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| Titel |
Shock Drift Acceleration in presence of turbulence: A simple model |
| VerfasserIn |
Karim Meziane, A. M. Hamza, M. Wilber, C. Mazelle |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250044256
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| Zusammenfassung |
| Analytic treatments of particle acceleration by collisionless shocks have commonly been
based on the assumption that the shock surface is quasi-planar with length scales larger than
the particle gyroradius. Within this framework, the derived particle distributions are not in
full agreement with the observations. Recent theoretical studies as well as numerical
simulations indicate that ion scales shock fluctuations could account for several features
observed in shock-associated energetic particle velocity distributions. We have developed a
simple model based on the shock drift acceleration mechanism in which both the shock
normal direction and the shock mirror ratio N = B2-B1 are subject to random
fluctuations. While, the maximum particle energy gain is dependent upon N, the pitch
angle distribution is dependent to both N and shock geometry. The fluctuations
could be induced by shock turbulence inherent to the shock nonstationarity occuring
for high Mach number and for a wide range of shock geometries. For low energy
particles, we derive the probability distributions functions f(v||) and f(v-¥) of ions
escaping upstream; we show that the distributions significantly deviate from the
conventional Maxwellian signature. At higher energy, the derived energy spectrum
extends to higher energy in comparison to the one obtained when the fluctuations are
suppressed. The derived results are compared to ion distributions and spectra observed
upstream of the Earth bow shock. They also in turn allow us to draw possible analogy
with neutral fluid turbulence involving higher moments of the particle distribution. |
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