Deriving the normalised ion-neutral collision frequency from EISCAT observations, Ann

• Main Authors: J. A. Davies, M. Lester, T. R. Robinson
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Published: 1997
• Subjects: EISCAT
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.371.6680; http://hal.archives-ouvertes.fr/docs/00/31/65/17/PDF/angeo-15-1557-1997.pdf

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 ®eld was signi®cantly enhanced, to values exceeding 2 km s)1 corresponding to a perpendicular electric ®eld of some 100 mV m)1. 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. 1