Abstract
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 field was significantly enhanced, to values exceeding 2 km sâ1 corresponding to a perpendicular electric field of some 100 mV mâ1. 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 field. 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, first-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 difference between predicted and observed enhancements in field-parallel ion temperature to thermospheric motion. The inclusion of neutral winds, which are themselves not inconsiderable, appears to have only a limited effect on the normalised collision frequencies derived.
Highlights
Various workers have attempted to derive estimates of the ion-neutral collision frequency from observations by incoherent scatter radar
Short periods during which meridionalow. Both the rotation in the ®eld-perpendicular ion velocity observed by European incoherent scatter facility (EISCAT) during the interval mentioned, from a near geomagnetic zonal westward direction towards the meridional north with decreasing altitude and the reduction in the magnitude of the ion velocity from that measured at F-region altitudes, illustrate the eect of increasing collisional coupling with the neutral atmosphere on the motion of the ion population
The intention is to demonstrate an alternative method to that of Nygre n et al (1987) by which the normalised ion-neutral collision frequency can be derived from EISCAT observations of ion velocity under conditions of enhanced electric ®elds. At this juncture, informative to point out the relative merits and disadvantages of this method over that of Nygre n et al (1987). These authors employed previously derived functional relationships between the electric ®eld and the vertical ion velocity to derive the ion-neutral collision frequency during an EISCAT UHF special programme run, in which the transmitter was pointed vertically and the remote sites remained at a single F-region tristatic volume
Summary
Various workers have attempted to derive estimates of the ion-neutral collision frequency from observations by incoherent scatter radar. In the presence of such a ®eld, the F-region ions move, in the frame of reference of the geomagnetic ®eld, approximately with the E  B drift in the ®eld-perpendicular direction such that vi It follows from Eq (1) that it is theoretically possible to determine the normalised ion-neutral collision frequency, at an altitude in the E-region where collisions have a signi®cant eect on ion dynamics, independently from both the rotation of the dierential ion-neutral velocity vector in the plane perpendicular to the magnetic ®eld from the Eeff  B direction at that altitude and from the reduction in the magnitude of the ®eld-perpendicular ion-neutral relative velocity from that of Eeff  B. The signal-to-noise ratio of the Fregion remote site and Tromsù long pulse observations employed in the present investigation are typically 20± 30% and 100±300%, respectively
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