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 field was significantly enhanced, to values exceeding 2 km s−1 corresponding to a perpendicular electric field 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 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.
Ion thermal effects on E-region instabilities: linear theory
Linear instabilities and turbulent processes in equatorial and high-latitude electrojets lead to formation of plasma density irregularities which create much of the common E-region radar clutter. Studies of these irregularities have focused on the role of the Farley-Buneman (FB) and gradient drift instabilities with the assumption that the plasma behaves isothermally or adiabatically. In the last decade, this restriction has been relaxed, resulting in the prediction of D and E-region thermally driven instabilities, which now have some supporting observational evidence. Even more recently, fully kinetic particle-in-cell (PIC) simulations show that ion thermal effects strongly modify the nonlinear behavior of the FB instability, especially at the top of the electrojet. This paper describes the linear theory of the combined FB and thermal instabilities. The theory predicts that instabilities will develop over a wider range of altitudes than predicted for the adiabatic or isothermal FB instability. It also predicts that preferred directions of thermally modified FB waves may differ significantly from that of standard Farley-Buneman waves. These theoretical developments have important observational consequences.
Ionospheric warming by neutral winds
Abstract The effect of frictional heating by means of neutral winds on the ion and electron temperature in the undisturbed ionosphere is studied theoretically by solving a system of basic ionospheric and atmospheric equations. The study shows that both the electron and ion temperatures are increased in the night-time ionosphere through friction. In the region between 150 and 200 km Ti may exceed T6 by as much as 130°. The increase of Ti due to friction amounts to about 100–200°, depending on the atmospheric model employed in calculating the neutral wind velocity. It is illustrated that frictional heating may be very important for the determination of the neutral temperature from measured ion temperature values.