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

摘要

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 Ep, the solar wind electric field Es, ram pressure Psand Alfven 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 E . The simulation results are well organized by a combined parameter f = E_swP_swMA,171vand approximately fitted by functions off and Ep. When the units are S for Ep, kV for Vpc and Vr, mV/m for E_sw, and nPa for Ps, the functions are found to be Vpc = 2.3 X 103(f+ 0.8)(Ep + 2)-1/(f+ 8.2) and Vr = 1.8 x 103 [f + 0.45(Ep + 1.2)] (Er + 0.45) -1/(f + 15.5). Three conclusions are made as follows: (1) Both Vpc and V_r saturate with respect to the increase off; any variation of the interplanetary conditions in favor of the increase off may cause the saturation. (2) The saturation point is found to be f_c = 6.6 for Vpc and fc= 14.4 .9EP for Vr, whereas the value of Ep controls the saturation levels. (3) The two -0 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.