We compare season-longitude distributions of > 8300 equatorial plasma bubbles (EPBs) observed during the last solar cycle with predictions of two simple models. Both models are based on considerations of parameters controlling the linear growth rate of the generalized Rayleigh-Taylor instability in the context of finite windows of opportunity during the prereversal enhancement periods near sunset. Tsunoda (1985) suggested that EPB occurrence rates should peak at times when the angle between the terminator and equatorial magnetic fields is smallest. Huang et al. (2001) argued that the very different rates of EPB occurrence in the Atlantic-African versus the Indian-Pacific Ocean sectors inversely reflect their different magnetic field strengths, B. The independence of and B from the phase of the solar cycle enables these comparisons.

Data acquired during >75,000 orbits of polar-orbiting Defense Meteorological Satellite Program (DMSP) satellites were sorted into 24 longitude and 12 one month bins, each containing ~250 samples. We show that: (1) in about half the longitude bins maximum rates of occurrence fall within ±15 days of when = 0. (2) Unpredicted phase shifts and asymmetries appear in occurrence rates near the two times per year when = 0. (3) At longitudes between -30° and 180° EPB occurrence rates correlate negatively with B. (4) In and to the west of the South Atlantic Anomaly (SAA) correlations vanish. EPB activity during the magnetic storm of 6 November 2001 in a region of eastward declination with a ˜ -27°, illustrates that random electric field penetration events can overwhelm damping effects of weak gradients in the Pedersen conductances near dusk. Results (2) and (4) indicate that other systematic forces operate in the low-latitude environment. Damping by interhemispheric winds appears to be responsible for phase shifts in maximum rate of EPB occurrence from days when = 0. Relatively low rates of EPB occurrence in and near the SAA result in part from enhanced Pedersen conductances due to precipitation of radiation belt electrons in the drift loss cone.