Adaptive Hybrid Modeling of Collisionless Plasma Shocks and Ion Acceleration

Abstract

We present a novel efficient technique for hybrid (kinetic ions, quasi-neutral fluid electrons) simulations of non-relativistic magnetized collisionless plasma shocks, frequently observed near the Sun, in the solar system, and beyond. This Adaptive Frame-Of-Reference Algorithm (AFORA) enables multi-dimensional simulations of plasma shocks along with concomitant ion acceleration in the shock frame, where shock evolution remains quasi-steady. Compared to moving shocks, this technique allows us to reduce the simulation time and domain size to a minimum while achieving converged shock dynamics and spectra of energetic ions. Using an event-driven (asynchronous) hybrid code, HYPERS, we demonstrate this approach in two spatial dimensions for different orientations of the background magnetic field with respect to the shock normal. Our results show excellent agreement of simulation shocks with observations of interplanetary (IP) shocks. We verify that different shock configurations (quasi-parallel, oblique, and quasi-perpendicular) convert bulk plasma flow energy into ion acceleration with varying degrees of efficiency. These findings underscore the importance of efficient and robust numerical algorithms for future high-resolution modeling of plasma shocks and ion acceleration in three dimensions. In addition to enabling efficient computational studies of collisionless shocks in general, this work paves the way for accurate prediction of seed populations of Solar Energetic Particles (SEPs), generated by coronal mass ejection (CME) shocks. The characteristics of seed ions can be used as inputs to Fokker-Planck models that simulate long-term transport and acceleration of ions along magnetic field lines through their interactions with background solar wind turbulence.