Study Of Large-scale Current Systems Of Earth's Magnetosphere And Ionosphere

Using the global PPMLR-MHD simulation model and Cluster data, thispaper presents the large-scale geometry of magnetospheric currents, putting theemphasis on the closure of cross-tail current and studies the intensi?cation ofionospheric Cowling current. The main achievements are summarized below.First, we performed a series of global MHD simulations to study the clo-sure of the cross-tail current in the magnetotail under di?erent solar wind con-ditions. The cross-tail current closes totally within the high-latitude magne-topause(HLMP), forming the classical'θ'structure when IMF is set to bezero. The situation changes for southward IMF cases: part of the cross-tail cur-rent passes through the magnetosheath and closes across the bow shock, formingan overlapped'θ'structure, when viewed from the Sun. Quantitative analysisshows (1)the total amount of cross-tail current and its percentage closing on thebow shock tend to grow with strength of the southward IMF BZ; (2)when solarwind speed increases, the total amount of cross-tail current increase, but thenearly a constant quantity of the cross-tail current comes from the bow shock;(3)an increase in the ionospheric Pedersen conductance leads to an increase ofthe bow shock contribution but a decrease of HLMP contribution to the cross-tail current, therefore the total cross-tail current seems independent with theionospheric conductance.Since the simulation has showed the current capacity of HLMP decreaseswith the strength of southward IMF BZ, while the bow shock current capacitydirectly opposites, we use Cluster data to check this result. After searching theHLMP and bow shock crossing events, a statistical study of the current capacityat the high-latitude magnetopause(HLMP) and bow shock under di?erent south-ward interplanetary magnetic ?eld (IMF BZ) conditions is carried out. With theenhancement of the southward IMF BZ, the current capacity at HLMP reduces,while it increases at the bow shock. Since the amount of the magnetosphericcurrent increases with the increase of the IMF BZ, a synthesis analysis based on the Cluster observations suggests the bow shock and HLMP together contributeto the magnetospheric current system, and the bow shock would replace HLMPto be an important current generator when the southward IMF BZ becominglarge. This Cluster observation con(?)rms the simulation results.At last, we examine the e(?)ects of the ionospheric conductance on the in-tensi(?)cation of the westward electrojet current in the ionosphere based on thePPMLR-MHD model. The ionospheric conductance is empirically linked withthe plasma sheet conditions. The simulation results are consistent with obser-vations: When the Pedersen and Hall conductances are small, the ionosphericcurrent shows a two-cell pattern; when the conductances increase, and the ra-tioΣH/ΣP≥2, an intense westward electrojet appears in the midnight sector.This intense westward electrojet is the Cowling current driven by the inducedsouthward electric (?)eld due to the blockage of the northward Hall current fromclosure in the equatorial plasma sheet. Meanwhile, the simulation shows over80% Cowling current is Hall component. Thus, when lack of radial current inthe midnight equatorial plane, the enhancement of ionospheric conductance isthe essential factor controlling the intensi(?)cation of Cowling current.