Seasonal-longitudinal variability of equatorial plasma bubbles

We compare seasonal and longitudinal distributions of more than 8300 equatorial plasma bubbles (EPBs) observed during a full solar cycle from 1989-2000 with predictions of two simple models. Both models are based on considerations of parameters that influence the linear growth rate, gammaRT, of the generalized Rayleigh-Taylor instability in the context Of finite windows of opportunity available during the prereversal enhancement near sunset. These parameters are the strength of the equatorial magnetic field, B-eq, and the angle alpha it makes with the dusk terminator line. The independence of alpha and B-eq from the solar cycle phase justifies our comparisons. We have sorted data acquired during more than 75 000 equatorial evening-sector passes of polar-orbiting Defense Meteorological Satellite Program (DMSP) satellites into 24 longitude and 12 one-month bins, each containing similar to250 samples. We show that: (1) in 44 out of 48 month-longitude bins EPB rates are largest within 30 days of when alpha = 0degrees; (2) unpredicted phase shifts and asymmetries appear in occurrence rates at the two times per year when alpha approximate to 0degrees; (3) While EPB occurrence rates vary inversely with B-eq, the relationships are very different in regions where B-eq is increasing and decreasing with longitude. Results (2) and (3) indicate that systematic forces not considered by the two models can become important. Damping by interhemispheric winds appears to be responsible for phase shifts in maximum rates of EPB occurrence from days when alpha = 0degrees. Low EPB occurrence rates found at eastern Pacific longitudes suggest that radiation belt electrons in the drift loss cone reduce gammaRT by enhancing E-layer Pedersen conductances. Finally, we analyze an EPB eve-it observed during a magnetic storm at a time and place where alpha approximate to -27degrees, to illustrate how electric-field penetration from high latitudes can overwhelm the damping effects of weak gradients in Pedersen conductance near dusk.