Geoscience Reference
In-Depth Information
4.3
High-Latitude Thermospheric Winds: CHAMP Satellite
Observations and UAM Modeling
4.3.1
Magnetosphere-Ionosphere Coupling Under External
Forcings
The largest magnitudes of neutral wind speed anywhere on the globe were found
to occur within the high-latitude upper thermosphere. Long-term observations
with ground-based Fabry-Perot interferometers (FPI) located at Thule and Søndre
Strømfjord, Greenland, for example, showed typical wind speeds of about 200 m/s
at solar minimum, rising to about 800 m/s at solar maximum, depending on the
geomagnetic activity level (Killeen et al.
1995
).
The high-latitude upper atmosphere and its embedded ionized layers are known
to be controlled by both solar EUV radiation and magnetospheric processes,
as precipitations of energetic particles and plasma convection, driven mainly
by reconnection processes at the magnetopause and in the magnetospheric tail.
Thermospheric winds respond rather directly via ion drag to plasma motions in
the ionosphere that are imposed by the solar wind-magnetosphere interaction
and the resulting large-scale magnetospheric plasma drift at high latitudes. The
thermospheric drag mode is particularly efficient with respect to the main outer
magnetospheric generator processes, mediated primarily via the Region-1 current
system of Iijima and Potemra. Vasyli unas (
2007
) talks in this context about
the mechanical advantage of the magnetosphere: the Lorentz force in the outer
magnetosphere is coupled with a Lorentz force in the ionosphere in such a way
that it becomes amplified by a factor given approximately by the square root of
the magnetic field magnitude ratio. The electromagnetic forces are very effective,
as has been shown, for example, with the energy input estimation by Siscoe and
Siebert (
2006
), concerning the strong neutral air accelerations during a particular
superstorm event, the so-called Bastille storm of mid-July (by accident, during this
day the CHAMP satellite was launched).
Relative motions of the neutral gas and ionized components result in Joule
heating of the ionosphere and thermosphere, being proportional to the square of
the velocity difference. A recent study showed mesoscale structures and variations
of the thermosphere on the scales of tens of kilometers and minutes that indicate a
very dynamic response to ionospheric forcing (Aruliah et al.
2004
).
Coupling between the ionized layers of the upper atmosphere and the ther-
mospheric neutral gas is mediated through magnetic field changes and both
field-aligned and ionospheric currents. This electrodynamic system is driven by
processes of momentum and energy transfer, mainly reconnection, between the
highly variable solar wind and its inherent interplanetary magnetic field with the
magnetosphere and by inner magnetospheric plasma processes. These processes are
particularly important at high latitudes, and they constitute a vital driving force for
the transpolar thermospheric wind circulation.
