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| Titel |
Comparison of Birkeland current observations during two magnetic cloud events with MHD simulations |
| VerfasserIn |
H. Korth, B. J. Anderson, J. G. Lyon, M. Wiltberger |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
0992-7689
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| Digitales Dokument |
URL |
| Erschienen |
In: Annales Geophysicae ; 26, no. 3 ; Nr. 26, no. 3 (2008-03-26), S.499-516 |
| Datensatznummer |
250016040
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| Publikation (Nr.) |
 copernicus.org/angeo-26-499-2008.pdf |
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| Zusammenfassung |
| Low altitude field-aligned current densities obtained from global
magnetospheric simulations are compared with two-dimensional
distributions of Birkeland currents at the topside ionosphere
derived from magnetic field observations by the constellation of
Iridium satellites. We present the analysis of two magnetic cloud
events, 17–19 August 2003 and 19–21 March 2001, where the
interplanetary magnetic field (IMF) rotates slowly
(~10°/h) to avoid time-aliasing in the magnetic
perturbations used to calculate the Birkeland currents. In the
August 2003 event the IMF rotates from southward to northward while
maintaining a negative IMF By during much of the
interval. During the March 2001 event the IMF direction varies from
dawnward to southward to duskward. We find that the distributions of
the Birkeland current densities in the simulations agree
qualitatively with the observations for northward IMF. For southward
IMF, the dayside Region-1 currents are reproduced in the simulation,
but appear on average 5° further poleward of their observed
location, while the nightside Region-1 currents and the Region-2
currents are largely under-represented. Comparison of the observed
and simulated Birkeland current distributions, which are intimately
related to the plasma drifts at the ionosphere, shows that the
ionospheric convection pattern in the MHD model and its dependence
on the IMF orientation is essentially correct. The Birkeland total
currents in the simulations are about a factor of 2 larger than
observed during southward IMF. For Bz>0 the
disparity in the total current is reduced and the simulations for
purely northward IMF agree with the observations to within 10%. The
disparities in the magnitudes of the Birkeland currents between the
observations and the simulation results are a combined effect of the
simulation overestimating the ionospheric electric field and of the
Iridium fits underestimating the magnetic perturbations. The
marginal difference in the latitude resolution of the ionospheric
grids in the simulations and the observations is shown to have only
secondary effect on the magnitudes of the Birkeland currents. The
electric potentials in the simulation for southward IMF periods are
twice as large as those obtained from measurements of the plasma
drift velocities by DMSP, implying that the reconnection rates in
the simulation are too large. |
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