Simulating the photospheric to coronal plasma using magnetohydrodyanamic characteristics II: reflections on non-reflecting boundary conditions

TL;DR

This paper implements and tests non-reflecting boundary conditions in an MHD simulation code, revealing their limitations and effects on simulation accuracy when modeling plasma phenomena.

Contribution

It introduces a flexible non-reflecting boundary condition method for MHD codes and evaluates its performance through various plasma simulations.

Findings

Non-reflecting boundary conditions can cause significant physical differences compared to larger domain simulations.

The implementation is compatible with arbitrary grid-based MHD schemes.

Differences arise from boundary condition reflections affecting plasma dynamics.

Abstract

We present our implementation of non-reflecting boundary conditions in the magnetohydrodynamics (MHD) code LaRe3D. This implementation couples a characteristics-based boundary condition with a Lagrangian remap code, demonstrating the generality and flexibility of such non-reflecting boundary conditions for use with arbitrary grid-based MHD schemes. To test this implementation for perturbations on a background state, we present simulations of a hot sphere in an angled magnetic field. We then examine a series of simulations where we advect a spheromak through a non-reflecting boundary condition at four speeds related to the fast and slow magnetosonic speeds and the Alfven speed. We compare the behavior of these simulations to ground truth simulations run from the same initial condition on an extended grid that keeps the spheromak in the simulation volume at all times. We find that the non-reflecting boundary condition can lead to severe, physical differences developing between a simulation using a non-reflecting boundary and a ground truth simulation using a larger simulation volume. We conclude by discussing the origins of these differences.