Numerical Simulation Study Of Particle Acceleration Mechanism At Interplanetary Shocks

It has been widely accepted that collisionless shock waves are strong sources of high-energy particles in space.The popular diffusive shock acceleration（DSA）theory,firstly proposed in the late 1970s by four independent groups,predicts a power-law distribution downstream of the shock with a spectral exponent that depends only on the shock compression ratio,and explains successfully the ob-served cosmic-ray spectrum.The shock acceleraiton mechanism includes mainly the first-order Fermi acceleration and shock drift acceleration（SDA）,which dom-inate in quasi-parallel and quasi-perpendicular shocks,respectively.In this thesis,we adopt test particle simulations to study particle acceleration at collisionless shocks.Firstly,we investigate proton acceleration by quasi-perpendicular shocks un-der a prescribed magnetic turbulence model.We select a series of quasi-perpendicular shock events withθ_Bn≥75^?during 1998–2005 from the ACE shock lists.With the observed shock parameters,the trajectories of particles are obtained by solv-ing the particle motion equation with a time-backward method,and then we get the energy spectrum of accelerated particles at the shock front through the as-sumed initial kappa and Maxwellian distributions.For each shock event in the simulations,we obtain steady-state spectra and the acceleraiton time,t_acc,and obtain the injection energy,E_inj,by fitting steady-state energy spectra from simu-lations well with observations.The simulation results indicate that in our magnetic tubulence model,regardless of whether the adopted initial distribution is kappa or Maxwellian,interplanetary quasi-perpendicular shocks can accelerate thermal particles to high energies up to MeV,and that for those quasi-perpendicular shocks with higher upstream speed and larger shock-normal angle approximate to 90^?,the acceleration time is shorter.Next,we study electron acceleration by shocks and discuss the effects of dif-ferent shock geometries and magnetic turbulence levels on the energy spectra of accelerated particles.Since low-energy electrons resonate within the dissipation range of the turbulence,we assume that the turbulence is composed of an iner-tial range and a dissipation range.It is found that at parallel and perpendicular shocks the acceleration of electrons is enhanced under the high and low turbulence levels,respectively,while at oblique shocks the acceleration is weak regardless of the magnitude of turbulence levels.We conclude that the particle acceleration at perpendicular shocks originates mainly from SDA,and at parallel shocks both SDA and first-order Fermi acceleration get enhanced in the case of a large mag-netic turbulence.We also found that the perpendicular shock acceleration with a large turbulence level is more effective than parallel shock acceleration with a small turbulence.This is likely due to the facts that SDA is more effective than first-order Fermi acceleration,and that at a perpendicular shock with low turbu-lence level,the shock acceleration is mainly from SDA,but at a parallel shock with high turbulence level,only part of shock acceleration is from SDA.In general,the acceleration efficiency increases with the increase of shock-normal angle.We also investigate the effect of shock thickness on the electron acceleration at perpen-dicular shocks.By examining the dependence of electron acceleration efficiency on the shock thickness,we identify a bend-over thickness L_diff,bin the scale of particles gyro-radii for perpendicular shocks.The acceleration efficiency does not change evidently if the shock thickness is much smaller than L_diff,b,but starts to drop abruptly if the shock thickness is much larger than L_diff,b.In the condition of this work,our simulations show that the bend-over thickness,L_diff,b,is in the order of ion inertial length.Finally,we focus on the electron acceleration by perpendicular shocks and study the evolution of the relative variation of particle momentum,?p/p,during each crossing the shock front,and the rate of particle momentum relative variation,?p/（p?t）,with varying particle velocity v.The simulation results show that both?p/p and?p/（p?t）decrease with the increase of particle velocity,which agrees well with the theoretical results when the velocity is smaller than 2×10⁸m/s.When the velocity is larger than 2×10⁸m/s,the rate of particle momentum relative variation displays a more noticeable decreasing tendency with particle velocity,clearly reflecting the relativistic effect caused by Lorentz factor.