Ion acceleration and its effect in shock-shock interaction

In space it often occurs that two collisionless shocks hit each other. In the near Earth environment it was actually observed in-situ by Hietala et al. [2011] that an interplanetary shock collides with the Earth's bow shock. They also observed energetic ions which may be accelerated through multiple interactions with the two approaching shocks. On the other hand, self-consistent numerical simulation of such a colliding shock system has been seldom performed so far. Almost the only example is given by Cargill et al. [1986, 1991] who reproduced a head-on collision of two shocks by using one-dimensional hybrid simulations.In this study a one-dimensional full particle-in-cell simulation of a head-on colliding two shock system is performed. We previously confirmed that electrons are strongly accelerated through multiply reflected by the approaching two shocks but ions are not when the two shocks are quasi-perpendicular. Here, the interaction of two quasi-parallel shocks is investigated. When a shock angle, the angle between upstream magnetic field and shock normal, is 30° (and 150°), not only some upstream electrons but also some upstream ions are reflected at the two shocks. Some of the reflected ions multiply interact with the shocks and are accelerated to relativistic energy, even when the two shocks are well separated. During their interaction with each shock, they gain energy through the so-called trapped ion acceleration mechanism [Sugiyama and Terasawa, 1999]. When the distance between the two shocks is comparable to the gyro radii of the energetic ions, they are further accelerated through the similar process previously seen in the two quasi-perpendicular shock interaction. The upstream energetic ions result in strong enhancement of local pressure which is higher than the pressure downstream. Influence of such a strong modification of the approaching shocks in the shock-shock interaction will also be discussed.