Seasonal variation of equatorial spread F in the American and Indian zones

Abstract

The occurrence of postsunset range type of spread F at Huancayo (in the American zone) shows a very consistent and strong seasonal variation with maximum around December and minimum around June solstices during any of the years of solar cycle epoch. The occurrence of range spread F at Kodaikanal (in the Indian zone) does not indicate any significant seasonal variation. It is suggested that the range type of equatorial spread F is generated by the action of eastward electric field in the ionospheric region with large plasma density gradient present after sunset, provided enough time is available for the development of irregularities before the electric field reverses to the nighttime westward direction. In the American zone the reversal of the electric field during June solstice occurs in general at about the same time as the sunset at the F region heights, and the spread F irregularities do not get enough time to develop, while during December solstice the electric field reverses more than an hour after the layer sunset, and enough time is provided to the irregularities to develop and produce spread F echoes in the ionograms. In the Indian zone the time of the reversal of electric field varies very little with season. The occurrence of spread F depends on the day to day variation of the time interval between sunset and the reversal of the electric field. The equinoctial maxima in the occurrence of spread F is due to the corresponding maxima in the magnitude of the postsunset peak in the electric field.