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

VHF radar interferometry had shown that PMSE irregularities can be accepted as passive tracersof dynamical processes in the mesopause region. Many observational features cannotprove that these dynamical processes, such as active turbulence, are the immediate cause ofPMSE. However, initial, even very weak, disturbances in the neutral atmosphere can translateinto disturbances in the ionospheric plasma (Schmidt number effect), and the correspondingcharge redistribution strongly increases the radar scatter cross section. The life cycle thencould be as shown in Fig. 5. A sheet-like PMSE irregularity structure is disturbed by atmosphericwaves and turbulence, which decays into fossil turbulence and fossil plasmairregularities remain. In the presence of ice particles (as described before) or any otherparticles of larger size than the common molecules in the mesosphere, the scatter cross sectionof these irregularities is further enhanced, which in turn causes sheet-like PMSE structures.These do not show any signs of active turbulence unless they are disturbed again by wavesbreaking into active turbulence. This is a repetitive process and can explain the life history ofPolar Mesosphere Summer Echoes.Many observations show that the lower PMSEregion is frequently aspect-sensitive. An explanationfor this anisotropic scattering is not knownyet. It is not immediately conceivable that thescattering structures in the electron gas, whichoriginally result from irregularities in the neutralgas, are horizontally elongated at the short Braggscale of MST VHF radars, namely a few meters.It is proposed here (sketched in Fig. 6) that somehorizontal structuring of the scattering medium atthese scales can result from polarization electricfields, which are generated by different chargingof heavy ions (NLC ice particles) of differentsizes. It is known that the larger ice particles arefalling faster than smaller ones, and that theselarge particles accumulate in the lower heights ofPMSE and NLC. Larger particles can attract orcarry a larger charge number. Consequently avertical electric field is established, and the equipotentialsurfaces are horizontally extended,which in turn would cause aspect sensitive scatter.Fig. 6 Sketch showing the verticaldistribution of charged particles. In summary we note that the Polar MesosphereSummer Echoes are resulting from the interplay of neutral dynamical (temperature),aeronomical (chemistry) and electro-dynamical (charge redistribution) processes. The neutraldynamical processes are not dominantly active turbulence resulting from wave breaking.References:Rapp, M., F.-J. Lübken, and T. Blix, The role of charged ice particles in the creation of PMSE:A review of recent developments, Adv. Space Res., 31(9), 2033-2043, 2003.Röttger, J., Middle atmospheric and lower thermosphere processes at high latitudes studiedwith the EISCAT radars, J. Atmos. Terr. Phys., 56(9), 1173-1195, 1994.Röttger, J., Observations of the polar D-region and the mesosphere with the EISCATSvalbard Radar and the SOUSY Svalbard Radar, Mem. Nat. Inst. Pol. Res., 54, 9-20, 2001.Rüster, R., J. Röttger, G. Schmidt, P. Czechowsky, and J. Klostermeyer, Observations ofMesospheric Summer Echoes at VHF in the Polar Cap Region, Geoph. Res. Lett., 28(8),1471-1474, 2001.57