SCINTILLATIONS, PLASMA DRIFTS, AND NEUTRAL WINDS IN THE EQUATORIAL IONOSPHERE AFTER SUNSET

An equatorial campaign was conducted during September 25 to October 7, 1994, to investigate the neutral and plasma dynamics in the equatoria l ionosphere after sunset in relation to the day-to-day variability of the occurrence of equatorial spread F (ESF). The campaign was organiz ed under the auspices of National Science Foundation's Multi-Instrumen ted Studies of the Equatorial Thermosphere Aeronomy program (MISETA), which included the Jicamarca radar, spaced-antenna satellite scintilla tion, digisonde all-sky imager, and Fabry-Perot interferometer (FPI) m easurements near the magnetic equator in Peru. During a part of the pe riod, September 27 to October 3, the Geophysics Directorate of Phillip s Laboratory performed measurements away from the magnetic equator at Aguaverde, Chile (magnetic latitude: 11 degrees S) located 800 km to t he east of the Jicamarca meridian using geostationary and GPS satellit e scintillation, digisonde and all-sky imager systems. The incoherent scatter radar results indicate that the postsunset enhancement of upwa rd plasma drift, even though of the order of only 20 m s(-1) during th e solar minimum period, is a necessary condition for the generation of ESF. In view of the extreme difficulty of determining the neutral win d speed during the early evening hours by the FPI due to low airglow i ntensity, it was not possible to unequivocally associate the observed postsunset enhancements with strong eastward neutral winds. However, c onsidering a few observations contiguous to the campaign period, it ap pears that such a causal relationship may exist: The scintillation dri ft measurements in Peru and Chile indicated that the zonal irregularit y drift was smaller away front the magnetic equator implying a variati on of neutral wind with latitude. This is reproduced in the altitude v ariation of zonal drift observed by the Jicamarca radar. During a magn etic storm, scintillation measurements indicated that eastward drifts near the magnetic equator are accompanied by westward drifts near the anomaly peak which is consistent with the effects of a disturbance dyn amo. The campaign results indicate that in order to resolve the Variab ility of ESF, a careful probing of neutral dynamics as a function of l atitude needs to be undertaken during the postsunset period.