Abstract
On the nature of PMSE: Electron diffusion in the vicinity of charged particles revisited
On the nature of PMSE: Electron diffusion in the vicinity of charged particles revisited
Markus Rapp
Optical Soundings and Sounding Rockets,
Leibniz Institute of Atmospheric Physics,
Kühlungsborn,
Germany
Franz-Josef Lübken
Optical Soundings and Sounding Rockets,
Leibniz Institute of Atmospheric Physics,
Kühlungsborn,
Germany
Triggered by recent experimental evidence showing that some parts of the
Cho et al. [1992]
theory describing electron diffusion in the vicinity of charged aerosol particles cannot be correct, we reconsider the process
of electron diffusion under the conditions of the polar summer mesopause region. The key idea is that perturbations in the
distribution of charged aerosol particles created for example by neutral air turbulence almost immediately lead to (anticorrelated)
perturbations in the electron number density due to simple charge neutrality and zero net current arguments. We obtain analytical
solutions of the coupled diffusion equations for electrons, charged aerosol particles, and positive ions subject to the initial
condition of anticorrelated perturbations in the charged aerosol and electron distribution. The main signatures of these solutions
are in line with available in situ evidence of small-scale plasma structures in the vicinity of polar mesosphere summer echoes
(PMSE), i.e., electron perturbations are anticorrelated to both perturbations in the distributions of negatively charged aerosol
particles and positive ions. The lifetime of these perturbations is proportional to the square of the aerosol particle radius
such that 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 initial creation mechanism has stopped. These results are almost independent of the ratio between
the aerosol charge number density and the number density of free electrons. These electron perturbations potentially give
rise to a radar reflectivity comparable to values observed with 50 MHz VHF radars. Our model results can readily explain why
in situ measurements of neutral air turbulence have repeatedly shown active turbulence only in the upper part of the PMSE
layer whereas turbulence was basically absent in the lower part. Furthermore, our model concept qualitatively yields the correct
altitude profile of the mean PMSE occurrence frequency based on the measured altitude profile of the turbulence occurrence
frequency.
Published 7
February
2003.
Citation: Rapp, M., and F.-J. Lübken
(2003),
On the nature of PMSE: Electron diffusion in the vicinity of charged particles revisited,
J. Geophys. Res.,
108(D8),
8437,
doi:10.1029/2002JD002857.