Physics
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmsh43b1510a&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #SH43B-1510
Physics
7514 Energetic Particles (2114), 7845 Particle Acceleration, 7851 Shock Waves (4455)
Scientific paper
We have studied the electron dynamics in high Mach number quasi-perpendicular collisionless shock waves by utilizing a one dimensional electromagnetic particle-in-cell code. The result of numerical simulation with θBn = 80 has shown that energetic electrons are generated through electron surfing acceleration (ESA) as well as purely perpendicular shocks. In addition to this, we have found that energetic electrons escaping to the upstream direction with higher energies than those observed in purely perpendicular shocks. Besides, their energies are also higher than those found in the downstream. Consequently, their higher energies cannot be explained by the leakage from the downstream. Thus, we can conclude that the energetic electrons are accelerated in the transition region and preferentially reflected back to the upstream. The observed energetic electrons orbits indicate that at first, they are accelerated perpendicular to the magnetic field at the leading edge of the transition region by ESA. Thus, their pitch angles increase. As a result, they are easily reflected by the mirror force around the magnetic overshoot where the compression ratio becomes maximum. The resulting particle energies are several times higher than those produced by ESA. The latter acceleration process accompanied by the reflection is known as shock drift acceleration (SDA) and have been extensively studied (e.g. Wu 1984; Leroy &Mangeney 1984). SDA can be understood as the mirror reflection process in the de Hoffman Teller frame where the mortional electric field vanishes. Therefore, the theory of SDA predicts that as the shock angle approaches to 90 degree, the acceleration efficiency (or the maximum energy) increases, whereas the number of accelerated particles decreases. In contrast to this prediction, our simulation results depict that the preacceleration perpendicular to the ambient magnetic field due to ESA increases the number of reflected electrons, hence the maximum energy. Because of this, electron acceleration efficiency in high Mach number quasi-perpendicular shocks will be higher than expected from the theories of SDA or ESA.
Amano Takanobu
Hoshino Masahiro
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