DERIVING THE NORMALIZED ION-NEUTRAL COLLISION FREQUENCY FROM EISCAT OBSERVATIONS

Common programme observations by the EISCAT UHF radar revealed an exte nded interval, post geomagnetic local noon on 03 April 1992, during wh ich the F-region ion velocity orthogonal to the geomagnetic field was significantly enhanced, to values exceeding 2 km s(-1) corresponding t o a perpendicular electric field of some 100 mV m(-1). Observations fr om this interval are used to illustrate a method by which estimates of the E-region ion-neutral collision frequency may be derived in the pr esence of enhanced electric field. From both the rotation of the ion v elocity vector and the reduction in the ion velocity magnitude relativ e to that in the F-region, independent estimates of the normalised ion -neutral collision frequency are made at the UHF E-region tristatic al titudes; the derived values are, in general, lower than model predicti ons. Although initial calculations assume a stationary neutral atmosph ere, first-order estimates of the E-region neutral wind are subsequent ly employed to calculate revised estimates of the normalised ion-neutr al collision frequency, these neutral winds are derived by attributing the difference between predicted and observed enhancements in field-p arallel ion temperature to thermospheric motion. The inclusion of neut ral winds, which are themselves not inconsiderable, appears to have on ly a limited effect on the normalised collision frequencies derived.