Electron content and other related measurements for a low latitude station obtained at sunspot maximum using a geostationary satellite

Abstract

Electron content measurements have been made at Hong Kong (lat. 22.2°N, dip 30°) for the period July 1967–December 1968 by continuously recording the polarisation angle of a signal from the geostationary satellite Syncom 3. Monthly median peak diurnal contents exhibited a semi-annual variation with maxima occurring at the equinoxes and a ‘winter anomaly’ effect was noted during the daytime. However this latter effect was not apparent for midnight values, though the semi-annual variation remained. The rate of increase in content with solar flux since 1964-5 was greatest for the winter and during this season the measured scale heights were also greatest, thus possibly implying a seasonal increase in the composition ratios [O][O2] and/or [O][N2]. Good short term correlation between the daily peak electron content and the daily solar flux has been established. The equivalent thickness exhibited a broad daytime increase during the summer but at other seasons sharp peaks occurred after sunset and before sunrise. The present results showed no seasonal variation of equivalent thickness, in contrast with the summer maximum obtained using data from lower height transit satellites; and also appreciably higher winter content values have been obtained with geosatellite data. A possible explanation of these facts may be that there is a significant increase of ionization above 1000 km during the winter season. No increase of equivalent thickness was observed between sunspot minimum and maximum. The integrated production rate for an overhead sun roughly followed a similar seasonal variation to that of the peak content if allowance was made for the solar flux control, which was found significant only during the winter. The zenith angle of the Sun corresponding to initial ionization was observed to vary between 97.0° in winter to 93.5° in summer. After sunset the ionosphere was found to decay in two stages giving two distinct effective loss rates with different seasonal variations. In interpreting these results it has been considered likely that after sunset during the equinoxes Mitra drifts of ionization occurred moving ionization from the magnetic equator to Hong Kong and around midnight during the summer a further appreciable influx of ionization occurred.