Axisymmetric particle-in-cell simulations of diamagnetic-cavity formulation in vacuum

Axisymmetric simulations of the expansion of a hot plasma suddenly introduced into a vacuum containing a weak magnetic field were performed using an electromagnetic particle-in-cell code. Both uniform and gradient fields have been used, with the simulation axis along the principal field direction. The formation of a diamagnetic cavity requires an initial plasma β>1; as the expansion proceeds, β diminishes, and the field eventually recovers. The maximum spatial extent of the cavity and its duration can be obtained from simple dynamical considerations. Field-aligned ion acceleration behind the electron front is observed in all field geometries and strengths. In the case of expansion into a divergent field, the plasma is found to move down the field gradient by ambipolar diffusion. These simulations are relevant to active release experiments in the Earth´s magnetosphere, to pellet ablation experiments, and to the naturally occurring diamagnetic bubbles observed at the Earth´s foreshock