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.