Ion Dynamics And Evolution Of Structures In Collisionless Shocks

Collisionless shocks in universe are of important interests, and treated as accelerators with high efficiency. For a particle, it can be accelerated diffusively by the first-order Fermi acceleration mechanism at a shock, and the acceleration mechanism is also called as Diffusive Shock acceleration (DSA). Therefore, shocks are commonly believed to be important sources for energetic particles, such as Galactic Cosmic Rays (GCRs) and Anomolous Cosmic Rays (ACRs), and become valuable physical structures which have been paid much attention on for decades. In this thesis, we performed hybrid simulations in order to investigate the excitation of waves in downstream of quasi-perpendicular shocks, behaviors of particles and formation of downstream High-speed Jets at a quasi-parallel shock. The main results are summaried as follows:1. Bunched ring-like distributions in the downstream of low Mach number quasi-perpendicular shocksAt low Mach number shocks, protons will form a bunched ring-like distribution after directly transmiting to the downstream side. The distribution results in oscillation of average velocity of protons and magnetic field, which make the pressure keep in constant. By using two-dimensional hybrid simulation with helium ions, behaviors of them are investigated. Results show that the radius of the bunched ring-like distribution of helium ions in phase diagram is larger than that of protons, and the relaxation of helium ions are slower.2. Excitation of downstream waves and relaxation of anisotropy distribution in quasi-perpendicular shocksWith two-dimensional hybrid simulation, excitation of waves are analyzed in downstream of quasi-perpendicular shocks with helium ions. At low Mach number shocks, bunched ring-like distributions of protons and helium ions are formed when they get downstream immediately. The distribution leads to temperature anisotropy that is T⊥>T∥, which will drive ion cyclotron waves and mirror waves under some specific conditions. However, bunched ring-like distributions of protons are relaxed by oscilation of magnetic field and have not driven ion cyclotron waves in dowstream. In contrast with protons, helium ions relax and excite ion cyclotron waves in further downstream. Helium ions are scattered to be a shell-like distribution by these waves and evolves into a bi-Maxwellian distribution finally. In medium Mach number shocks, both protons and helium ions excite ion cyclotron waves in downstream with temperature anisotropy. And helium cyclotron waves are driven in further downstream and scatter helium ions to be a shell-like distribution and a bi-Maxwellian distribution finally. For both low and medium Mach number shock, a shell-like distribution of helium ions means helium cyclotron waves are dominant in downstream, and mirror waves have not been excited due to low plasam beta. On the contrary, the amplitude of ion cyclotron wave driven by protons and helium ions is sufficiently strong so that a shell-like distribution of helium ions can be generated by either of them in downstream of a high Mach number shock. In addition, mirror waves are excited in downstream with high plasma beta.3. Ion dynamics and formation of downstream High-speed Jets in quasi-parallel shocksIn quasi-parallel shocks, ultralow frequency waves result from the interaction between backstreaming particles and injected plasma and are convected back to shock front. These waves contribute to the formation of ripples along shock front when they encounter it. As a consequence, the reformation of the shock is not coherent and ion dynamics along shock front behave differently. The results from two-dimensional hybrid simulation show that injected particles are easier to be reflected by the shock in lower part of the ripple. And, in upper part of the ripple, injected particles tend to transmit through the shock front directly and form a downstream high-speed jet with larger bulk velocity. The characteristics of high-speed jet in simulation are in agreement with experiment observation.4. Particle acceleration in quasi-parallel shocks with two-dimensional simulationIn our simulation, all the particles, which will be accelerated later, must be reflected when they meet the shock front for the first time and suffer several stage acceleration. At the beginning, particles move along the shock front and get accelerated, which is the first stage acceleration; later, for the second accelerating stage, they leave the shock front and are trapped between upstream waves and the shock front; then, some of them go to further upstream or further downstream, and others are trapped again by by further upstream waves and downstream waves and suffer more than two stages of acceleration. Besides, the last stage is similar to the description of Diffusive Shock Acceleration and the energy of particles suffering three accelerating stages is larger than others. Finally, they also move to futher upstream or further downstream finally.