One type of low-latitude ionization trough in the Southern hemisphere

It is deduced from the Intercosmos-19 satellite topside sounding data that at solar maximum at low latitudes of the Southern hemisphere at night time there can occur a peculiar type of ionization trough which seems not to be described in the literature yet. This trough occurs usually in the interval from the second half of November to the beginning of February in the region of the Southern crest of the equatorial anomaly. Its occurrence probability is quite high and at postmidnight hours at longitudes 75 degrees to 3000 degrees is about 50%. In other seasons as well as at the anomaly Northern crest this trough was not practically observed. It is suggested that the cause of this trough formation is the thermospheric wind from the summer (Southern) hemisphere to the winter one which at latitudes under consideration lifts the ionization along the geomagnetic field lines and can thereby reduce its density at the hmF2 level. According to the most widely used HWM90 model of neutral winds, at night the wind from the Southern hemisphere to the Northern one in December is somewhat stronger than from the Northern hemisphere to the Southern one in June. But this difference is too little to explain the absence of a trough similar to the one described herein at the anomaly Northern crest in June solstice. This suggests that the asymmetry of the hemispheres is really stronger than represented in the HWM90 model. (C) 1999 COSPAR. Published by Elsevier Science Ltd.