Abstract
Abstract. The occurrence of blanketing type Es (Esb) 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 Es 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 Esb 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 Esb 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 Esb 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 Esb layer depending on the phase of the solar cycle. The gradient of the Esb 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.
Highlights
Equatorial ionograms show the presence of layer type irregularities in the ionospheric E-region known as “Equatorial sporadic E” (Esq )
Its presence is strongly associated with the day-time “Equatorial Electrojet” (EEJ), which consists of an enhanced flow of current over the magnetic equator in the eastward direction driven basically by a large vertical polarization electric field (Ep) set up by the eastward electric field (Ey) which has its origin in the global wind dynamo electric fields (Reddy, 1989; Stening, 1995)
EdsepaetndTernivcaenwdriuthmthwe amsaxr3ei--232 small noticeable differences during the solar minimum epoch of 1994–1997. This is because of a natural consequence that mum number of days of occurrence confined to the solstitial the solstitial months of low solar activity periods have Esb months of May, June, July and August (Devasia, 1976) which occurrences even on days with Ap>15, whereas during high was essentially based on the ionograms of Trivandrum for the solar activity periods, the occurrence of Esb events are mostly solar quiet periods of 1969–1972.The present study indicates restricted to magnetically quiet days only
Summary
Equatorial ionograms show the presence of layer type irregularities in the ionospheric E-region known as “Equatorial sporadic E” (Esq ). The wind-induced polarization electric fields in the same direction as the vertical polarization electric field due to Ey can lead to the ionization convergence and the eventual formation of the Esb layers This establishes the close linkage between the generation of a CEJ event and the simultaneous appearance of an associated blanketing Es layer as resulting from the local interaction of east-west winds with large vertical shears with the electrojet plasma when the electrojet is of comparatively lower intensity in the afternoon hours. Such favorable conditions appear to exist in the magnetic equatorial location of Trivandrum during the summer solstitial months of May, June, July and August. Where λr =5.4 m, corresponding to the 54.95 MHz VHF radar at Trivandrum
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