CHARACTERISTICS OF IONOSPHERIC BUBBLES DETERMINED FROM ASPECT SENSITIVE SCATTER SPREAD-F OBSERVED WITH ALOUETTE-1

Aspect sensitive scatter from bubbles is observed on a large percentage of equatorial Alouette 1 ionograms. The occurrence frequency of bubbles increases sharply (from zero) after sunset and reaches a maximum at about 23-24 hours local time. Near midnight, bubbles are observed in the topside ionosphere on about 65% of American and African satellite passes and on about 50% of Asian passes. The frequency of occurrence decreases gradually throughout the postmidnight and morning periods and becomes almost zero near noon. Averaged over the whole day, the occurrence of bubbles at Asian longitudes is about one half that at American and African longitudes. The occurrence of bubbles peaks at the December solstice for American longitudes and at the June solstice for Asian longitudes. The above observations imply that bubbles are initiated by field-aligned currents at the steep density gradient at the bottom of the postsunset F layer.