Simulation of the polar cap potential during periods with northward interplanetary magnetic field

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.