An A3 Investigation of Multi-Frequency Absorption in the Equatorial Region
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The A3 method is an experimental technique by which radio wave absorption in the ionosphere is measured by recording the field strength of a distant HF trasmitter. For multifrequency A3 studies, radio transmissions on four different frequencies are considered adequate. The multifrequency A3 method employing five broadcast frequencies: 4.87MHz, 5.047MHz, 6.09MHz, 9.60MHz and 15.40Mhz has been used to investigate respectively, the variation of absorption with frequency, the seasonal dependence of absorption and the influence of geomagnetic activity in the equatorial region, at 1000 hrs LT. The analysis of the multifrequency absorption data suggests: (i) it may be misleading to assume that in the equatorial region HF absorption obeys frequency laws such as L = Af-1 or L = Af-2. Rather the ionospheric absorption in the equatorial region appear to follow the law L = Af-n where A is a constant and 0.8 < n < 2.4 (ii) Solar activity does not significantly affect the mechanism of frequency dependence of non-deviative absorption. (iii) There appears to be a semiannual variation in the frequency index with equinoctial minima and maxima near the solstices. (iv) There exists distinct semi-annual variation in absorption with equinoctial maxima and solstial minima at 4.87 MHz (v) At higher frequencies the variation of equatorial radio wave absorption appears more of a non-seasonal phenomenon. (vi) Seasonal variation of HF radio wave absorption at higher frequencies appears to be less amenable to theoretical interpretations. (vii) Geomagnetic activity has no significant effect on the equatorial HF radio wave absorption.