Other
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufmsa71a..08r&link_type=abstract
American Geophysical Union, Fall Meeting 2002, abstract #SA71A-08
Other
0305 Aerosols And Particles (0345, 4801), 2439 Ionospheric Irregularities, 3332 Mesospheric Dynamics, 5729 Ionospheres (2459), 6984 Waves In Plasma
Scientific paper
Since several years very strong radar echoes from the upper polar summer mesosphere called `polar mesosphere summer echoes' (PMSE) are observed. These PMSE are difficult to understand since electron density irregularities must have been produced in the D region at very small spatial scales of meters only. Neutral air turbulence was proposed as the key mechanism for this structuring earlier but in-situ measurements have frequently shown the absence of turbulence at PMSE altitudes. We reconsider microphysical processes of electron diffusion in the presence of positive ions and negatively charged aerosols. We obtain analytical solutions of the coupled diffusion equations and find that the main characteristics of these solutions are in line with available in situ measurements of electrons, ions, aerosols, and neutral air turbulence. The lifetime of the plasma perturbations is proportional to the square of the aerosol particle radius. For example, the presence of particles with radii larger than ~10~nm allows for the existence of electron number density perturbations up to several hours after the creation mechanism has ceased. Contrary to other studies we find that this result is almost independent of the ratio between the aerosol charge number density and the number density of free electrons, again in agreement with observations. The electron perturbations potentially give rise to a radar reflectivity comparable to values observed with 50~MHz VHF radars. Our model readily explains why in situ measurements of neutral air turbulence have repeatedly shown active turbulence in some part of the PMSE layer whereas turbulence was basically absent at other parts. We compare our model results with ground-based observations of PMSE and find that the model yields the correct altitude profile of the mean PMSE occurrence frequency.
Luebken F.
Rapp Michael
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