Impulsive plasmoid penetration of a tangential discontinuity: Two-dimensional ideal and Hall magnetohydrodynamics

Impulsive plasmoid penetration of a tangential discontinuity (e.g., the magnetopause) is studied using two-dimensional ideal and Hall magnetohydrodynamics. The model assumes a unidirectional magnetic field and describes the plasma and field dynamics transverse to the field; this is an idealized model and not typical of the magnetopause. Two magnetic field configurations are studied: the parallel field case where the magnetic fields in the magnetosheath and magnetosphere are in the same direction and the antiparallel field case where they are in opposite directions. The evolution of the plasmoid is symmetric about its propagation direction in ideal MHD. One new ideal MHD result is that after the plasmoid penetrates the discontinuity it undergoes vortex shedding. The Hall MHD results are similar to those obtained by Savoini et al. [1994]. The major similarity is that the plasmoid penetration process is asymmetric. The Hall antiparallel field case, although asymmetric, evolves very much like the ideal MHD case. However, the Hall parallel field case evolves much differently from the ideal MHD case. The plasmoid undergoes a substantial deformation in the direction orthogonal to the propagation direction and does not penetrate as far into the magnetosphere. We suggest that the Hall term can be an important mechanism to provide asymmetric plasmoid propagation through the discontinuity and that Hall MHD can capture important macroscopic effects observed in hybrid simulations that are not described by ideal MHD.