The thermal and dynamical state of the atmosphere during polar mesosphere winter echoes F.-J. Lübken1, B. Strelnikov1, M. Rapp1, W. Singer1, R. Latteck1, A. Brattli2, U.-P. Hoppe2, and M. Friedrich3 1Leibniz Institute of Atmospheric Physics, Kühlungsborn, Germany 2Norwegian Defence Research Establishment (FFI), Kjeller, Norway 3Institute of Communications Networks and Satellite Communications, Graz University of Technology, Austria
Abstract. In January 2005, a total of 18 rockets were launched from the
Andøya Rocket Range in Northern Norway (69° N) into strong
VHF radar echoes called "Polar Mesosphere Winter Echoes" (PMWE).
The echoes were observed in the lower and middle mesosphere during large
solar proton fluxes. In general, PMWE occur much
more seldom compared to their summer counterparts PMSE
(typical occurrence rates at 69° N are 1–3% vs. 80%, respectively).
Our in-situ measurements by falling sphere, chaff, and instrumented payloads
provide detailed information about the thermal and dynamical
state of the atmosphere and therefore allow an unprecedented study
of the background atmosphere during PMWE.
There are a number of independent
observations indicating that neutral air turbulence has caused PMWE.
Ion density fluctuations show a turbulence spectrum within PMWE and
no fluctuations outside. Temperature lapse rates close to the adiabatic
gradient are observed in the vicinity of PMWE indicating persistent
turbulent mixing. The spectral broadening of radar echoes is consistent
with turbulent velocity fluctuations. Turbulence also explains the
mean occurrence height of PMWE (~68–75 km): viscosity increases
rapidly with altitude and destroys any small scale fluctuations
in the upper mesosphere, whereas electron densities are usually too low in the
lower mesosphere to cause significant backscatter. The seasonal variation of
echoes in the lower mesosphere is in agreement with a turbulence climatology
derived from earlier sounding rocket flights.
We have performed model calculations to study the radar backscatter
from plasma fluctuations caused by neutral
air turbulence. We find that volume reflectivities observed during PMWE
are in quantitative agreement with theory. Apart from turbulence the most crucial
requirement for PMWE is a sufficiently large number of electrons,
for example produced by solar proton events.
We have studied the sensitivity of the radar echo strength on various
parameters, most important electron number density and turbulence intensity.
Our observational and theoretical considerations do not provide any
evidence that charged aerosol particles are needed to explain PMWE,
in contrast to the summer echoes which owe their existence to
charged ice particles.
Citation: Lübken, F.-J., Strelnikov, B., Rapp, M., Singer, W., Latteck, R., Brattli, A., Hoppe, U.-P., and Friedrich, M.: The thermal and dynamical state of the atmosphere during polar mesosphere winter echoes, Atmos. Chem. Phys., 6, 13-24, doi:10.5194/acp-6-13-2006, 2006.