Development of a Multi-Grids Approach into a Parallelized Hybrid Model to Describe Ganymede's Interaction with the Jovian Plasma

Abstract:

Ganymede is the only satellite which has its own magnetosphere, which is embedded in the Jovian magnetosphere (Kivelson et al. 1996). This peculiar interaction has been investigated by means of a 3D parallel multi-species hybrid model based on a CAM-CL algorithm (Mathews et al. 1994). In this formalism, ions have a kinetic description whereas electrons are considered as an inertialess fluid which ensures the neutrality of the plasma and contributes to the total current and electronic pressure. Maxwell’s equations are solved to compute the temporal evolution of electromagnetic field. Hybrid simulations are performed on a uniform cartesian grid with a spatial resolution of about 240 km. Our results are globally consistent with other models and Galileo measurements. Nevertheless, our description of the magnetopause and the ionosphere is not satisfying enough due to the low spatial resolution. Indeed, we want to describe scale heights of 125 km in the ionosphere whereas the best spatial resolution that we are allowed to use is about 240 km. Therefore, in order to obtain more efficient and relevant results, it is necessary to improve the size of the grid. In this optic, we are introducing a multi-grids approach in order to refine the spatial resolution by a factor 2 (~120km) near Ganymede. The creation of a finer mesh in the simulation grid leads to make some peculiar computations at the interfaces between the two different grids, whether for the calculation of moments, such as charge density or current, or the computation of electromagnetic fields. Moreover, the parallelization of the code, based on domain decomposition methods, imposes us to take care of boundary conditions. In the hybrid model, macroparticules, which represent a kind of cloud of physical particles, have a volume equal to that of a grid cell. Then, the macroparticules entering into the higher spatial resolution region are splited into smaller macroparticules whose the volume corresponds to the volume of a cell of the finer mesh. The improvement of the spatial resolution in the hybrid model will also allow us to relevantly couple the results of this model with those of our 3D multi-species exospheric model (Turc et al. 2014), into a test-particle model that describes the ionosphere of Ganymede. Basic tests and validation results of the multi-grids approach are presented.