Radar Scattering from the Summer Polar Mesosphere: Theory and Observations
The anomalously large radar reflectivities observed in the summer
polar mesosphere have eluded satisfactory explanation until now. We
propose that the following chain of causality is responsible for the
so-called polar mesosphere summer echoes (PMSE): The uniquely low
temperatures in the summer mesopause produce ice aerosols. Because the
aerosols exist in a plasma, they become electrically charged. The
ambient electrons become coupled to the aerosols through electric
fields and their effective diffusivity is retarded due to the large
size of the aerosols. The reduction in diffusivity allows electron
density inhomogeneities to be maintained at the radar Bragg
scales. The radar waves are then scattered by the inhomogeneities. We
support the above concept by developing a quantitative theory of
ambipolar diffusion in the mesosphere. We then apply the results to
isotropic turbulence and Fresnel radar scatter to show that the
observed radar reflectivities can be explained by the theory. We show
that the presence of realistic charged aerosols are sufficient to
explain PMSE. We also show that dressed aerosol radar scatter,
proposed by others as a generation mechanism for PMSE, can only apply
to echoes detected by UHF radars. We present data taken with the
Sondrestrom 1.29-GHz radar, which we believe to be the first PMSE
event observed above one gigahertz, and attribute it to dressed
aerosol scatter. In the summer of 1991, we used the Cornell University
portable radar interferometer (CUPRI) to observe the mesosphere while
rockets carrying in situ sensors were flown through two PMSE
occurrences and a noctilucent cloud/PMSE event. We present a selection
of first results from this campaign (NLC-91). The first simultaneous
height comparison between noctilucent clouds and PMSE show that the
radar scattering region was near or slightly above the visible cloud
layer. We also infer from aspect sensitivity measurements and Doppler
spectrograms that there were two distinct types of PMSE: enhanced
turbulent scatter and partial (Fresnel) reflection from steep edges in
the electron density. Both mechanisms require an anomalously low
electron diffusion coefficient.
Use interlibrary loan to obtain a copy of this thesis from the Cornell
University library system.
Return to publications list.