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| Titel |
MESSENGER Observations of Magnetic Flux Ropes in Mercury's Plasma Sheet |
| VerfasserIn |
Gina A. DiBraccio, James A. Slavin, Suzanne M. Imber, Daniel J. Gershman, Jim M. Raines, Scott A. Boardsen, Brian J. Anderson, Haje Korth, Thomas H. Zurbuchen, Ralph L. Jr. McNutt, Sean C. Solomon |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250092471
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| Publikation (Nr.) |
 EGU/EGU2014-6821.pdf |
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| Zusammenfassung |
| MESSENGER orbital observations provide a new opportunity to investigate magnetic
reconnection in the cross-tail current sheet of Mercury’s magnetotail. Here we use
measurements collected by the Magnetometer and Fast Imaging Plasma Spectrometer
(FIPS) during “hot seasons,” when the orbital periapsis is on Mercury’s dayside
and MESSENGER crosses the plasma sheet at distances of ~1.5 to 3 RM (where
RM is Mercury’s radius, or 2440 km). These data frequently contain signatures of
large-scale magnetic reconnection in the form of plasmoid-type magnetic flux ropes and
southward magnetic fields in the post-plasmoid plasma sheet. In the cross-tail current
sheet, which separates the north and south lobes of the magnetotail, flux ropes are
formed by reconnection at two or more X-lines and are then transported either
toward or away from the planet by the Alfvénic flow emanating from the X-lines.
Here we present a survey of 49 plasmoid-type flux ropes identified during seven
MESSENGER “hot seasons,” for which minimum variance analysis indicates that
the spacecraft passed near the central axis of the structure. The locations of the
selected flux ropes range between 1.7 and 2.8 RM down the tail from the center of
the planet. With FIPS measurements, we determined an average proton density
of 2.55 cm-3 in the adjacent plasma sheet surrounding the flux ropes, implying
an Alfvén speed of ~450 km s-1. Under the assumption that the flux ropes are
moving at the local Alfvén speed, we used the mean duration of 0.74 ± 0.15 s
to calculate a typical diameter of ~0.14 RM, or ~340 km. We have modeled the
plasmoids as force-free flux ropes in order to confirm this result. A superposed epoch
analysis demonstrates that the magnetic structure of the flux ropes is similar to what is
observed at Earth, but the timescales are 40 times faster at Mercury. The results of this
flux rope survey indicate that intense magnetic reconnection occurs frequently in
the cross-tail current layer of this small but extremely dynamic magnetosphere. |
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