Authors: Lucas Tarr (National Solar Observatory), Mark Linton (Naval Research Laboratory), Peter Schuck (Goddard Space Flight Center), James Leake (Goddard Space Flight Center)

A crucially important and often overlooked uncertainty in magnetohydrodynamics (MHD) simulations is how the boundary conditions are treated. While the boundary conditions bear the task of representing the physical impact of the full external universe on the simulation volume, it is not uncommon for the choice of boundary condition to receive little or no discussion. In this contribution, we use our characteristics-based boundary condition implementation in the 3D MHD code LARE3D to demonstrate the profound impact that a boundary condition in fact has on the full simulation dynamics. To emphasize how important the boundary condition is, we compare two frequently used methods for specifying ‘non-reflecting’ boundary conditions and demonstrate that even the subtle difference between them is sufficient to cause otherwise identical simulations to dramatically diverge from one another.