Oblique Incidence Ionospheric Absorption Near the Magnetic Equator

Shamsi, S. (1984-07)

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Measurement of equatorial ionospheric absorption made by the A3 Method for quiet conditions near a period of maximum solar activity (1979 to 1981) are used for a detailed examination of Absorption of Radio Waves in equatorial ionosphere. Assuming an index law of the form L = L0COSmX for the absorption (X being zenith angle of the sun), the diurnal and seasonal characteristics of the variation are obtained for two frequencies (4.9 MHz and 7.1 MHz) and compared with existing theories. Problems posed for an index law for seasonal variation of absorption by the experimental evidence as indicated. Effect of solar activity on absorption of radio Waves is studied and an improved form of the variation of absorption with solar activity is presented. The new form of variation of absorption with solar activity also explains the seasonal variation of absorption. Examples of solar flare effects on the ionospheric absorption are presented for the two frequencies. A study has been made for the correlation between the occurrence of solar flares as reported in Solar Geophysical Data (Noaa Boulder Colorado) and the observed ionospheric effect. Special features observed include a pronounced absorption near layer sunrise, and an apparent enhancement of signal strength around sunset. Arguments are advanced for possible physical mechanism governing these phenomena.