Further effects of charged aerosols on
summer mesospheric radar scatter
In an earlier paper we showed that charged aerosols play a crucial
role in enhancing radar echoes from the summer polar mesosphere
through reduced diffusion turbulent scatter and dressed aerosol
scatter [Cho et al., 1992]. Here we explore the effects of
charged aerosols on radar scatter through fossil turbulence and
electron density depletion layers. We find that the former can
produce radar scatter even after the decay of neutral gas turbulence.
The latter, which are likely produced by the scavenging of free
electrons by ice particles, are a candidate for causing partial
reflection or Fresnel scatter. Furthermore, we examine the mutual
aerosol interaction restriction on dressed aerosol scatter more
closely. We find that a high ambient electron density and low aerosol
number density are needed for effective dressed aerosol scatter to
occur. We then show that very small (less than 1 nm radii),
negatively charged aerosols enhance electron diffusivity, and thus
inhibit radar scatter. Also ice aerosol sedimentation, in light of
the reduced diffusion theory, leads us to conclude that the
statistical peak in PMSE power should be located between the mean
mesopause and the average NLC height, which agrees with observations.
Finally, we invoke time lags in the ice particle formation cycle to
account for the observed non-correlation between PMSE and NLC
occurrence.
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