Study Of The Axis Orientations Of Magnetic Flux Transfer Events At The Earth's Magnetopause

Flux transfer events(FTEs)are reconnection-related phenomena characterized by bipolar variation in the magnetic field component normal to the local magnetopause().To date,the basic problems about FTE structure such as its nature,formation mechanism and generation location are still in dispute.Determining FTE's central axis orientation is not only essential to investigate its geometrical structure and understand the plasma dynamics,but also can help us to distinguish various formation mechanisms and generation locations.This article focuses its study on the axis orientation of FTEs.The main contents and conclusions are as follows:(1)A new multi-point analysis method is developed to determine the FTE axis orientation.The method is devised based on the purely geometrical assumption that the structure is left-right symmetric.For FTEs which are embedded within magnetopause,regardless of their actual physical properties,the left-right symmetry should be roughly satisfied,and therefore the new method will have wide applications.Model test and actual applications demonstrates that,in inferring axial orientation of such structures,the new method is more efficient and reliable than the traditional techniques such as the minimum variance analysis of the magnetic field,the Grad-Shafranov(GS)reconstruction and the more recent method based on cylindrically symmetric assumption.(2)A new multi-point method is used to reconstruct magnetic field of a FTE structure when the axis orientation is well determined and the observing spacecraft are in a proper configuration.The applications to two FTEs observed by THEMIS and Cluster respectively shows that the method can quickly and efficiently reconstruct the spatial distribution of the magnetic field structure,which will help us to infer the magnetic field line configurations,to understand the temporal variation of measured data and to obtain the spatial distribution characteristics of other plasma parameters relative to the magnetic field.(3)The first statistical study of FTE axial orientations has been performed by using the new axis-orientation-inferring technique.The results reveal that: FTE axial orientations are basically along the magnetopause;Although FTEs observed at the duskside low latitude mostly have nearly north-south orientated axes,they are still generated along the subsolar component merging line;The subsolar component merging line is not like a splitter with constant direction as described in traditional views,instead it has large curvature.(4)For the first time the magnetic signal of the observed FTE being formed through multiple-X line reconnection has been found.The magnetic signal was characterized by multi-polar variations(e.g.double-or triple-bipolar variation)of .GS reconstruction maps of events with such magnetic signals show that each event contains multiple substructures,which is consistent with the multiple-X line reconnection model.However,the substructures are aligned along the magnetopause in descending order of size,which is not predicted in the traditional multiple-X line reconnection view.To explain this phenomenon,we proposed a new model to describe the process of FTE formation: Magnetic reconnection is continuously active on the magnetopause,making the magnetic field near the main X-line to form wedge-shaped(or X-type)configuration.Reconnection rate changes periodically,when it grows inside the wedge-shaped magnetic field many small-scale magnetic islands are generated through tearing instability.These magnetic islands coalesce with each other,and form multiple FTE structures aligned in order of size due to the limitation of the wedgeshaped magnetic field lines.Under the combined effect of magnetosheath flow pressure and magnetic curvature forces,the structures move away from the reconnection locations.(5)A series of FTEs which “anomalously” move sunward at the duskside lowlatitude magnetopause are studied.Comparing the structure velocity with FTE motion model,we could find that to counter the large tailward-moving magnetosheath flow,the actual magnetic tension should be much larger that the model value,showing that when the magnetohydrodynamics effect plays an important role,the exsiting model fails to describe the magnetosheath environment accurately and thus needs modification.