Proceedings with Extended Abstracts (single PDF file) - Radio ...

Incoherent scatter spectracompared with model (T, p, m)modelmeasurement? ?Fig. 4 Plot of incoherent scatter spectra. The solid line gives the half-powerwidth of a fitted Lorentzian, and the dashed lines those from models.There is another method to deduce the neutral temperatures from incoherent scatter radar observations.The spectrum width depends on the ion-neutral collision frequency (related to pressure pand ion mass m) and the temperature T. Larger temperatures result in a larger spectrum width. InFig. 4 we show a 4-minutes average of the spectra as function of the altitudes between 60 km and95 km. The dashed lines are contours for different levels of a model Lorentzian. The lowerhorizontal bar at 78 km denotes a similarity of model and measurement. The upper horizontal barat 86 km shows that the measured spectrum is wider than the model. This is an indication of arise in temperature with height, as we would expect from the Joule heating. Care has to be taken,however, when there are heavy ions or ice particles, which cause PMSE. These have a narrowerspectrum than the incoherent scatter spectrum, i.e. could be interpreted as lower temperatures.The observations shown in Fig. 4 are not affected by PMSE. We also have chosen the comparisonlevel at 78 km, where we never detected PMSE. The broad spectra below 65 km, denoted byquestion-marcs, cannot be investigated here; they may be due to a change of ion mass.Since we still assume that the observed temperature increases are too small to explain thedisappearance of PMSE during this strong ionospheric disturbance, we need to search for anothermechanism. Chilson et al. (2000) have observed a significant reduction of scatter cross section ofPMSE during artificial electron heating of the PMSE plasma environment. We suspect that naturalelectron heating, which is quite pronounced during ionospheric disturbances, might haveplayed a role as well. This needs much further investigations.References:Banks, P.M., Joule heating in the high-latitude mesosphere, J. Geophys. Res., 6709-6712, 1979.Chilson, P.B., E. Belova, M.T. Rietveld, S. Kirkwood, and U.P. Hoppe, First artificially inducedmodulation of PMSE using the EISCAT heating facility, Geophys. Res. Lett., 27 , 3801-3804,2000.Rapp.M., J. Gumbel, F.J. Lübken, and R. Latteck, D-region electron number density limits for theexistence of polar mesosphere summer echoes, J. Geophys. Res., 107(D15), doi:10.1029/2001JG000915, 2002.225