COMMENT ON "THE EFFECT OF STRONG VELOCI'FY SHEARS ON INCOHERENT SCATTER SPECTRA: A NEW INTERPRETATION OF UNUSUAL HIGH LATITUDE SPECTRA" W.Kofman

The authors discuss in their article (Swartz et al., 1988) the distortion of incoherent scatter spectra due to highvelocity shears in the scattering volume. This is a well-known effect (see, for example, the discussion of one aspect of this problem in Kofman et al., 1984). It is our opinion that the demonstration and analysis of one single event of such a distortion is not very conclusive. A better justification of such a paper would have been a systematic modelling of this effect, giving the experimenter some tool at hand to estimate the shear from the distortion in a more quantitative way. With their paper the authors cast some doubts on the interpretation of incoherent scatter data in terms of unusual high ion temperatures (Kofman and Lathuillere, 1987), nonMaxwellian ion velocity distributions (Moorcroft and Schlegel, 1988; Lockwood et al., 1987 ) and anisotropic ion velocity distributions (Lovhaug and Fla, 1986; Perraut et al., 1984). We would like to state the following in this context: 1. The authors term these effects as "exotic", but these phenomena re at least as common as high-velocity shears. Indeed, Farmer et al. (1988) have employed a coupled ionosphere-thermosphere model to show how the supersonic flows required to produce non-Maxwellian plasma arise in certain locations at even very low magnetic activity. 2. The work cited above was performed with EISCAT which generally uses much smaller scattering volumes than those of 300 km length described by Swartz et al. (1988). Usually the effective range resolution is 54 km or, in the case of tristatic measurements, of the order of only a few km. This reduces the probability of having strong velocity shears in a single scattering volume. 3. The EISCAT measurements yielding high ion temperatures were made while the antenna pointing was along the magnetic field line. We do not expect to have large velocity shears in this direction. 4. The fact that the effects described in the above cited