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| Titel |
Origin and acceleration mechanisms of backstreaming ion populations in the Earth's quasi-perpendicular Ion Foreshock: Full-particle 2D simulation results |
| VerfasserIn |
Philippe Savoini, Bertrand Lembège |
| 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 |
250088656
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| Publikation (Nr.) |
 EGU/EGU2014-2793.pdf |
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| Zusammenfassung |
| The ion foreshock located upstream of the Earth’s bow shock is populated with
ions reflected back by the shock front with an high energy gain. In-situ spacecraft
measurements have clearly established the existence of two distinct populations
in the foreshock upstream of the quasi-perpendicular shock region (i.e. for 45°
-¤ ÎBn -¤ 90°, where ÎBnis the angle between the shock normal and the upstream
magnetostatic field): (i) field-aligned ion beams (or “FAB”) characterized by a gyrotropic
distribution, and (ii) gyro-phase bunched ions (or “GPB”) characterized by a
NON gyrotropic distribution, which exhibits a non-vanishing perpendicular bulk
velocity.
The use of 2D PIC simulations of a curved shock, where full curvature effects, time of
flight effects and both electrons and ions dynamics are fully described, has evidenced that
the shock front itself can be the possible source of the different backstreaming
ions.
Our analysis evidences the importance of the interaction time (Δinter) with the shock
front, in particular “GPB” and “FAB” populations are characterized by a short (Δinter=
1 to 2 Ïci) and much larger (Δinter= 1 to 10 Ïci) interaction time respectively, where
Ïci is the ion upstream gyroperiod. This discrimination allows a deeper statistical
analysis:
(i) backstreaming ions are splitted into both “FAB” and “GPB” populations
depending on their injection angle when hitting the shock front (i.e. defined between the local
normal to the shock front and the gyration velocity vector at the time ions hit the
front).
(ii) As a consequence, ion trajectories strongly differ between the “FAB” and “GPB”
populations at the shock front. In particular,“FAB” ions suffer multi-bounces along the
curved front whereas “GPB” ions make only one bounce. Such differences can explain
why the “FAB” population loses their gyro-phase coherency and become gyrotropic which
is not the case for the “GPB”.
As also evidenced by these simulations, the origin of both populations can be
associated directly to their interaction with the shock front itself and do not require any
upstream instability which can be another source for such backstreaming ions. |
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