In a test of the generally accepted Rayleigh‐Taylor (R‐T) instability mechanism for equatorial spread F (ESF), a linear instability growth rate γRT is derived following the formalism of Haerendel (preprint, 1973) which takes into account the variations of physical parameters along geomagnetic flux tubes. The resulting form of γRT extends the results of previous work by including direct dependencies on transequatorial neutral winds, zonal electric fields, vertical and horizontal ionospheric density gradients, the presence of an E region, and chemical recombination. Realistic atmospheric and ionospheric density model inputs are used for the first time to make quantitative calculations of R‐T growth rates for a range of geophysical conditions. The key result of this study is that time/altitude domains having positive calculated instability growth rates are found to coincide with observed time/altitude patterns of ESF occurrence over both a monthly and a yearly time frame. This success in being able to model the climatological occurrence of ESF lends support to the physical model adopted for the instability mechanism and opens up new avenues of research into ESF predictability on a night‐to‐night and even an hour‐to‐hour basis.

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[1] We have examined more than 75,000 latitudinal profiles of plasma densities measured by ion detectors on five Defense Meteorological Satellite Program (DMSP) satellites in the evening local time (LT) sector between 1989 and 2001. This survey established detection frequencies of equatorial bubbles (EPBs) at 840 km over the recent solar cycle. The annual rate of EPB detections decreased by more than an order of magnitude from >1000 during solar maximum to <100 during solar minimum years. EPB data were divided into 24 longitude sectors to determine seasonal and solar cycle variability in rates of encounter by DMSP. During the ascending and descending portions of the solar cycle, each longitude sector showed repeatable seasonal variations. The envelope of seasonally averaged rates of EPB encounters resembles the solar cycle variability for similar averages of the F 10.7 index. On both global and longitude sector scale sizes, annual rates of EPB encounters correlate with the yearly averages of F 10.7 . We also find that throughout the solar cycle the EPB detections were overrepresented during times of high geomagnetic activity signified by Kp ! 5. During solar minimum years, about one third of the EPBs occurred when traces of the Dst index had significant negative slopes (dDst/dt À5 nT/hr). This suggests that electric field penetration of the inner magnetosphere is responsible for driving many EPBs. Comparisons of plasma and neutral density profiles in the evening sector, calculated using the Parameterized Ionospheric Model (PIM) and MSIS-86 Model, indicate that the height of the bottomside of the F layer is >100 km lower during solar minimum than solar maximum. However, the overall effect is to increase the growth rate of the Rayleigh-Taylor instability at solar maximum in the bottomside F layer only by about a factor of 2. We suggest that the variability of electric fields in the postsunset equatorial ionosphere is the source of the observed discrepancy between EPB detections under solar maximum/minimum conditions.