 |
| Titel |
Production of non-gyrotropic and gyrotropic backstreaming ion distributions in the quasi-perpendicular ion foreshock region : Origin and acceleration mechanisms. |
| VerfasserIn |
Philippe Savoini, Bertrand Lembège |
| Konferenz |
EGU General Assembly 2015
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250112615
|
| Publikation (Nr.) |
 EGU/EGU2015-12780.pdf |
|
|
|
|
|
| Zusammenfassung |
| Collisionless shock waves are well-known structures in the environment of planetary
magnetospheres which decelerate the bulk flow of the solar wind and converts it in thermal
energy. Spacecrafts have firmly established the existence of the so-called foreshock region
magnetically connected to the shock and filled with energized particles backstreaming from
the shock front. One of the important unresolved problem is the exact origin of these
high-energy backstreaming charged particles.
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 45Ë
≈¤ ÎBn ≈¤ 90Ë , where ÎBn is the angle between the shock normal and the upstream
magnetostatic field): (i) field-aligned ion beams (or «ÂFAB») characterized by a gyrotropic
distribution, and (ii) gyro-phase bunched ions (or «ÂGPB») 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.
A recent analysis has evidenced that both populations can be discriminated in terms of
interaction time (δinter) with the shock front. "GPB" and "FAB" populations are
characterized by a short (δinter ~ 1 Ïci) and much larger (δinter ≈¥ 2 Ïci) interaction time
respectively, where Ïciis the ion upstream gyroperiod.
In addition, present statistical results evidence that:
(i) backstreaming ions are splitted into "FAB" and "GPB populations " depending
on their injection angle when hitting the shock front (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,Â"FAB" ions suffer multi-bounces along the
curved front whereas "GPB" ions make only one bounce. Such differences may explain why
the "FAB" population loses their gyro-phase coherency and become gyrotropic which is not
the case for the "GPB".
Then, the differences observed between “FAB” and “GPB” populations do not involve
some distinct reflection processes as often claimed in the literature but follow from different
particle time histories at the shock front. Both “FAB” and “GPB” ions suffer the same
reflection process but only “FAB” population loose their initial phase coherency by suffering
several bounces.
This important result was not expected and greatly simplifies the question of their origin. |
|
|
|
|
|
|