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

Abstract

Abstract. The occurrence of blanketing type E s (E sb ) layers and associated E-region irregularities over the magnetic equatorial location of Trivandrum (8.5 • N; 77 • E; dip ∼0.5 • ) during the summer solstitial months of May, June, July and August has been investigated in detail for the period 1986-2000 to bring out the variabilities in their characteristics with the solar cycle changes. The study has been made using the ionosonde and magnetometer data of Trivandrum from 1986-2000 along with the available data from the 54.95 MHz VHF backscatter radar at Trivandrum for the period 1995-2000. The appearance of blanketing E s layers during these months is observed to be mostly in association with the occurrence of afternoon Counter Electrojet (CEJ) events. The physical process leading to the occurrence of a CEJ event is mainly controlled by the nature of the prevailing electro dynamical/neutral dynamical conditions before the event.Hence it is natural that the E sb layer characteristics like the frequency of occurrence, onset time, intensity, nature of gradients in its top and bottom sides etc are also affected by the nature of the background electro dynamical /neutral dynamical processes which in turn are strongly controlled by the solar activity changes. The occurrence of E sb layers during the solstitial months is found to show very strong solar activity dependence with the occurrence frequency being very large during the solar minimum years and very low during solar maximum years. The intensity of the VHF radar backscattered signals from the E sb irregularities is observed to be controlled by the relative roles of the direction and magnitude of the prevailing vertical polarization electric field and the vertical electron density gradient of the prevailing E sb layer depending on the phase of the solar cycle. The gradient of the E sb layer shows a more dominant role in the generation of gradient instabilities during solar minimum periods while it is the electric field that has a more dominant role during solar maximum periods.

Occurrence of blanketing E<sub>s</sub> layer (E<sub>sb</sub>) over the equatorial region during the peculiar minimum of solar cycle 24

“…It is observed that the occurrence of E sb is maximum during the summer solstice of both 2007 and 2009. This is in general agreement with the earlier studies on E sb (Bhargava and Subrahmanyan, 1964;Chandra and Rastogi, 1975;Devasia, 1976;Devasia et al, 2006). 4.…”

“…For the winter solstice (January-February, November-December) of 2009, the peak E sb occurrence is seen at around 14:00 IST. Devasia et al (2006) have shown that the E sb occurrence in the equatorial region is higher during low solar activity than that during high solar activity. Also, at Indian equatorial stations, the maximum E sb occurrence is predominantly reported during the summer solstice with a minor secondary peak during the winter solstice (Bhargava and Subrahmanyan, 1964;Devasia, 1976).…”

“…Reddy and Devasia (1973) proposed theories to explain the formation of E sb at equatorial latitudes, which are based on the horizontal convergence of ions. The most widely observed features of E sb are its seasonal occurrence, which indicates a peak during the local summer (Bhargava and Subrahmanyan, 1964;Oyinloye, 1969;Chandra and Rastogi, 1975;Devasia, 1976;Devasia et al, 2006), and its anti-correlation with equatorial electrojet (EEJ) intensity (Tsunoda, 2008). The theories proposed for the formation of E sb do not explain its seasonal dependence and the role of EEJ.…”

“…Nevertheless, there is a report of an equinoctial peak E sb occurrence at American and African sectors during a high solar activity period (Oyinloye, 1971). The past investigations of E sb at low-latitude stations provide two important observational aspects of E sb occurrence: (i) the occurrence decreases as we move from low latitude to equatorial stations where EEJ current is stronger (Oyinloye, 1971;Abdu et al, 1996) and (ii) E sb at equatorial regions is mainly seen during periods of CEEJ or weak EEJ (Bhargava and Subrahmanyan, 1964;Chandra and Rastogi, 1975;Sen Gupta and Krishna Murthy, 1975;Devasia et al, 2006 winter (January-February) and vernal equinox of 2009 as compared to 2007 is an interesting observation. In order to understand this discrepancy, we examine the dependence of E sb occurrence on EEJ and CEEJ.…”

“…The present work is motivated by two observational aspects of equatorial E sb reported earlier: (i) the occurrence of E sb is higher during low solar activity periods (Devasia, 1976;Devasia et al, 2006) and (ii) the occurrence of E sb is likely on a counter equatorial electrojet (CEEJ) day (Bhargava and Subrahmanyan, 1964;Chandra and Rastogi, 1975;Sen Gupta and Krishna Murthy, 1975;Devasia et al, 2006).…”