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

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.

[1]  H. Booker,et al.  Scattering of radio waves by the F-region of the ionosphere , 1938 .

[2]  B. W. Osborne.  Ionospheric behaviour in the F2 region at Singapore , 1951 .

[3]  J. W. Dungey,et al.  Convective diffusion in the equatorial F region , 1956 .

[4]  D. Martyn.  The Normal F Region of the Ionosphere , 1959, Proceedings of the IRE.

[5]  D. F. Martyn.  Large‐scale movements of ionization in the ionosphere , 1959 .

[6]  A. Lyon,et al.  Equatorial spread-F at Ibadan, Nigeria , 1961 .

[7]  R. Cohen,et al.  On the nature of equatorial spread F , 1961 .

[8]  George C. Reid,et al.  The formation of small‐scale irregularities in the ionosphere , 1968 .

[9]  D. Cunnold.  Drift‐dissipative plasma instability and equatorial spread F , 1969 .

[10]  B. Balsley.  Equatorial electrojet - Seasonal variation of the reversal times , 1970 .

[11]  H. Chandra,et al.  Solar cycle and seasonal variation of spread-F near the magnetic equator , 1970 .

[12]  Ronald F. Woodman,et al.  Equatorial spread F: Implications of VHF radar observations , 1970 .

[13]  R. Rastogi,et al.  Daily Variation of E-Region Drifts at the Dip Equator during 1968–69 , 1971 .

[14]  B. Balsley.  Electric fields in the equatorial ionosphere: A review of techniques and measurements , 1973 .

[15]  J. Röttger.  Wave-like structures of large-scale equatorial spread-F irregularities , 1973 .

[16]  Chun-Ming Huang.  On the ionization transport in the low-latitude ionosphere , 1974 .

[17]  M. Hudson,et al.  Linear theory of equatorial spread F , 1975 .

[18]  Anthony J. Scannapieco,et al.  Nonlinear equatorial spread F , 1976 .

[19]  P. Kaw,et al.  An interpretation for the power spectrum of spread F irregularities , 1976 .

[20]  Ronald F. Woodman,et al.  Radar observations of F region equatorial irregularities , 1976 .

[21]  B. C. Edgar,et al.  Equion, an Equatorial Ionospheric Irregularity Experiment , 1977 .

[22]  R. G. Rastogi,et al.  Range and frequency spreadF at Huancayo , 1977 .

[23]  S. L. Ossakow,et al.  Morphological studies of rising equatorial Spread F bubbles. Interim report , 1977 .

[24]  R. Rastogi.  On the equatorial spreadF , 1978, Proceedings / Indian Academy of Sciences.

[25]  S. Zalesak,et al.  Nonlinear equatorial spread F: dependence on altitude of the F peak and bottomside background electron density gradient scale length. Interim report , 1978 .

[26]  R. K. Jain,et al.  Nonlinear Rayleigh-Taylor instability in partially ionized plasma and the equatorial spread-F , 1978 .

[27]  E. Ott.  Theory of Rayleigh-Taylor bubbles in the equatorial ionosphere , 1978 .