Deriving the normalised ion-neutral collision frequency from EISCAT observations

Common programme observations by the EISCAT UHF radar revealed an extended interval, post geomagnetic local noon on 03 April 1992, during which the F-region ion velocity orthogonal to the geomagnetic ®eld was signi®cantly enhanced, to values exceeding 2 km s corresponding to a perpendicular electric ®eld of some 100 mV m. Observations from this interval are used to illustrate a method by which estimates of the E-region ion-neutral collision frequency may be derived in the presence of enhanced electric ®eld. From both the rotation of the ion velocity vector and the reduction in the ion velocity magnitude relative to that in the F-region, independent estimates of the normalised ion-neutral collision frequency are made at the UHF E-region tristatic altitudes; the derived values are, in general, lower than model predictions. Although initial calculations assume a stationary neutral atmosphere, ®rst-order estimates of the E-region neutral wind are subsequently employed to calculate revised estimates of the normalised ion-neutral collision frequency; these neutral winds are derived by attributing the di€erence between predicted and observed enhancements in ®eldparallel ion temperature to thermospheric motion. The inclusion of neutral winds, which are themselves not inconsiderable, appears to have only a limited e€ect on the normalised collision frequencies derived.