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compared with the Southern, although the variances differ significantly. Expressed
as standard deviation, they have about one-fourth (between 20% and 40%) larger
values at South with respect to North (cf. F orster et al.
2008
).
The standard deviation of the flow direction (Fig.
4.4
, lower panel) shows
remarkable differences between IMF B
y
C
and IMF B
y
conditions for both
hemispheres. In the Northern Hemisphere, the flow is much more aligned for the
latter, with a minimum value for sector 5, where the wind amplitude maximizes. In
the Southern Hemisphere, there is a clear tendency for the opposite IMF
B
y
behavior
with smaller standard deviations for IMF B
y
C
, coinciding with the maximum wind
amplitudes. The formation of the large round-shaped plasma convection cell that is
present under IMF B
y
C
B
y
conditions at the Northern (Southern) Hemisphere
at the dusk side traces in the thermospheric wind pattern. It acts apparently as an
obstacle for the neutral wind flow across the polar region at high latitudes. The
curl-free noon-midnight aligned plasma flow seems on the contrary to operate as
a “pressure valve”, allowing the largest cross-polar thermospheric wind amplitudes
for IMF angle ranges corresponding to sector 5 at North and sector 3 at South.
4.3.3
Electric Fields and Thermospheric Winds
from UAM Modeling
We performed model calculations for both geomagnetically quiet and moderately
perturbed days during medium to high solar activity conditions near the fall equinox
on 12 and 28 October 2003, respectively. The latter (28 October) coincides with
our case study example day (see Fig.
4.7
, below), which is discussed in the next
section and compared there with direct observations of CHAMP during overflights
over the polar regions. This day has directly preceded the huge famous superpower
magnetic storm of 29-30 October 2003, the so-called Halloween superstorm, the
global complex modeling of which is a big challenge for solar-terrestrial research
(see, e.g., Toth et al.
2007
).
In our view, it represents mean geomagnetic conditions of 2003, although the
solar radiation conditions were somehow exceptional on this day. Some of the most
intense solar flares in recent history (measured in the X-ray range 0.1-0.8 nm)
occurred near the end of 2003. The solar flare event of 28 October 2003 at
11
UT was the fourth most intense (X17), launching a coronal mass ejection (CME)
that arrived Earth's environment on the subsequent day (
06 UT) causing the huge
geomagnetic storm (Tsurutani et al.
2005
). The mean EUV radiation level at that
time was moderate. Expressed with the 10.7-cm wavelength radio radiation fluxes as
proxy index, the 81-day average was about 140, while its daily value reached
271.
The X17 flare of 28 October 2003 determined a remarkable increase in the EUV
bands and X-ray bands and caused increased photoionization effects in the dayside
ionosphere, particularly in the lower ionospheric layers (Villante and Regi
2008
).
