ALBERT

Description: [1]  The two primary methods responsible for solar wind magnetosphere coupling are magnetic reconnection and the viscous interaction. The viscous interaction is generated due to the antisunward dragging of plasma inside the magnetopause by the plasma flowing in the magnetosheath, creating a return flow deeper inside the magnetosphere and producing a circulation pattern. This viscous circulation pattern is mapped into the ionosphere via magnetic field lines, which results in ionospheric electric field in the non-rotating Earth's frame. We measure this interaction in terms of an electric potential, the viscous potential. In this paper, we use the results obtained from the LFM simulation model during periods of purely northward IMF for different solar wind velocity and ionospheric conductivity, showing a reduction of the viscous potential with increasing magnitude of northward IMF. The viscous potential is found to settle around 5-10 kV for large + B z values. The decrease in viscous potential was found to be associated with a weak or non-existent sunward plasma flow in the nightside plasmasheet. Instead, the return flow to the dayside occurs at high latitudes and is associated with the reconnection topology and dynamics that occur during northward IMF periods. We also show that the magnetosphere remains closed during purely northward IMF, except for two small regions-one on each hemisphere, where the magnetic reconnection occur. We argue that the reduction of the viscous potential is due to a reduction of the velocity shear across the magnetopause and the lack of sunward convection in the equatorial tail.

Description: In this paper we examine the response of the ionospheric cross-polar cap potential to steady, purely northward interplanetary magnetic field (IMF) using the Lyon-Fedder-Mobarry global magnetohydrodynamic simulation of the Earth's magnetosphere. The simulation produces the typical, high-latitude “reversed cell” convection that is associated with northward IMF, along with a two cell convection pattern at lower latitude that we interpret as being driven by the viscous interaction. The behavior of the potential can be divided into two basic regions: the viscous dominated region and the reconnection dominated region. The viscous dominated region is characterized by decreasing viscous potential with increasing northward IMF. The reconnection dominated region may be further subdivided into a linear region, where reconnection potential increases with increasing magnitude of northward IMF, and the saturation region, where the value of the reconnection potential is relatively insensitive to the magnitude of the northward IMF. The saturation of the cross-polar cap potential for northward IMF has recently been documented using observations and is here established as a feature of a global MHD simulation as well. The region at which the response of the potential transitions from the linear region to the saturation region is also the region in parameter space at which the magnetosheath transitions from being dominated by the plasma pressure to being dominated by the magnetic energy density. This result is supportive of the recent magnetosheath force balance model for the modulation of the reconnection potential. Within that framework, and including our current understanding of the viscous potential, we present a conceptual model for understanding the full variation of the polar cap potential for northward IMF, including the simulated dependencies of the potential on solar wind speed and ionospheric conductivity.