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| Titel |
Non-stationarity of the quasi-perpendicular bow shock: comparison between Cluster observations and simulations |
| VerfasserIn |
H. Comişel, M. Scholer, J. Soucek, S. Matsukiyo |
| 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. 2 ; Nr. 29, no. 2 (2011-02-03), S.263-274 |
| Datensatznummer |
250016965
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| Publikation (Nr.) |
 copernicus.org/angeo-29-263-2011.pdf |
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| Zusammenfassung |
| We have performed full particle electromagnetic simulations of a quasi-perpendicular
shock. The shock parameters have been chosen to be appropriate for the
quasi-perpendicular Earth's bow shock observed by Cluster on 24 January 2001
(Lobzin et al., 2007). We have performed two simulations with different ion to
electron mass ratio: run 1 with mi/me=1840 and run 2 with mi/me=100.
In run 1 the growth rate of the modified two-stream instability (MTSI) is large
enough to get excited during the reflection and upstream gyration of part of the
incident solar wind ions. The waves due to the MTSI are on the whistler mode
branch and have downstream directed phase velocities in the shock frame.
The Poynting flux (and wave group velocity) far upstream in the foot is also
directed in the downstream direction. However, in the density and magnetic
field compression region of the overshoot the waves are refracted and the
Poynting flux in the shock frame is directed upstream. The MTSI is
suppressed in the low mass ratio run 2. The low mass ratio run shows
more clearly the non-stationarity of the shock with a larger time scale
of the order of an inverse ion gyrofrequency (Ωci): the
magnetic field profile flattens and steepens with a period of
~1.5Ωci−1. This non-stationarity is different
from reformation seen in previous simulations of perpendicular or
quasi-perpendicular shocks. Beginning with a sharp shock ramp the large
electric field in the normal direction leads to high reflection rate of
solar wind protons. As they propagate upstream, the ion bulk velocity
decreases and the magnetic field increases in the foot, which results
in a flattening of the magnetic field profile and in
a decrease of the normal electric field. Subsequently the reflection
rate decreases and the whole shock profile steepens again. Superimposed
on this 'breathing' behavior are in the realistic mass ratio case the
waves due to the MTSI. The simulations lead us to a re-interpretation of
the 24 January 2001 bow shock observations reported by Lobzin et al. (2007).
It is suggested that the high frequency waves observed in the magnetic
field data are due to the MTSI and are not related to a nonlinear phase
standing whistler. Different profiles at the different spacecraft are
due to the non-stationary behavior on the larger time scale. |
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