DERIVATION OF THE ION TEMPERATURE PARTITION COEFFICIENT-BETA-PARALLEL-TO FROM THE STUDY OF ION FRICTIONAL HEATING EVENTS

This paper reports the results of a study evaluating the ion temperatu re partition coefficient beta(parallel-to) over a range of F region al titudes. The data have been selected from three EISCAT CP-0 experiment s, each of which displayed clear evidence of ion frictional heating. T he data were averaged at a time resolution of 30 s. These observations , made in an observing direction parallel to the Earth's magnetic fiel d, have the advantage that the line-of-sight ion thermal velocity dist ribution can be closely approximated to a Maxwellian, while still givi ng a Ene-of-sight ion temperature which can be interpreted on the basi s of a well-known energy balance equation. The technique for determini ng beta(parallel-to) depends upon fitting the variation of the field-p arallel ion temperature to simplified forms of this balance equation. The method is an extension of the curve-fitting approach previously us ed by Glatthor and Hernandez (1990) to deduce both ion temperature par tition coefficients at a single F region altitude. The results of this procedure are compared to the theoretical predictions for beta(parall el-to) obtained from an equation originally due to St.-Maurice and Han son (1982). By introducing measured and modeled parameters, it is foun d that values of beta(parallel-to) close to those expected for resonan t charge exchange collisions are predicted around the F region peak. A t greater heights, however, the increased influence of Coulomb collisi ons is predicted to give rise to an increase in beta(parallel-to) with altitude, corresponding to ion thermal velocity distributions which t end toward isotropy. This height dependence, which has been theoretica lly predicted in some other recent studies, would be an important fact or in calculations of the energy balance in the upper F region. While the experimentally derived values of beta(parallel-to) are close to th e RCE predictions near the F region peak, the expected increase in bet a(parallel-to) with altitude is seen in only one of the three selected events. In the remaining cases, the increases are notably smaller. Po ssible reasons for this discrepancy are discussed. Changes in ion and neutral composition, together with effects such as heat conduction and thermal diffusion can act to bias the curve-fitting technique. The de gree of coupling between the ionized and neutral atmospheres is also a n important factor, and there is considerable uncertainty in some of t he terms used in the theoretical part of the study. Nonetheless, the r esults suggest that Coulomb collisions play a potentially important ro le in determining the energy balance of the upper F region. A clear ne ed exists for further studies in this area to establish more fully the contribution of Coulomb processes.