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

Fig. 3 Cumulative heating rate (temperature increase as functionof time) at different altitudes around the mesopause.Fig. 2 shows the results of our deductions: (a) The electron density profile exhibited an increasearound 90 km and significant reduction above 90 km. This result shows a good agreement withthe numerical estimation of electron density calculated with the GEOS satellite data of protonprecipitation (private communication, Rapp 2001).Fig 2 also shows (b) the calculated heating rate as function of altitude, and in Fig. 3 we show theaccumulated neutral heating rate. i.e the temperature increase over a few days. Of course, thisdoes not include any radiation or heat transport, which likely would reduce the temperature increase.We recognize that there is very little heating of a small fraction of one Kelvin around80 km but it gets larger with altitude, and it reaches 8 Kelvin at 92.7 km. These calculations arefor fairly high D-region electron densities, which are only observed during strong solar protonevents. Also the ionospheric electric field of 60 mV/m, which was estimated from the ion driftmeasurements with the ESR incoherent scatter radar, is fairly high, but representative for thishighly disturbed period.The mean temperature (dashed line in Fig. 2) raised a little due to the heating above 85 km. Wehave also included in Fig. 2 the limiting curves of water vapor mixing ratio as function of altitude.Only in the regions with temperatures below these curves ice particles can be formed. In themodel, which we used in Fig. 2, this means between about 82 and 92 km, where we detect thePMSE. The ice particles interact with the ionospheric plasma, which results in the formation ofplasma irregularities raising the radar scatter cross section. When the temperature raises, the iceparticles would melt, which in turn will result in a disappearance of the PMSE. At least for thismodel, the peak temperatures do not change sufficiently strong to cause this melting effect. We,thus, cannot provide full prove from these observations that the disappearance of PMSE in theupper heights can be exclusively explained by a neutral temperature increase due to heating.Rapp et al. (2002) had recalled that the ratio of the product of mean charge and number density ofheavy particles to electron density has an influence on electron diffusion and hence a too highelectron density would prevent PMSE.224