Equatorial ionospheric plasma density bubbles observed by ESRO-4

It is shown that the ESRO-4 satellite, in a polar orbit, was able to observe equatorial plasma bubbles using the fixed-bias total ion current probe. Experimental conditions were suitable for observations during a period at the vernal equinox 1973, when 21 plasma density depletion events were identified, extending across the equator. Statistical analysis suggests that the bubble cross-sections, transformed on to the zero-dip surface, were elongated, with an axial ratio of the order of 7:1, and were tilted upwards by about 10° to the east. The observations were made selectively near the F-layer peak; no movement of bubbles was measured.