The magnetohydrodynamics (MHD) treats the cometary plasma environment in the fluid approximation using the global, 3D magnetohydrodynamic (MHD) code BATSRUS (Block Adaptive Tree Solarwind Roe-Type Upwind Scheme). The code solves the governing equations of ideal magnetohydrodynamics. In the MHD limit, the plasma is treated as a single species plasma. Effects related to finite gyroradius, resistivity and other kinetic effects are neglected. However, we are able to describe some non-MHD effects by describing these deviations from ideal MHD (sources and sinks of mass, momentum and energy, or resistive and diffusive processes) through appropriate source terms. Here we present a few details about the aspects of BATSRUS that are relevant to the solar wind--comet interaction and refer the reader to an extensive literature related to the numerical algorithms used in the code. A review of many of the fundamental aspects of the model algorithms and their application to comets can be found in papers listed above an references contained in them.
One of the most important features of BATSRUS is the ability to easily adapt the grid to resolve specific regions of space. BATSRUS utilizes the approach of adaptive blocks. Adaptive blocks partition space into regions, each of which is a regular Cartesian grid of cells, called a block. If the region needs to be refined, then the block is replaced by 8 child sub-blocks, each of which contains the same number of cells as the parent block. The use of cells of varying sizes allows us to resolve features of vastly different length scales. In the simulations presented in this paper, we are able to resolve the body, the diamagnetic cavity and the solar wind--neutral cloud interaction even though these occur at vastly different length scales Note that the adaptive mesh allows us to use an extremely large simulation box and therefore self-consistently model the mass loading and subsequent slowing of the plasma upstream of the bow shock.
In the model all processes associated with mass loading (ionization, charge exchange, recombination, etc.) are handled through appropriate sources terms. These are implemented in the same manner as the source terms used in our previous study of the comet Hale-Bopp Differences from that paper only require adjusting the gas production rate to that of comet 67P/CG and adjusting the photoionization scale length for heliocentric distance.