DMSP observations of equatorial plasma bubbles in the topside ionosphere near solar maximum

The Defense Meteorological Satellite Program (DMSP) flights F9 and F10 crossed postsunset local time sectors approximately 14 times per day in Sun-synchronous orbits at an altitude of similar to 840 km. We have examined a large database of postsunset plasma density measurements acquired during similar to 15,000 equatorial crossings made by DMSP F9 in 1989 and 1991 and DMSP F10 in 1991. On 2086 of these crossings equatorial plasma bubbles (EPBs) were observed as intervals of depleted and irregular plasma densities. We have analyzed these EPB events to determine their distributions with season, longitude (S/L), and levels of geomagnetic activity. The global S/L distributions of EPBs observed by the DMSP satellites are shown to be in general agreement with results from discrete ground based measurements. That is, the, seasonal variations detected at 840 km in longitude bins hosting radar/scintillation observatories appear similar to those reported from the g round. Over the Atlantic sector where EPBs occur frequently. we found good agreement with predictions of a simple model proposed by Tsunoda [1985]. In the Pacific sector the frequency of EPB occurrence is considerably lower, and poor counting statistics preclude confident predictions regarding the absolute value of seasonal variations. We suggest that relatively large equatorial magnetic fields at F layer altitudes in the Pacific (similar to0.34 G) sector more strongly inhibit the growth of the Rayleigh-Taylor instability than at Atlantic (similar to0.25 G) longitudes. Contrary to general belief, we found that EPB occurred regularly during geomagnetic storms, especially in the initial and main phases. EPB activity appears to have been suppressed from many hours to days during storm recovery phases.