The ISSI team #520 discussed the physics of shock waves in collisionless plasmas (collisionless shocks), with a particular focus on the issue of energy partition among different constituents, including ions and electrons, thermal and non-thermal particles, and turbulence. Contrary to a shock wave in a collisional medium, the dissipation at a collisionless shock is realized through collective nonlinear wave-particle interactions, which makes the issue extremely challenging. We discussed the topic from various perspectives, including remote-sensing astronomical observations, in-situ spacecraft observations, laboratory experiments, kinetic simulations, and theory.
One of the most intense topics of discussion in the team was the equilibration between ions and electrons. It is known that some fraction of the ion energy is converted to electrons at the shock, but it has yet to be understood quantitatively. We compiled state-of-the-art knowledge of kinetic simulations and measurements with various means (astronomical, in-situ, and laboratory experiments). We found supernova remnant shocks show, in general, less efficient equilibration than expected from the other approaches. A possible reason for this is that astronomical observations typically sample much larger volumes behind the shock compared with the others.
The shock acceleration of nonthermal particles is another topic that was extensively discussed. Diffusive shock acceleration has long been the standard theory of shock acceleration of cosmic rays. The so-called injection problem states that a pre-accelerated seed population is required to explain the observed cosmic-ray flux with this theory. Recent kinetic simulations with various shock parameters found similar kinds of particle acceleration processes that, however, occur within the transition layer. We had intense discussions on simulation, observations, and theoretical modeling of the particle acceleration mechanism, which may lead to a solution to the injection problem.
The team meetings over the two years truly helped to nurture interdisciplinary collaborations among team members. This will undoubtedly shape the future of collisionless shock research.