BISTATIC MEASUREMENTS OF INCOHERENT-SCATTER PLASMA LINES

According to the classical theory of incoherent scatter plasma lines, it is possible to determine the electron drift velocity from the diffe rence in offset frequencies between the up and downshifted plasma line s. Together with the electron density and the ion drift velocity deriv ed from the ion line it should then be possible to determine the field aligned current. However, measurements interpreted with theory assumi ng a Maxwellian electron distribution have indicated Ear too large cur rents. A recent theory by Kofman et al. (1993) J. geophys. Res. 98, 60 79-6085, takes into account the temperature-gradient induced heat flow which significantly influences the result. We show measurements of pl asma lines, for both high and low solar activity, measured with both t he monostatic, F-region cutoff (Tromso) and the bistatic, intersection -volume (Kiruna) techniques. These are compared to the standard Maxwel lian theory as well as to the theory including heat flow. The Kiruna d ata give the same results as do the Tromso measurements, within the me asurement accuracy, showing that the discrepancy between theory and me asurements is not an artefact of the technique of measuring the plasma line cutoff at the peak of the F region. The additional heat flow ter m makes up for most of the discrepancy between the theoretical predict ion and the measurements, and the latter indicate that it may be sligh tly overestimated in the high solar activity case. For the high solar activity conditions the surrounding neutral atmosphere is much denser, which might reduce the heat flow. For low solar activity there is sti ll a remaining discrepancy between theory and measurements after accou nting for the heat flow term. This may be explained by an enhanced hea t flow caused by runaway electrons, as proposed by Mishin and Hagfors (1994) J. geophys. Res. 99, 6537-6539, or by a thermal electron drift balancing a net photoelectron escape flux.