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| Titel |
Polar mesosphere summer echoes (PMSE): Review of observations and current understanding |
| VerfasserIn |
M. Rapp, F.-J. Lübken |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 4, no. 11/12 ; Nr. 4, no. 11/12 (2004-12-21), S.2601-2633 |
| Datensatznummer |
250001434
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| Publikation (Nr.) |
 copernicus.org/acp-4-2601-2004.pdf |
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| Zusammenfassung |
| Polar mesosphere summer echoes (PMSE) are very
strong radar echoes primarily studied in the VHF
wavelength range from altitudes close to the
polar summer mesopause. Radar waves are scattered
at irregularities in the radar refractive index
which at mesopause altitudes is effectively determined
by the electron number density. For efficient
scatter, the electron number density must reveal
structures at the radar half wavelength (Bragg
condition for monostatic radars; ~3 m for
typical VHF radars). The question how such small
scale electron number density structures are
created in the mesopause region has been a
longstanding open scientific question for almost
30 years. This paper reviews experimental and
theoretical milestones on the way to an advanced
understanding of PMSE. Based on new experimental
results from in situ observations with sounding
rockets, ground based observations with radars
and lidars, numerical simulations with
microphysical models of the life cycle of
mesospheric aerosol particles, and theoretical
considerations regarding the diffusivity of
electrons in the ice loaded complex plasma of the
mesopause region, a consistent explanation for
the generation of these radar echoes has been
developed. The main idea is that mesospheric
neutral air turbulence in combination with a
significantly reduced electron diffusivity due to
the presence of heavy charged ice aerosol
particles (radii ~5–50 nm) leads to the
creation of structures at spatial scales
significantly smaller than the inner scale of the
neutral gas turbulent velocity field itself.
Importantly, owing to their very low diffusivity,
the plasma structures acquire a very long
lifetime, i.e., 10 min to hours in the presence
of particles with radii between 10 and 50 nm.
This leads to a temporal decoupling of active
neutral air turbulence and the existence of small
scale plasma structures and PMSE and thus readily
explains observations proving the absence of
neutral air turbulence at PMSE altitudes. With
this explanation at hand, it becomes clear that
PMSE are a suitable tool to permanently monitor
the thermal and dynamical structure of the
mesopause region allowing insights into important
atmospheric key parameters like neutral
temperatures, winds, gravity wave parameters,
turbulence, solar cycle effects, and long term
changes. |
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