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| Titel |
Statistical visualization of the Earth's magnetotail based on Geotail data and the implied substorm model |
| VerfasserIn |
S. Machida, Y. Miyashita, A. Ieda, M. Nosé, D. Nagata, K. Liou, T. Obara, A. Nishida, Y. Saito, T. Mukai |
| 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 ; 27, no. 3 ; Nr. 27, no. 3 (2009-03-02), S.1035-1046 |
| Datensatznummer |
250016437
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| Publikation (Nr.) |
 copernicus.org/angeo-27-1035-2009.pdf |
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| Zusammenfassung |
We investigated the temporal and spatial development of the near-Earth
magnetotail during substorms based on multi-dimensional superposed-epoch
analysis of Geotail data. The start time of the auroral break-up (t=0) of
each substorm was determined from auroral data obtained by the Polar and
IMAGE spacecraft. The key parameters derived from the plasma,
magnetic-field, and electric-field data from Geotail were sorted by their
meridional X(GSM)–Z(proxy) coordinates.
The results show that the Poynting flux toward the plasma-sheet center
starts at least 10 min before the substorm onset, and is further enhanced at
X~−12 RE (Earth radii) around 4 min before the onset. Simultaneously,
large-amplitude fluctuations occurred, and earthward flows in the central
plasma sheet between X~−11 RE and X~−19 RE and a duskward flow
around X=−10 RE were enhanced. The total pressure starts to decrease around
X=−16 RE about 4 min before the onset of the substorm. After the substorm
onset, a notable dipolarization is observed and tailward flows commence,
characterised by southward magnetic fields in the form of a plasmoid.
We confirm various observable-parameter variations based on or predicted by
the relevant substorm models; however, none of these can explain our results
perfectly. Therefore, we propose a catapult (slingshot) current-sheet
relaxation model, in which an earthward convective flow produced by catapult
current-sheet relaxation and a converted duskward flow near the Earth are
enhanced through flow braking around 4 min before the substorm onset. These
flows induce a ballooning instability or other instabilities, causing the
observed current disruption. The formation of the magnetic neutral line is a
natural consequence of the present model, because the relaxation of a highly
stretched catapult current-sheet produces a very thin current at its
tailward edge being surrounded by intense earthward and tailward magnetic
fields which were formerly the off-equatorial lobe magnetic fields. This
location is the boundary between a highly stressed catapult current sheet
and a Harris-type current sheet characterized by little stress. In addition,
the flows induced around the boundary toward the current-sheet center may
enhance the formation of the magnetic neutral line and the efficiency of
magnetic reconnection. After magnetic reconnection is induced, it plays a
significant role in driving the substorm. |
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