Geoscience Reference
In-Depth Information
7
Dynamics and Electrodynamics
of the Mesosphere
By definition, the mesosphere is the region of the atmosphere located between
the stratopause and the mesopause, between roughly 50 and 90 km, in which
the temperature decreases with height. The transition between the mesosphere
and the thermosphere is called the mesopause and is the altitude at which the
temperature reaches a minimum before increasing with height in the thermosphere.
The mesopause also delineates the boundary between the earth's neutral atmo-
sphere and the ionosphere. The latter is collocated with the thermosphere, which
has temperatures ten times higher than those at the mesopause. The primary sub-
ject in this chapter is the polar summer mesosphere which is by far the most
interesting mesospheric region. The highest clouds on earth are found here, and
the temperature often reaches values as low as 110K with one measurement as
low as 90K. This is clearly the coldest place on earth. Most of this chapter is
devoted to this region since it exhibits the very interesting electrodynamics and
plasma physics associated with dusty or icy plasmas. We present a brief discussion
of atmospheric electricity and upwardly propagating lightning and close with obser-
vations and theory of mesospheric bore waves, both of which are of great scientific
interest.
7.1 Noctilucent Clouds (NLC) and the Solstice
Temperature Anomaly
Interest in the polar summer mesosphere began on June 18, 1885, two years
after Krakatoa's eruption in 1883. A set of silvery-blue clouds was observed in
the Northern Hemisphere under twilight conditions when the sun was below
the horizon, but it was still illuminating the mesosphere—that is, just before
dawn or just after sunset (Gadsden and Schröder, 1989). Using photographic
triangulation, it was found that the height of these clouds was about 82 km—the
highest clouds ever seen on earth. Due to their unusual nighttime brightness, they
were named “noctilucent clouds” (NLC). It is believed that this major volcanic
eruption introduced a considerable amount of water vapor and possibly dust
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