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| Titel |
Viscosity in the thermosphere: Evidence from gravity wave, neutral wind and direct lab measurements that the standard viscosity coefficients are too large in the thermosphere; and implication for gravity wave propagation in the thermosphere |
| VerfasserIn |
Sharon Vadas, Geoff Crowley |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250146389
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| Publikation (Nr.) |
 EGU/EGU2017-10413.pdf |
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| Zusammenfassung |
| In this paper, we review measurements of 1) gravity waves (GWs)
observed as traveling ionospheric disturbances (TIDs) at z~283 km
by the TIDDBIT sounder on 30 October 2007, and 2) simultaneous
rockets measurements of in-situ neutral winds at z~320-385 km. The
neutral wind contains a ~100 m/s peak at z~325 km in the same direction as
the GWs, but oppositely-directed to the diurnal tides. We hypothesize
that several of the TIDDBIT GWs propagated upwards and created this neutral
wind peak. Using an anelastic GW ray trace model which includes
thermospheric dissipation from molecular viscosity and thermal conductivity
with mu proportional to the temperature to the power of 0.7, we forward
ray trace the GWs from z_i=220 km. Surprisingly, the GWs dissipate
below z~260 km, well below the altitude they were observed. Furthermore,
none of the GWs could have propagated high-enough to create the neutral
wind peak. In our opinion, this constitutes a significant discrepancy
between observations and GW dissipative theory. We perform sensitivity
experiments to rule out background temperature and wind effects as being
the cause. We propose a modification to the formula for mu, and show
that this yields ray trace results that agree reasonably well with the
observations. We examine papers and reports for laboratory experiments
which measured mu at low pressures, and find similar results. We conclude
that the standard formulas for mu routinely used in thermospheric models
must be modified in the thermosphere to account for this important effect.
We also show preliminary GW ray trace results using this modified formula
for mu, and compare with previous theoretical results. |
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