The early morning development and the evening decay of electron content-latitude profiles at low latitudes and their dependence upon solar declination

Abstract

Electron content-latitude profiles from 20°S to 30°N have been obtained for 1975 (a period of low solar activity) from an analysis of Faraday fading recordings of INTASAT taken at Hong Kong (long. 114.1°E, lat 22.2°N, dip 30°N) and Bali (long. 115.2°E, lat. 8.9°S, dip 35.5°S) during the periods 0800–1000 and 2000–2200 LST (120°E). Hourly median profiles have been computed for both the morning and evening periods for the northern summer and winter, the March/April equinox and also corresponding to a period during which the sun was overhead the magnetic equator (9°N). Profiles were most symmetric with respect to the magnetic equator at the equinox. In the morning at the solstices the formation of a single crest of the equatorial anomaly was observed on the winter side of the magnetic equator, though at higher latitudes the electron content values were greatest in the summer hemisphere compared with those of winter hemisphere for the same dip location. The former result is explainable by the action of a wind blowing from the summer hemisphere across the magnetic equator while the latter is probably due to the proximity of the sun giving rise to a faster ionization production rate in the summer hemisphere. In the evening at the solstices the electron content was greatest in the summer hemisphere and for the northern winter a single crest of the equatorial anomaly remained at about 35°S dip as late as 2200 LST. These results are explainable as being a consequence of the movement of ionization to high levels in the summer hemisphere and low levels in the winter hemisphere by the daytime wind, re-combination after sunset being the most rapid for that ionization in the winter hemisphere. At the equinox the profiles showed the greatest content values and the two equatorial anomaly crests were symmetric with respect to the magnetic equator for 2000–2100 LST. However from 2100–2200 LST the northern crest collapsed near to the magnetic equator and it is thought that this was due to a southward wind blowing at that time. Representative individual profiles corresponding to the varying overhead position of the Sun throughout the year have been given. The evening profiles, in particular, show marked changes of form reflecting the effects of prevailing equatorward winds. From the December solstice to the March equinox no such a wind was apparent but between the September equinox and the December solstice this wind did occur in the summer hemisphere, having a maximum effect on the evening profiles around the end of October. With the movement of the Sun (and the thermal pressure bulge) towards the most southward position in the summer hemisphere during the day the effect of the evening equatorward wind on the electron content profiles decreased.