Orestis A. Karapiperis, Matthieu Schaller

Physical problems with a solution that can be expressed analytically are scarce; this holds even more true for problems set in a cosmological context. Such solutions are, however, invaluable tools for making comparisons between theory, numerical experimentation and observations. In this work we present what to our knowledge is the first set of non-trivial closed-form expressions describing the behaviour of a system governed by the equations of non-ideal Magnetohydrodynamics (MHD), where the effects of Ohmic diffusion are considered, in a cosmologically expanding frame. We provide analytical solutions that describe the time evolution of linear perturbations to a homogeneous background in a radiation-dominated universe, yielding dissipative Alfv\'en waves. Although in our base framework solutions for any other cosmology of interest cannot be expressed in a closed form, they can still be obtained reliably through numerical integration of the coupled system of ordinary differential equations we provide. We compare our analytical solutions to numerical results obtained using our novel implementation of Smoothed Particle Magnetohydrodynamics (SPMHD) in the SWIFT astrophysical simulation code, to find good agreement between the two. We find the code to display good convergence behaviour, its predictions agreeing with theory to within 0.1% for a modest number of resolution elements and at a negligible computational cost. We aim this work as a companion and supplement to the cosmological ideal MHD wave tests recently presented in the literature, and suggest that it be adopted as part of standard testing of code implementations of MHD.