Simulation Of Electrodynamic Magnetosphere-Ionosphere Coupling Based On Finite Element Method

Magnetosphere-Ionosphere coupling(MI Coupling) play an important role in the chainof Sun-Earth relationship, and the key electric factor in it is the electric current which fowalong feld lines between these two regions. The interaction between the solar wind andthe magnetosphere generate feld-aligned currents(FACs) in magnetosphere.FACs transmitenergy from magnetosphere into ionosphere along the magnetic feld lines, and then dissi-pated in the form of Joule heating and electron precipitation. The state change, especiallyconductivity change, of the ionosphere, in turn feedback on magnetospheric feld-alignedcurrents, resulting in a change in the amplitude of the magnetospheric Alfven waves, evengenerate feedback instability in the presence of background driving electric feld. Obser-vations of Cluster and FAST revealing that, Alfven standing wave and traveling wave areintrinsic part of the large-scale MI coupling. Numerical simulation of MI coupling is ofgreat value to the study of plasma convection in auroral ionosphere.This paper systematically summarizes and analyzes the physical processes and simu-lation results of nonlinear dispersive scale Alfven waves in MI coupling.1. The frequency, temporal and spatial characteristics, as well as the ponderomotiveand dispersive efects are summarized. The ponderomotive force and electron inertiabring the resonance location earthward, while the thermal efect bring it outward.2. The nonlinear electron heating by shear Alfven waves is shown. When ionosphericcurrent is small than the critical current, the ionization is not important. Whenionospheric current is great than the critical current, ionization became importantand Pedersen conductivity will change.3. Two mechanisms are shown, which are commonly used to explain the formation ofFAC, auroral arcs, density cavity and bump, et al. Field line resonance(FLR), inwhich magnetosphere plays primary role, performs well in explaining the very largescale and large amplitude density cavity. Ionospheric feedback instability (IFI), whichis triggered by ionosphere, is more suitable to explain the higher frequency densityfuctuation.There are lots of3D magnetosphere model (LFM, BATS-R-US, GUMICS et al.),almost all of which are dealing ideal or reduced MHD equations, make it difcult to addsmall scale efects. Currently, there are rare studies which numerically simulating shear Alfven waves in3D magnetosphere. This paper focus on the3D extending of2D fniteelement method program TOPO, including the mesh generation based on magnentosphericmodel, redesign of data format, rewriting of the fortran codes, and the parallelization ofcalculation.Based on the newly developed3D TOPO program, a trial simulation of magnetosphericAlfven wave resonance is given, and the FLRs structure are successfully achieved. Theresonance shell is set to L=8.0Re and the period is calculated to be159.18S. The resultsshow that a steady feld line resonance is formed after8periods. As the time goes by, thewidth of Afven wave is getting narrower and the amplitude of the wave is getting stronger,fnally reach a steady situation. The max parallel current in polar area is2.9μA/m~2, whichmatches well with auroral observations.Unfortunately, the parallelization of3D TOPO is not well down yet, and the problemsolved by it is limited to spare meshes. In the following work, we will get the3D TOPOparallelized thoroughly, and add small scale efects into3D MI coupling simulation.