A collisionless shock may be strongly modified by the presence of neutral atoms through the processes of charge exchange between ions and neutrals and ionization of the latter. These two processes lead to exchange of energy and momentum between charged and neutral particles both upstream and downstream of the shock. In particular, neutrals that suffer a charge exchange downstream with shock-heated ions generate high velocity neutrals that have a finite probability of returning upstream. These neutrals might then deposit heat in the upstream plasma through ionization and charge exchange, thereby reducing the fluid Mach number. A consequence of this phenomenon, that we refer to as the "neutral return flux", is a reduction of the shock compression factor and the formation of a shock precursor upstream. The scale length of the precursor is determined by the ionization and charge exchange interaction lengths of fast neutrals moving towards upstream infinity. In the case of a shock propagating in the interstellar medium, the effects of ion-neutral interactions are especially important for shock velocities < 3000 km/s. Such propagation velocities are common among shocks associated with supernova remnants, the primary candidate sources for the acceleration of Galactic cosmic rays. We then investigate the effects of the return flux of neutrals on the spectrum of test-particles accelerated at the shock. We find that, for shocks slower than ~3000 km/s, the particle energy spectrum steepens appreciably with respect to the naive expectation for a strong shock, namely E^-2.
Collisionless shock in a partially ionized medium. II. Balmer emission
传播到部分电离介质中的强激波通常与光学巴耳末线有关。这种发射是由于受冲击气体中的热质子和电子对中性氢的碰撞激发。在之前的论文（Blasi et al. 2012）中已经计算了这种Balmer主导冲击的结构，其中中性粒子的分布函数是从适当的Boltzmann方程推导出来的，包括通过电荷交换和电离与离子和电子耦合。这个计算显示了中性粒子的存在如何通过在激波前形成“中性粒子诱导”的前兆来显著改变激波结构。在这里，我们跟进我们以前的工作，并调查由此产生的巴耳末辐射的性质，目的是使用所观察到的辐射作为诊断工具的冲击参数。我们主要关注超新星残余激波，我们发现，对于典型参数，Hα发射通常具有三分量光谱轮廓，其中：1）窄分量来源于上游冷氢原子，2）宽分量来源于已经与激波下游的激波质子进行电荷交换的氢原子，以及3）中间组分是由于与中性诱导前体中的暖质子进行了电荷交换的氢原子。中间线的宽度反映了前体中的温度，而窄线的宽度不被前体改变。此外，我们表明，这两个中间和广泛的组件的配置文件一般偏离热分布。最后，我们表明，如果有效的电子加热发生在前体区域可以产生大量的巴耳末发射。
Collisionless relaxation as the origin of the anisotropic, non-thermal, and multi-temperature momentum distributions observed in space plasmas
Max Planck Institute for Astrophysics（马克斯·普朗克天体物理学研究所）；Deutsches Zentrum für Astrophysik（德国天体物理学中心）；Ludwig-Maximilians-Universität München（慕尼黑大学）；Excellence Cluster ORIGINS（ORIGINS卓越集群）；Leibniz-Institute for Astrophysics Potsdam (AIP)（波茨坦莱布尼茨天体物理研究所）
PIC simulations of nonrelativistic high-Mach-number oblique shocks propagating in a turbulent medium
University of Wisconsin-Madison（威斯康星大学麦迪逊分校）；University of Potsdam（波茨坦大学）；Deutsches Elektronen-Synchrotron DESY（德国电子同步辐射中心）；Institute of Physics of the Czech Academy of Sciences（捷克科学院物理研究所）；Max-Planck-Institut für Plasmaphysik（马克斯·普朗克等离子体物理研究所）；The University of Tokyo（东京大学）
