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

Fig. 2 PMSE plot (SOUSY Svalbard Radar, 17 June, 17:21 UT,to 18 June, 00:04 UT, 2001) showing multiple layering of thin sheets.Regardless of the scattering mechanism causing the PMSE, there must be a reason why thecorresponding irregularities occur in thin sheets. Röttger (1991) had discussed the possibilitiesand summarized that “the physical mechanisms leading to PMC (Polar Mesospheric Clouds)and NLC (Noctilucent Clouds) on the one hand and PMSE on the other are surely somewhatdifferent, but they must be undoubtedly related”. During the past years this has been provedby many experiments and theoretical work (e.g., Rapp et al., 2003), which cannot be summarizedin this extended abstract. Röttger (1994) also suggested that cooling in certain phases ofgravity waves is a likely mechanism to create heavy ions being responsible for PMSEirregularities. Ice particles and aerosols are now believed to interact with the ionosphericplasma and this results in PMSE as well. Ice particles form when a certain water vapor saturationtemperature is reached, and this cooling can be achieved by gravity wave modulation ofthe mean background temperature profile as sketched in Fig. 3. The cool regions occur quasiperiodicallywith height. We recognize this in the PMSE plots of Fig. 2.It is argued that verticaltemperature variations dueto long period waves causethese sheets in PMSEIn the stratosphere theyincrease the verticalgradient of potentialtemperature, whichenhances the reflectivityFig. 3 A simple model how a gravity wave can cause temperature variationsof the mean temperature profile of the mesopause region resulting in local regionsof cooling where in turn ice particles and PMSE irregularities form.Gravity waves propagate vertically and horizontally as sketched in Fig. 4. This means thatthese cool regions (where conditions for PMSE can be formed) move up or down on the meanprofile, depending on the gravity wave and background parameters. Longer period waveswould remain fairly persistent at certain altitudes, whereas shorter-period waves and interactionof waves which each other would result in quite some variability, as we can see in theplots of Figs. 1 and 2. As Röttger (1994) suggested, there can even be steepening of gravitywaves and this in turn could explain the very thin sheets of PMSE, which we often observe.55