An Adaptive Approach for Modeling Collisionless Plasma Shocks and Ion Heating in the Shock Frame

Abstract

We present a novel computational method for hybrid (kinetic ions, quasi-neutral fluid electrons) simulations of non-relativistic magnetized collisionless plasma shocks, observed near the Sun, in the solar system, and beyond. This Adaptive Frame-Of-Reference Approach (AFORA) enables simulations of shocks in their co-moving frames of reference, where shock evolution remains quasi-steady for quasi-stationary upstream conditions, in contrast to moving shocks. We demonstrate our method in two spatial dimensions for a range of angles between the background magnetic field and the shock normal (obliquities). Shock-frame simulations, conducted with a time-adaptive hybrid code, HYPERS, are shown to self-consistently capture upstream and downstream magnetic fluctuations commonly observed by spacecraft at interplanetary shocks and Earth's bow shock. These fluctuations, which may be crucial for estimating the efficiency of ion acceleration at heliospheric and astrophysical shocks, are notably missing from many hybrid simulations of shocks, unless upstream external turbulence is imposed. The quasi-stationary behavior of numerical shocks in the shock frame is confirmed in a series of animations and by demonstrating converged ion energy spectra in the downstream region. Our approach underscores computationally efficient and physically more accurate modeling of shock-driven turbulence, downstream plasma relaxation, and concomitant ion heating in the shock frame. The new method can be used for exploring ion relaxation at interplanetary (IP) shocks, more efficient studies of cosmic rays (CRs) and plasma turbulence, and more accurate predictions of seed populations of Solar Energetic Particles (SEPs) at heliospheric shocks driven by coronal mass ejections (CMEs).

Accepted for publication in The Astrophysical Journal. 27 pages, 11 figures, 1 table. Animations available at https://doi.org/10.5281/zenodo.21422353