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460 km, and it is managed by the GFZ German Research Centre for Geosciences,
Helmholtz Centre Potsdam. This mission was designed to perform detailed studies
of the Earth's gravitational and geomagnetic field with unprecedented accuracies
and space-time resolutions as well as GPS atmosphere and ionosphere profiling
(Reigber et al. 2002 ). One key scientific instrument on board CHAMP was a triaxial
accelerometer for in situ measurements at F-layer heights ( 300-450 km). Located
at the spacecraft's center of mass, it effectively probed the in situ acceleration with
an accuracy of 3 10 9 ms 2 (Doornbos et al. 2010 ). Accelerometers carried
by satellites provide valuable data for improving our understanding of the thermo-
spheric density and neutral winds at upper atmosphere altitudes (> 250 km).
From the air drag observations, thermospheric mass density and cross-track
neutral wind have been obtained using a first set of calibrations and, as far as
it concerns the wind measurements, a simplified methodology (Liu et al. 2006a ).
This method neglects lift and sideways forces on the spacecraft or requires that
these forces were modeled and removed from the acceleration beforehand, as was
done later by Sutton et al. ( 2007 ), who named it a “dual-axis method.” Using
these first cross-track wind estimations, a preliminary study on the algorithm to
deduce statistical pattern of the high-latitude thermospheric wind circulations for
1 year of moderate to high solar activity (2003) and its dependence on the IMF
orientation has been performed by Forster et al. ( 2008 ). This analysis has been
repeated here in Fig. 4.2 (cf. also Forster et al. 2011 ) with a newly calibrated
CHAMP data set, performed recently within an European Space Agency (ESA)
project study, where an improved methodology of neutral wind determination was
implemented. The new method employs a sophisticated iterative algorithm for
determining density and the crosswind component simultaneously from multi-axis
accelerometer measurements. It makes use of detailed numerical models of the
spacecraft's surface interaction with various radiation sources and aerodynamic
forces (Doornbos et al. 2010 ).
Figure 4.2 shows the average circulation at the Northern Hemisphere for
moderate to high solar activity conditions of the full year 2003 without sorting
for any IMF dependence as measured by the CHAMP accelerometer (for the south
polar cap see, e.g., Forster et al. 2011 ). The method for deriving full vector plots
from the single-component (cross-track) measurements was explained by Forster
et al. ( 2008 ). Some peculiarities, which are the result of the tight coupling to the
plasma drift and the specific high-latitude electrodynamics, are already obvious
in the average circulation pattern. Among them are the large-amplitude anti-solar
cross-polar streaming over the central polar cap with the occurrence of an extended
clockwise vortex in the dusk sector, a stream stagnation region equatorward of the
cusp position, and some deflection in the counterclockwise sense on the dawn side.
The large dusk side circulation cell is nearly co-located with the position of the
plasma drift dusk cell for southward IMF (see Fig. 4.1 , left panel). Vorticity studies
of the high-latitude neutral wind confirm the circulation cell structure with opposite
vorticity orientations at the dawn and dusk sides, some prevalence in magnitude of
the dusk over the dawn side vortex, and the strong dependence of the circulation
pattern on IMF B z and B y (Forster et al. 2011 ).
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