The Study Of Energetic Particles' Acceleration Mechanism At Shocks With Magnetic Turbulence

We calculated large amount of test particles' trajectories at MHD shocks of different geometries(θ_BN～[0,90°]) with magnetic turbulence.At perpendicular shocks, we investigated the effect of shock strength and particles' initial energy on the average energy change per shock-crossing. We studied test particle's energy gain from shock drift acceleration(SDA) under different conditions. Based on our numerical results, we gave a theoretical formula for SDA, which agrees with our simulation results. We showed that, although magnetic turbulence was included, particles would still be accelerated mainly by SDA at perpendicular shock, while the diffusive shock acceleration, based on the guiding center assumption, is not valid.At parallel shocks, we studied test particle's energy gain from both SDA and diffusive shock acceleration(DSA) under different conditions. The numerical results showed that the times of shock-crossing and the energy of some particles increased drastically with greater energy gain at parallel shocks. With the magnetic field turbulence included, particles would be affected by DSA more significantly, which was based on the guiding center assumption at parallel shock than they would at perpendicular shocks. However, DSA would not be the main reason of particles' acceleration at parallel shocks. On the other hand, SDA, which might be caused by the randomly induced electric field generated by the magnetic field turbulence, would be more effective for the particles' remarkable energy gain while crossing the shock frequently.At oblique shocks with differentθ_BN, we showed that particles' average energy gain per shock-crossing had two peaks, at parallel and perpendicular shocks respectively. The peak at perpendicular shocks was much higher, but parallel shocks could accelerate particles to much higher energy in the end than shocks with any otherθ_BN could. We also found that particles could be accelerated by two Both SDA and DSA at oblique shocks withθ_BN～0&90°, however, the SDA would be the main factor while the effect of DSA was less than our expectation of paticle acceleration at shocks.In conclusion, it would be unreasonable to analyze particles' acceleration at shocks in the time and length scale larger than those in particles' gyration. Models about particles' acceleration in much smaller time and length scale should be constructed instead. Furthermore, the DSA, based on the guiding center assumption, is not valid at perpendicular shock, while it is not as effective for particles' acceleration as SDA in parallel shock. In addition, although particles' energy gain per shock crossing is larger at perpendicular shock, it is parallel shock which might accelerate particles most dramatically in the end(more than 10 times), if compared with oblique shocks of arbitraryθ_BN.