Kinetic Simulations of Particle Acceleration in Collisionless Supercritical Shock-Shock Interaction

Understanding ion energization during the interaction between an Interplanetary (IP) shock and a Bow shock remains an important and intriguing problem in space plasma physics. In this context, we present hybrid particle-in-cell simulations using the 2D EPOCH code to investigate particle acceleration during supercritical collisionless shocks interactions. In order to estimate the level of particle energization in two shocks interaction, we consider two cases. First, we present an example of particle acceleration induced by an isolated bow shock resulting from the solar wind-Earth’s magnetosphere interaction. Second, we present a case study of a Coronal Mass Ejection (CME)–driven IP shock interaction with the Earth’s bow shock for both quasi-parallel and quasi-perpendicular geometries. During the interaction of two shocks, ions undergo multiple reflections between the converging magnetic fields, enabling efficient energy gain through Fermi acceleration. By modelling the system using hybrid simulations, we can further observe how this acceleration is modified and enhanced in the presence of ion-kinetic scale structures and non-stationary developed self-consistently at both shocks. As expected, the shock–shock configuration produces substantially stronger ion energization than a single isolated collisionless shock. Our simulations show that as the two shocks approach and overlap, their highly structured magnetic ramps, reflected-ion populations, and upstream waves interfere, producing time-dependent variations in shock thickness, amplitude, and position. By analyzing ion velocity distributions, bulk flow, temperature, and electromagnetic fields, we characterize key features of the interaction region, including shock evolution, reformation, ion reflection, and particle energization. These results provide new insight into how shock–shock interactions influence the turbulent shock transition and enhance ion acceleration compared with a single shock.