Hydromagnetic waves in an expanding universe -- cosmological MHD code tests using analytic solutions

TL;DR

This paper presents analytic solutions for linear hydromagnetic waves in an expanding universe and uses them to test and validate cosmological MHD codes like AREPO, highlighting the importance of grid resolution and numerical viscosity effects.

Contribution

It derives analytic solutions for hydromagnetic waves in a cosmological setting and demonstrates their use in testing and validating cosmological MHD simulation codes.

Findings

Good agreement between analytic solutions and simulations with sufficient grid points.

Numerical damping can be modeled by a scale-factor-dependent viscosity coefficient.

Analytic solutions serve as effective benchmarks for code development and testing.

Abstract

We describe how analytic solutions for linear hydromagnetic waves can be used for testing cosmological magnetohydrodynamic (MHD) codes. We start from the comoving MHD equations and derive analytic solutions for the amplitude evolution of linear hydromagnetic waves in a matter-dominated, flat Einstein-de-Sitter (EdS) universe. The waves considered are comoving, linearly polarized Alfv\'en waves and comoving, magnetosonic (fast) waves modified by self-gravity. The solution for compressible waves is found for a general adiabatic index and we consider the limits of hydrodynamics without self-gravity in addition to the full solution. In addition to these analytic solutions, the linearized equations are solved numerically for a $\Lambda$CDM cosmology. We use the analytic and numeric solutions to compare with results obtained using the cosmological MHD code AREPO and find good agreement when using a sufficient number of grid points. We interpret the numerical damping clearly evident in simulations with few grid points by further deriving the Alfv\'en wave solution including physical Navier-Stokes viscosity. A comparison between Alfv\'en wave simulations and theory reveals that the dissipation can be described by a numerical viscosity coefficient $\eta_\mathrm{num} \propto a^{-5/2}$ where $a$ is the scale factor. We envision that our examples could be useful when developing a new cosmological MHD code or for regression testing of existing codes.