Low-latitude seasonal anomalies in ionospheric absorption of radio waves

Abstract

Ionospheric absorption of radio waves at low latitudes is investigated. Harmonic analysis of noon time data from a number of stations shows the annual and semiannual periodicities to be important. The annual component is very weak at the geographic equator and by 30° north latitude increases to a peak amplitude of close to 40 per cent of the annual average noon time absorption. The 6-monthly component is nearly constant with latitude and forms about 15 per cent of the mean value. At the Equator the predominant 6-monthly component gives rise to the so called ‘seasonal and summer’ anomalies in ionospheric absorption. Besides solar zenith angle variations the origin of these anomalies is traced largely to similar variations in the incident solar ionizing X-ray flux.