Investigation Of The Solar Wind-magnetosphere-ionosphere Coupling In Terms Of Global MHD Simulations

The evolution and inherent coupling of the solar wind-magnetosphere-ionosphere (SMI) system serves as one of crucial issues of space weather. This thesis applies global MHD simulations to make a quantitative study of several important electrodynamic parameters of the SMI system,i.e.,the ionospheric transpolar potential,the magnetopause reconnection voltage,and the ionospheric region 1 field-aligned current.The main achievements are summarized below.1.The saturation of ionospheric transpolar potential and magnetopause reconneetion voltageThe ionospherie transpolar potential is defined as the difference between the maximum and minimum of the ionospherie electric potential,and it can be observed by ground radar and space satellites.Previous observations and global magnetohydrodynamic(MHD) sinmlation studies showed that the transpolar potential(V_pc) saturates with increasing solar wind electric field. The energy transfer is believed to be implemented mainly by magnetic reconnection, and the total reconnection rate is characterized by the reconnection voltage(V_r).In contrast with the transpolar potential,the reconnection voltage cannot be directly observed,so it can only be evaluated through global MHD simulations.Previous simulation studies revealed a similar saturation for V_pc.This thesis presents a quantitative investigation of the saturation of both V_pc and V_r in terms of a large number of simulation runs.In the simulations,we assume that the ionospheric Pedersen conductance is uniform, the Hall conductance vanishes,and the interplanetary magnetic field (IMF) is due southward.Each run is characterized by four parameters:the Pedersen conductance∑_p for the ionosphere,and the electric field E_sw,ram pressure P_sw,and Alfv(?)n Mach number M_A for the solar wind.With the use of more than 100 simulation runs we show that V_pc and V_r not only depends on E_sw,but also increases monotonically with increasing P_sw and decreasing M_A.Through a synthetical analysis of all simulation examples,a combined parameter f is found,given by f= E_swP_swM_A^-1/2,and both V_pc and V_r can be approximately fitted by functions of f and∑_p.When the units are S for∑_p,kV for V_pc and V_r,mV/m for E_sw,and nPa for P_sw,these functions are found to be V_pc= 2.3×10³(f+0.8)(∑_p+2)^-1/(f+8.2) and V_r=1.8×10³[f+0.45(∑_p^1/2+1.2)](∑_p^1/2+0.45)^-1/(f+15.5).Three conclusions are made as follows based on these formulas.(1) Both V_pc and V_r saturate with respect to the increase of f;any variation of the interplanetary conditions in favor of the increase of f may cause the saturation.(2) The saturation point is found to be f_c=6.6 for V_pc and f_c=14.4-0.9∑_p^1/2 for V_r, whereas the value of∑_p controls the saturation levels.(3) The two potentials, V_pc and V_r,exhibit similar saturation behaviors,and are positively correlated because of sharing the same driving source and the coupling inherent in the SMI system.2.Magnetic merging line and reconnection voltage versus the IMF clock angleMagnetic merging line is the separator between the Earth's closed field lines and the IMF open field lines,and it marks the magnetic reconnection sites.The potential drop along the merging line is called reconnection voltage, which denotes the total reconnection rate.We propose a set of diagnosis methods to trace the magnetic merging line and to calculate the electric potential along it for the magnetospheric magnetic field,from which the reconnection voltage is determined.The magnetic merging line is constructed by minimum points of magnetic field strength along a large number of last closed magnetic field lines and several properly selected last closed field lines,whereas the radial ray integration of convectional electric field is used to calculate the electric potential on the merging line.The diagnosis methods are successfully applied to magnetospheric magnetic fields associated with different IMF clock angles (θ_IMF),and a preliminary analysis is presented on the clock angle response of the geometry of the merging line and the associated reconnection voltage.It is shown that(1) The magnetic merging lines of the magnetospheric magnetic field and the compound field superposed by the Earth's dipole field and the IMF are similar in geometry.(2) The reconnection voltage is approximately fitted by sin^3/2(θ_IMF/2) for its response toθ_IMF.The ionospheric transpolar potential and the voltage along the polar cap boundary show different depondencies from that of the reconnection voltage,so it is not justified to take them as substitutes for the reconnection voltage in measuring the total reconnection rate.(3) The length of the sunward merging line between the two peaks of reconnection potential shows a non-monotonic variation in response toθ_IMF,peaked atθ_IMF=90°,so it is also not justified to take electric fields along the merging line to characterize the the total reconnection rate and the coupling strength between the solar wind and the magnetosphere.(4) The reconnection nearby the magnetic nulls closest to the subsolar point is negligible,implying that anti-parallel reconnection expected to occur nearby the nulls does not actually take place.This gives support to the component reconnection hypothesis for quasi-steady states of the SMI system.3.Contribution from the Earth's bow shock to region 1 current under low Alfv(?)n Math numbersRecent studies revealed the important contribution of the bow shock to the region 1 current of the ionosphere,and concluded that such a contribution is positively correlated to the southward IMF strength B_s.This thesis extends the study to low Alfv(?)n Mach number cases.It is found that the contribution of the bow shock to the region 1 current,denoted by I_1bs,is positively correlated to B_s as M_A＞2,a similar result as obtained by previous authors,However,if M_A becomes close to or falls below 2,I_1bs will decrease in both magnitude and percentage(i.e.,I_1bs/I₁,I₁ is the total region 1 current) with increasing B_s.The surface current density at the subsolar point of the bow shock and the total bow shock current have a similar behavior:it shows a non-monotonic variation with respect to M_A or B_s,peaked at M_A=2.7. Three conclusions are then made as follows:(1) The surface current density nearby the subsolar point,which is relatively easier to be observed,may be used to largely describe the behavior of the bow shock instead of the total bow shock current.(2) The peak of the total bow shock current is reached at about M_A=2.7 when only B_s is adjusted.(3) The non-monotonic variation of the bow shock current with M_A causes a similar variation of its contribution to the region 1 current,with the turning point at about M_A=2.