Geoscience Reference
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Based on global first-principle modeling, Lin et al. ( 2005 ) performed various
numerical experiments to identify the relative importance of storm-time ionospheric
drivers, primarily for mid- and low-latitude regions, and with respect to the
equatorial ionization anomaly. Lei et al. ( 2011 ) analyzed thermospheric density
observations of the CHAMP and GRACE satellites to study the recovery of
the thermosphere during the October 2003 storm. They found that neither the
empirical models such as NRL-MSISE00 nor the global theoretical modeling with
TIEGCM reproduced the observed rapid thermosphere relaxation times adequately.
Comparing the CHAMP observations with model predictions, Sutton et al. ( 2005 )
had also demonstrated some empirical model shortcomings of the NRL-MSISE00
thermospheric density and HWM-93 neutral wind predictions in the course of
this disturbance event. As global simulation tasks, one has to keep in mind the
complexity and the global interrelations of the described phenomena, although
the focus of our studies is on the high-latitude upper atmosphere physics and
its relationship to external drivers. UAM performs calculations of the neutral
and ionized components of the upper atmosphere self-consistently and starts the
simulations at a quasi-equilibrium state obtained beforehand.
Figures 4.8 and 4.9 show UAM model results for the same four time moments
(06, 12, 18, and 24 UT) of the case study day 28 October 2003, at high latitudes
of the Northern Hemisphere. Figure 4.8 presents the calculated horizontal electric
potential distributions, and Fig. 4.9 shows the corresponding results for the dynamic
response of the thermospheric neutral wind with snapshots at the same time
moments. Note that the isolines of the electric potential distributions in Fig. 4.8
correspond practically to the almost horizontal electromagnetic drift trajectories of
the F-layer plasma at high latitudes.
Figure 4.8 clearly shows a strengthening of the potential difference across the
polar cap at 18 and 24 UT. These increased plasma drift velocities come along
with accordingly enhanced horizontal thermospheric wind speeds in Fig. 4.9 .The
comparison of the neutral wind patterns in Fig. 4.9 with the statistical average
of 2003 in Fig. 4.2 corroborates the similarity of the modeled thermospheric
circulations during moderate geomagnetic disturbances with the mean pattern and
the approximate coincidence of the characteristic speed values. They exceed the
statistical average pattern somehow during the more active periods (18 and 24 UT),
and the peak values are at different places compared with the mean pattern (see
Fig. 4.2 ); this is indicative for the mutual dynamic reactions between the neutrals
and the plasma motion. At 18 UT we find the maximum neutral wind speed in the
afternoon sector at about 70 ı magnetic latitude, whereas at 24 UT the neutral wind
velocities maximize over the central polar cap.
The calculated vector plots of the thermospheric circulation at a 400-km altitude
as shown in Fig. 4.9 for four different UT moments are related both to the
characteristic disturbance effects (i.e., essential the IMF dependencies, cf. Haaland
et al. 2007 ;Weimer 2005 ) and also to a regular (background) variation, which is the
result of the offset between geographic and geomagnetic coordinates. This offset
causes a regular variation of the solar luminosity with respect to its coverage of
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