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The problem of day-to-day variability in the occurrence of equatorial spread F (ESF) is addressed using multidiagnostic observations and semiempirical modeling. The observational results are derived from a two-night case study of ESF onset conditions observed at Kwajalein Atoll (Marshall Islands) using the ALTAIR incoherent scatter radar and all-sky optical imaging techniques. The major difference between nights when ESF instabilities did not occur (August 14, 1988) and did occur (August 15, 1988) in the Kwajalein sector was that the northern meridional gradient of 6300-A airglow was reduced on the night of limited ESF activity. Modeling results suggest that this unusual airglow pattern is due to equatorward neutral winds. Previous researchers have shown that transequatorial thermospheric winds can exert a control over ESF seasonal and longitudinal occurrence patterns by inhibiting Rayleigh-Taylor instability growth rates. Evidence is presented to suggest that this picture can be extended to far shorter time scales, namely, that 'surges' in transequatorial winds acting over characteristic times of a few hours to a day can result in a stabilizing influence upon irregularity growth rates. The seemingly capricious nature of ESF onset may thus be controlled, in part, by the inherent variability of low-latitude thermospheric winds.

The physical mechanisms by which negative ions change the shape of the incoherent scatter spectrum, and the way in which shape changes may be used to detect the presence of heavy positive and negative ions in an ambient ionosphere are investigated. In order to detect heavy negative ions, the temperature structure of the ionosphere is fixed to a prevent average measurement, and any changes in spectral shape during the experiment are interpreted as being caused by changes in composition, and not by changes in the temperature ratio Te/Ti. The spatial and temporal development of heavy negative ion plasma clouds created during four active chemical release experiments was observed. Concentrations of 10-40-percent SF6(-) were detected in SPINEX 1, SPINEX 2, and IMS data sets. An average uncertainty of +/-10-percent SF6(-) is present in all three experiments. Concentrations of 30-percent Br(-) were detected in the NICARE 1 release, with uncertainties of +/-4 percent.