Bistatic measurements of incoherent scatter plasma lines

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According to the classical theory of incoherent scatter plasma lines, it is possible to determine the electron drift velocity from the difference in offset frequencies between the up and downshifted plasma lines. Together with the electron density and the ion drift velocity derived from the ion line it should then be possible to determine the field aligned current. However, measurements interpreted with theory assuming a Maxwellian electron distribution have indicated far too large currents. A recent theory by Kofman et al. (1993) J. geophys. Res. 98, 6079–6085, takes into account the temperature-gradient induced heat flow which significantly influences the result. We show measurements of plasma lines, for both high and low solar activity, measured with both the monostatic, F-region cutoff (Tromsø) and the bistatic, intersection-volume (Kiruna) techniques. These are compared to the standard Maxwellian theory as well as to the theory including heat flow. The Kiruna data give the same results as do the Tromsø measurements, within the measurement accuracy, showing that the discrepancy between theory and measurements is not an artefact of the technique of measuring the plasma line cutoff at the peak of the F region. The additional heat flow term makes up for most of the discrepancy between the theoretical prediction and the measurements, and the latter indicate that it may be slightly 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 still a remaining discrepancy between theory and measurements after accounting for the heat flow term. This may be explained by an enhanced heat 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.