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| Titel |
Comparison of accelerated ion populations observed upstream of the bow shocks at Venus and Mars |
| VerfasserIn |
M. Yamauchi, Y. Futaana, A. Fedorov, R. A. Frahm, J. D. Winningham, E. Dubinin, R. Lundin, S. Barabash, M. Holmström, C. Mazelle, J.-A. Sauvaud, T. L. Zhang, W. Baumjohann, A. J. Coates, M. Fraenz |
| 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 ; 29, no. 3 ; Nr. 29, no. 3 (2011-03-10), S.511-528 |
| Datensatznummer |
250016988
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| Publikation (Nr.) |
 copernicus.org/angeo-29-511-2011.pdf |
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| Zusammenfassung |
| Foreshock ions are compared between Venus and Mars at energies of
0.6~20 keV using the same ion instrument, the Ion Mass Analyser, on
board both Venus Express and Mars Express. Venus Express often observes
accelerated protons (2~6 times the solar wind energy) that travel away
from the Venus bow shock when the spacecraft location is magnetically
connected to the bow shock. The observed ions have a large field-aligned
velocity compared to the perpendicular velocity in the solar wind frame, and
are similar to the field-aligned beams and intermediate gyrating component of
the foreshock ions in the terrestrial upstream region. Mars Express does not
observe similar foreshock ions as does Venus Express, indicating that the
Martian foreshock does not possess the intermediate gyrating component in the
upstream region on the dayside of the planet. Instead, two types of gyrating
protons in the solar wind frame are observed very close to the Martian
quasi-perpendicular bow shock within a proton gyroradius distance. The first
type is observed only within the region which is about 400 km from the bow shock
and flows tailward nearly along the bow shock with a similar velocity as the
solar wind. The second type is observed up to about 700 km from the bow shock and
has a bundled structure in the energy domain. A traversal on 12 July 2005,
in which the energy-bunching came from bundling in the magnetic field
direction, is further examined. The observed velocities of the latter
population are consistent with multiple specular reflections of the solar
wind at the bow shock, and the ions after the second reflection have a
field-aligned velocity larger than that of the de Hoffman-Teller velocity
frame, i.e., their guiding center has moved toward interplanetary space out
from the bow shock. To account for the observed peculiarity of the Martian
upstream region, finite gyroradius effects of the solar wind protons compared
to the radius of the bow shock curvature and effects of cold ion abundance in
the bow shock are discussed. |
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