Electron energization at quasi-perpendicular oblique shocks in Supernova remnants

由于电子的高辐射效率，大多数关于天体物理物体中粒子加速的现有信息都与电子有关。然而，对于电子是如何加速的，我们知道的还不多。电子参与超新星残余激波的扩散激波加速，要求它们被预加速到洛伦兹因子超过100。在哪里以及如何发生是这个项目的重点。超新星激波具有非相对论性的传播速度，并具有高音速和阿尔夫尼亚马赫数的特征。非相对论性激波的物理学是由粒子的反射控制的，粒子与入射等离子体的相互作用激发了激波上游的各种不稳定性。如果磁场的方向与激波法向准平行，离子可以移动很远，并在远上游区域驱动不稳定。对于准垂直激波，反射的离子速度不够快，基本上只经过一个拉莫尔轨道，但电子可能沿着磁场向远上游区域流动，并在那里驱动波。在严格垂直冲击的方法和结果的基础上，我们使用完全动力学的细胞内粒子模拟来研究这些过程，特别是在倾斜准垂直冲击下的电子加速度。在激波中，上游离子动能的一部分通过不同的渠道流向电子：激波冲浪加速、激波漂移加速、激波电位加速、磁重联、与磁湍流的随机相互作用等。改变大尺度磁场与激波法线之间的夹角，我们期望确定单个加速机制对粒子能量贡献的变化，从而能够建立电子注入扩散激波加速度的速率作为激波倾角的函数。该项目的基本输出将是上游和下游电子能谱，电子在冲击下的反射效率，以及它们驱动的等离子体不稳定性的类型和行为。我们还将探讨模拟中使用的离子与电子质量比对结果的影响。我们的结果将为信噪比冲击下电子加速的全球模型提供信息。众所周知，冲击的后期演化涉及冲击锋面和预冲击介质的大规模扰动，这些扰动是由上游介质中反射粒子携带的电流等更快的过程所造成的，而上游介质的存在和性质可以用全PIC方法很好地捕捉到，这就是我们要做的。我们假设，一旦电流开始，长波长的干扰几乎是不可避免的，因为电流总是会有小的变化。我们的项目将确定电流产生的效率。

Most of the available information about particle acceleration in astrophysical objects relates to electrons, on account of their high radiation efficiency. Not as much is known, however, as to how electrons get accelerated. Electron participation in diffusive shock acceleration at Supernova-remnant shocks requires that they be pre-accelerated to Lorentz factors above 100. Where and how that happens is the focus of this project. Supernova shocks have nonrelativistic propagation velocities and are characterized by high sonic and Alfv ́enic Mach numbers. The physics of non-relativistic shocks is governed by reflection of particles, the interaction of which with the incoming plasma excites a variety of instabilities upstream of the shock. If the magnetic field is oriented quasi-parallel to the shock normal, ions can travel far and drive instabilities in the far-upstream region. For quasi-perpendicular shocks the reflected ions are not fast enough and essentially conduct only one Larmor orbit, but electrons may stream along the magnetic field toward the far upstream region and drive waves there. We use fully kinetic Particle-In-Cell simulations to study these processes, and electron acceleration in particular, at oblique quasi-perpendicular shocks, building on methods and results obtained for strictly perpendicular shocks. At shocks a fraction of the upstream ion kinetic energy goes to electrons though different channels: shock surfing acceleration, shock drift acceleration, acceleration by shock potential, magnetic reconnection, stochastic interaction with magnetic turbulences, etc. Changing the angle between the large-scale magnetic field and the shock normal, we expect to determine the changes in the contribution of individual acceleration mechanisms to particle energization and hence be able to establish the rate of electron injection into diffusive shock acceleration as function of the shock obliquity. The essential output of the project will be the upstream and downstream electron spectra, the efficiency of electron reflection off the shock, and the type and behaviour of plasma instabilities they drive. We shall also explore the scaling of the results with the ion-to-electron mass ratio used in the simulation. Our results will inform global models of electron acceleration at SNR shocks.It is known that the late-time evolution of shocks involves large-scale disturbance of the shock front and the pre-shock medium that are seeded by faster processes like currents carried by reflected particles in the upstream medium whose existence and properties are well captured with the full PIC method, and that is what we are going to do. We posit that the long-wavelength disturbance is almost inevitable once the current startsbecause there will always be small variations in that current. Our project will establish with what efficiency the currents are generated.