Testing the thermospheric neutral wind suppression mechanism for day-to-day variability of equatorial spread F

The determination of the physical processes that cause the day-to-day varia bility of equatorial spread F (ESF) has long been one of the outstanding pr oblems in terrestrial space physics. Within the context of the Rayleigh-Tay lor instability model for ESF, mechanisms that either enhance or inhibit th e growth of a seed perturbation offer potential sources of variability that can be tested. In this study the hypothesis that enhanced thermospheric me ridional winds play a critical role in suppressing ESF is examined during t he Multi-Instrumented Studies of Equatorial Thermospheric Aeronomy (MISETA) campaign of September 1998. New high time-resolution Fabry-Perot interfero meter (FPI) observations at 6300-Angstrom nightglow made at Arequipa (Peru) provided the neutral wind measurements during the critical postsunset hour s that had been sampled only sparsely in earlier morphology studies. Eviden ce of local ESF activity was obtained using GPS-based observations of phase fluctuations (Fp) and 6300-Angstrom all-sky optical images from the same s ite. Additional GPS measurements of Fp and total electron content (TEC) fro m Bogota (Colombia) and Santiago (Chile) were used to determine the full fl ux tube development of ESF plumes and to characterize the F region morpholo gy of the interhemispheric Appleton anomaly. Correlative studies between th e nightly magnitudes of the meridional winds (U-m), ESF activity (Fp), and indices describing the strength (I-s) and asymmetry (I-a) of the Appleton a nomaly offered no convincing evidence for the wind suppression mechanism. T he best available precursor for premidnight ESF appeared to be the strength of the electrodynamically driven Appleton anomaly pattern at sunset. If on e assumes that the required seed perturbation for ESF onset is essentially always available, then for all practical purposes, the magnitude of the eas tward electric field that causes upward drift is both the necessary and suf ficient parameter to forecast ESF with reasonable success. These results re confirm 60 years of study pointing to the dominance of electrodynamical pro cesses in the onset and growth of plasma instabilities at low latitudes.