics of the equatorial blanketing E<sub><i>s</i></sub> layers and associated irregularities

“…Whereas during solstitial months the appearance of a CEJ event and associated blanketing E s layer during the weakened EEJ condition (or CEJ) in the afternoon hours provide the necessary condition of electron density gradients in the right direction so as to generate gradient drift instabilities. Radar observations of many such events of blanketing type E s layer occurrences during the solstitial months have shown that such layer type E sb irregularities in the EEJ region give rise to strong backscattering of 54.95 MHz radar signals (Reddy and Devasia, 1977;Devasia et al, 2004). Remarkable changes in the amplitude and Doppler frequency spectra of the radar backscattered signals were observed coinciding with the appearance of blanketing E s layers in the ionograms simultaneously obtained from the colocated ionosonde at Trivandrum.…”

“…It may be mentioned that larger values of f b E s at the onset and growth phases of an E sb layer are indicative of the presence of very sharp electron density gradients of the E sb layer (Reddy and Devasia, 1977), which can give a large growth rate to the plasma instabilities even in the presence of a very weak electric field. The variations of f b E s with time as well as the height locations of the E sb layer in the EEJ region have important roles in the growth and sustenance of gradient instabilities during Counter Electrojet events when the east-west electric field is very weak (Devasia et al, 2004).…”

“…These are known as Blanketing E s (E sb ) layers. The above facts imply that, just as the daytime electrojet is a seat of both type I and type II irregularities associated with the E s , the CEJ on certain occasions can also be a seat of type I and type II irregularities associated with E sb (Crochet et al, 1979;Somayajulu et al, 1994;Woodman and Chau, 2002;Devasia et al, 2004).…”

“…However, the observation of type II irregularities during CEJ events using the 54.95 MHz VHF radar at Trivandrum has been relatively infrequent and such observations are in general associated with the appearance of very sharp ionization layers or E sb layers manifested by the Trivandrum ionograms (Reddy and Devasia, 1977;Somayajulu and Viswanathan, 1987;Reddy, 1989;Devasia et al, 2004).…”

Seasonal dependence of the "forecast parameter" based on the EIA characteristics for the prediction of Equatorial Spread F (ESF)

“…Instead, blanketing Es was seen to persist over Trivandrum even during 19:45 IST (not illustrated). The occurrence of blanketing Es is also very common during the June-July period in the low-latitude ionosphere over Indian longitudes, in the solar minimum epoch (Devasia, 1976;Devasia et al, 2004). It is known that for estimating the growth rate, the field-line integrated ionospheric conductivity should be taken into account (Kelly, 1989;Sultan, 1996).…”