Study Of Variations Of Open-closed Field Line Boundary And Open Magnetic Flux In Terrestrial Polar Region

The terrestrial open field lines are produced by the magnetic reconnection be-tween interplanetary magnetic field and magnetospheric magnetic field.Therefore,the total flux of open field lines and the location of the boundary between open and closed field lines at the height of polar ionosphere are the highly important parameters which characterize the solar wind-magnetosphere(SW)coupling.The studies of dependence of open magnetic flux(Fpc)and open-closed field line bound-ary(OCB)on the solar wind conditions are very significant for understanding the physical process of SW coupling and forecasting these two parameters.In this dis-sertation,using the global PPMLR-MHD model,we investigate the changing of Fpc and OCB with the upstream solar wind conditions.1.In order to check whether the OCB determined by the PPMLR-MHD model matches the observations quite well,we compare our simulation results with DMSP boundary crossings.(1)We choose two typical OCB-crossing periods observed by the DMSP satellites.The upstream solar wind conditions and polar cap flux basically remained steady,wherefore the magnetosphere is ba-sically under quasi-steady state during these two periods.Using the PPMLR-MHD model,we simulate these two quasi-steady states,diagnose the OCB lo-cations,and compare the simulation OCB with all DMSP boundary crossings during the two periods.The model-data comparison shows that the PPMLR-MHD code can reproduce the OCB location quite well under the steady in-terplanetary conditions.(2)Magnetospheric substorm is a brief disturbance in the Earth's magnetosphere.Based on the PPMLR-MHD model,we simu-late a period on 8 March,2008 which includes two isolated substorm events.We employ DMSP particle observations to determine the OCB crossings,and present the comparison of the OCB location calculated from PPMLR-MHD model with those determined by DMSP measurements.The model-data com-parison shows us that the PPMLR-MHD model can depict the variation of OCB locations well during a real substorm event.2.Using the global PPMLR-MHD simulation,we investigate the effect of the interplanetary magnetic field(IMF)on the location of OCB,in particular the duskside and dawnside OCBs and their asymmetry.The IMF conditions have significant effects on the topology of reconnection at dayside magnetopause.Thus,it plays an important role in determining the dawnside and duskside OCBs and dawn-dusk OCB asymmetry.Our model results show that the dawnside OCB has the same latitude as the duskside under a due southward IMF.However,when IMF By is nonzero,the OCB has an obvious displace-ment along the dawn-dusk meridian,and the dawnside and duskside OCBs respond differently to IMF conditions.Based on the quantitative analysis of simulated data,we obtain two empirical expressions:one describes the rela-tion between the dawn-dusk OCB latitudes and the IMF conditions;the other one describes the relation between the asymmetric extent and the IMF con-ditions.By the model-data comparison,we also found that the differences between the dawnside and duskside OCB latitudes from model predictions are in good agreement with the observations.3.We use the global PPMLR-MHD model to study the dependence of steady-state open magnetic flux on the solar wind density and southward IMF.During periods when the magnetosphere is under quasi-steady state,the input solar wind conditions determine the dayside merging rate,and the tail reconnec-tion region adjusts itself to match the dayside merging rate.Therefore,the steady-state open magnetic flux is controlled in a same fashion by the solar wind conditions like the dayside merging rate.It is shown that the behav-iors of open flux can be explained by the control of the geoeffective length(the y scale of solar wind flowing into merging region)by the magnetosheath flow around the magnetosphere.For a high Mach number solar wind flow,the magnetosheath flow is dominated by the plasma pressure gradient force.Under such conditions,if the southward IMF magnitude increases,the mag-netosheath flow and geoeffective length are essentially unaffected so that the dayside merging rate increases linearly and the open flux increases essentially linearly as well.If the solar wind density increases the merging line and geoef-fective length shortens due to the shrinking magnetosphere,so that the open flux decreases for the higher solar wind density.However,this behavior does not follow the Chapman-Ferraro scaling.The lack of Chapman-Ferraro scaling in the open flux is because the magnetosheath flow pattern is not self similar under compression by the solar wind.On the other hand,for a low Mach number flow,the magnetic force dominates the magnetosheath flow.Under such circumstances,if the southward IMF increases,the magnetic force divert-ing the magnetosheath flow also becomes larger,the geoeffective length gets smaller,and the dayside merging rate is essentially unchanged.Therefore,the open flux saturates regardless of the increasing southward IMF.Moreover,the density dependence is reversed.A higher density produces a longer geoeffective length,which results in a larger open magnetic flux.