GRAVITY-WAVE SEEDING OF EQUATORIAL PLASMA BUBBLES

Some examples from the Atmosphere Explorer E data showing plasma bubbl e development from wavy ion density structures in the bottomside F lay er are described. The wavy structures mostly had east-west wavelengths of 150-800 km; in one example it was about 3000 km. The ionization tr oughs in the wavy structures later broke up into either a multiple-bub ble patch or a single bubble, depending upon whether, in the precursor wavy structure, shorter wavelengths were superimposed on the larger-s cale wavelengths. In the multiple-bubble patches, intrabubble spacings varied from 55 km to 140 km. In a fully developed equatorial spread F case, east-west wavelengths from 690 km down to about 0.5 km were pre sent simultaneously. The spacings between bubble parches or between bu bbles in a patch appear to be determined by the wavelengths present in the precursor wave structure. In some cases, deeper bubbles developed on the western edge of a bubble patch, suggesting an east-west asymme try. Simultaneous horizontal neutral wind measurements showed wavelike perturbations that were closely associated with perturbations in the plasma horizontal drift velocity. We argue that the wave structures ob served here that served as the initial seed ion density perturbations were caused by gravity waves, strengthening the view that gravity wave s seed equatorial spread F irregularities.