Transpolar potential and reconnection voltage of the Earth from global MHD simulations

Abstract

The ionospheric transpolar potential (Vpc) and the magnetospheric reconnection voltage (Vr) of the Earth are investigated in terms of global MHD simulations of the solar wind-magnetosphere-ionosphere (SMI) system. A spherical shell approximation is used for the ionosphere with a uniform Pedersen conductance and a zero Hall conductance, the interplanetary magnetic field is due southward, and quasi-steady solutions are obtained for the system. Each solution is characterized by the following four parameters: the ionospheric Pedersen conductance Σp, the solar wind electric field Esw, ram pressure PSW, and Alfvén Mach number MA. More than 100 cases are treated separately and Vpc and Vr are calculated in each case. It is found that both Vpc and Vr increase monotonically with increasing Psw and decreasing MA regardless of the magnitude of Esw. The simulation results are well organized by a combined parameter f= ESWPSWM A-1/2 and approximately fitted by functions of f and Σp. When the units are S for ΣP kV for V pc and Vr, mV/m for Esw and nPa for P SW, the functions are found to be Vpc = 2.3 x 10 3 (f+ 0.8)(ΣP + 2)-1/(f+ 8.2) and V r = 1.8 x 103[f + 0.45(Σp1/2 + 1.2)] (Σp1/2 + 0.45)-1/(f + 15-5). Three conclusions are made as follows: (1) Both VpC and Vr saturate with respect to the increase of f, any variation of the interplanetary conditions in favor of the increase of may cause the saturation. (2) The saturation point is found to be fc = 6.6 for Vpc and fc= 14.4 0.9Σp1/2 for Vr, whereas the value of ΣP controls the saturation levels. (3) The two potentials, Vpc and Vr, stem from the SMI coupling and exhibit similar saturation behaviors. They are positively correlated because of sharing the same driving source and the close coupling between the ionosphere and the magnetosphere. Copyright 2009 by the American Geophysical Union.