Iw. Mccrea et al., DERIVATION OF THE ION TEMPERATURE PARTITION COEFFICIENT-BETA-PARALLEL-TO FROM THE STUDY OF ION FRICTIONAL HEATING EVENTS, J GEO R-S P, 98(A9), 1993, pp. 15701-15715
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.