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| Titel |
Magnetic clouds' structure in the magnetosheath as observed by Cluster and Geotail: four case studies |
| VerfasserIn |
L. Turc, D. Fontaine, P. Savoini, E. K. J. Kilpua |
| 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 ; 32, no. 10 ; Nr. 32, no. 10 (2014-10-15), S.1247-1261 |
| Datensatznummer |
250121119
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| Publikation (Nr.) |
 copernicus.org/angeo-32-1247-2014.pdf |
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| Zusammenfassung |
| Magnetic clouds (MCs) are large-scale magnetic flux ropes ejected from the
Sun into the interplanetary space. They play a central role in
solar–terrestrial relations as they can efficiently drive magnetic activity
in the near-Earth environment. Their impact on the Earth's magnetosphere is
often attributed to the presence of southward magnetic fields inside
the MC, as observed in the upstream solar wind. However, when they arrive in
the vicinity of the Earth, MCs first encounter the bow shock, which is
expected to modify their properties, including their magnetic field strength
and direction. If these changes are significant, they can in turn affect the
interaction of the MC with the magnetosphere. In this paper, we use data from
the Cluster and Geotail spacecraft inside the magnetosheath and from the
Advanced Composition Explorer (ACE) upstream of the Earth's environment to
investigate the impact of the bow shock's crossing on the magnetic structure
of MCs. Through four example MCs, we show that the evolution of the MC's
structure from the solar wind to the magnetosheath differs largely from one
event to another. The smooth rotation of the MC can either be preserved
inside the magnetosheath, be modified, i.e. the magnetic field still rotates
slowly but at different angles, or even disappear. The alteration of the
magnetic field orientation across the bow shock can vary with time during the
MC's passage and with the location inside the magnetosheath. We examine the
conditions encountered at the bow shock from direct observations, when
Cluster or Geotail cross it, or indirectly by applying a magnetosheath model.
We obtain a good agreement between the observed and modelled magnetic field
direction and shock configuration, which varies from quasi-perpendicular to
quasi-parallel in our study. We find that the variations in the angle between
the magnetic fields in the solar wind and in the magnetosheath are
anti-correlated with the variations in the shock obliquity. When the shock is
in a quasi-parallel regime, the magnetic field direction varies significantly
from the solar wind to the magnetosheath. In such cases, the magnetic field
reaching the magnetopause cannot be approximated by the upstream magnetic
field. Therefore, it is important to take into account the conditions at the
bow shock when estimating the impact of an MC with the Earth's environment
because these conditions are crucial in determining the magnetosheath
magnetic field, which then interacts with the magnetosphere. |
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