Effects Of Shear Flows Perpendicular To Magnetic Reconnection Plane On Magnetic Reconnection In Two Dimensional Hybrid Simulation Studies

Magnetic reconnection is the basic process for the magnetospheric plasmas. Many explorsive phenomena, such as magnetic storm, aurora, geomagnetic substorm, are relative to the reconnection. Large-scale plasma convections are often found in space plasmas due to the drastic plasma jets from flares, cronas, prominences, etc. Such flows have high randomicity. Flow shear will be formed when the solar wind impacts on the magnetosphere. Shear flows widely exsit in high-latitude and flank region of magnetosphere and magnetotail. The effects of shear flows on magnetic reconnection are also very important for the process. The magnitude and direction of flow shear are various for different region of magnetopause. The second magnetic islands are frequently seen in the observations and studies of magnetopause and magnetotail reconnection processes. But the mechanism of second islands is still not clear. In the studies of the effects of shear flows on magnetic reconnection, second islands induced by shear flows perpendicular to reconnection plane are viewed to be relative to the flux-transfer events (FTEs). This thesis focuses on the effects of shear flows perpendicular to reconnection plane on magnetic reconnection.In this thesis, considering ion dynamic effects,2D hybrid simulation, where ions are treated as discrete charged particles and electrons are assumed as massless fluid, is carried out to further study the effects of the shear flows perpendicular to reconnection plane on symmetric and asymmetric reconnection. Considering the complexity of plasmas environment in magnetosphere and the high randomicity of shear flows, four different shear flow profiles are applied in the initial equilibria of symmetric and asymmetric magnetic reconnection. In order to understand the effects of shear flows, magnetic flux, magnetic field Bv component and vorticity ?v perpadicular to the reconnection plane, ion flow velocity and ion number density are compared with the reconnection without shear flow. Some significant results are gained.In Chapter ?, a brief review of some backgrounds is introduced, which involves the basic theories of magnetic reconnection, the reconnection processes in magnetosphere, recent progress of effects of shear flows on magnectic reconnections, and the motivations of this thesis.In Chapter ?, numerical simulation methods in plasmas are briefly described.In Chapter ?, two dimensional hybrid simulation model and numerical algorithm used in this thesis are given.In Chapter ?. two dimensional hybrid simulation model including ion dynamic effects is used to study the effects of shear flows perpendicular to reconnection plane on symmetric magnetic reconnection. For the equilibrium shear flow varying in normal of current sheet, the effect of the symmetric flow on the reconnection process is not significant, while the effect of the antisymmetric flow is obvious. The structure of Bv induced by the Hall effect in reconnection progress is reshaped. The quadrupolar patter is replaced by bipolar patter. The structure of reconnection layer is changed and the reconnection rate is decreased. For the driven shear flow varying along current sheet, the effect of symmetric flow on reconnection process is significant. The X-point in single point reconnection turns into O-point. Consequently, a "multiple X-point" reconnection process is triggered. For antisymmetric driven shear flow varying along current sheet, the effect of flow is also observable. One can clearly see the formation of second islands in outflow region, and the reconnection rate is increased. The second islands induced by shear flows in this thesis may be helpful in understanding the second islands observed in magnetotail.In Chapter V, two dimensional hybrid simulation model is used to study the effects of shear flows perpendicular to reconnection plane on asymmetric magnetic reconnection. For the symmetric shear flows, the effects are insignificant due to the fact that at the X-point, the flow shear vanishes. But for the symmetric flow varying in normal of current sheet, second islands are generated, thus the reconnection rate is increased?20%. For antisymmetric shear flows, the common effect is to generate the second islands which are frequently seen in subsolar magnetopause reconnection as FTEs and increase the reconnection rate. Applying the results to satellite observations at subsolar magnetopause crossing, can conclude that the shear flows perpendicular to the magnetic reconnection plane may be one of the major causes for FTEs. During the events the reconnection rate can be as low as?0.01-0.03 if the flow shear around the X-point is weak; or as fast as more than 0.08 if the flow shear around the X-point is strong. The coalesced effect of shear flow and asymmetric reconnection is discussed. It is concluded that, as the asymmetry increases the reconnection rate can be reduced notably.Finally, the summary of this thesis and an outlook of the future work are presented.