Investigation Of The Magnetopause Structure Based On THEMIS Observations

The magnetopause is not only the interface between the magnetosphere and the so-lar wind,but also controls the mass,momentum and energy transport between them.All magnetospheric structures and phenomena are directly or indirectly influenced by mag-netopause processes.Various explosive phenomena occurring in the magnetosphere will affect various ground-based and space-based systems,and even cause irreparable damage.Therefore,the study of the magnetopause can help to improve the early warn-ing of space disastrous weather to reduce the impact on human activities.In this paper,the small-scale structure of the magnetopause and the dynamic motion characteristics of the magnetopause are analyzed based on the observation of THEMIS.The main re-search contents and conclusions are summarized as follows:(1)Time History of Events and Macroscale Interactions during Substorms(THEMIS)data are used to investigate the magnetic field structures in the vicinity of the magne-topause.Generally,the tendency that the farther away from the Earth,the weaker the detected magnetic field is expected inside the subsolar magnetopause.Here we show two cases which conflict with the expectation that the magnetic field gradient direc-tion reverses from inward to outward in a short time interval.After a further analysis,it is found that the THEMIS probes encountered a magnetopause indentation in these two cases.To explain this anomalous magnetic structure,a three-dipole model is con-structed.By assuming the motion of the virtual satellite in this model,the main features of the magnetic field observed by THEMIS adjacent to the indented magnetopause are reproduced.Finally,the origin of the magnetopause indentation is discussed through the case of indented magnetopause caused by the fast flow observed by THEMIS.(2)A new four-step magnetopause crossing events(MCEs)automatic identifica-tion algorithm is developed.Manual searching for MCEs is a time-consuming and energy-consuming task.The existing automatic identification methods all have their own limitations.By synthetically consider the particle and magnetic field data observed by satellite,we first determine which region(magnetosphere,magnetosheath or the so-lar wind)the satellite is located through particle flux data,then the candidate MCE time is obtained based on the definition of MCE.The accurate MCE time is obtained through magnetic field data.Finally,the MCE time is reexamined by the particle and field data on both sides.By checking the results of our identification procedure,it is find that our four-step identification algorithm can avoid the false identification to the best and this provides us with great convenience for our next work.This identification algorithm is applied to the observation of THEMIS between 2007 and 2018 when THEMIS’ apogee is located near the subsolar point and some key parameters(magnetopause normal,mag-netopause velocity,etc.)are calculated automatically to construct a database of MCEs.(3)The 16758 MCEs found in second work are classified into three types based on the variation characteristics of the magnetospheric magnetic field observed near the magnetopause:(1)the magnetic disturbance is large and there is no clear trend?(2)the farther away from the magnetopause,the weaker the magnetic field is?(3)the far-ther away from the magnetopause,the stronger the magnetic field is.The relationship between the change rate of the magnetospheric magnetic field and the velocity of the magnetopause in the second and third types instances are studied respectively.It is found that there is a linear relationship between the two variables in the second type instances and the absolute value of the slope is larger when the magnetosheath mag-netic field is northward,which implies that the subsolar magnetospheric magnetic field is more compressed under northward magnetosheath magnetic field? For the third type instances,there is no significant relationship between the two variables.Finally,we construct a simple model to explain the linear relationship in the second type instances,and why there is no significant relationship between the two variables in the third type instances is discussed.(4)In addition to the magnetopause studying,the magnetotail plasma sheet ULF waves triggered by interplanetary shock are analyzed indetail.Two cases of ULF wave in the magnetotail plasma sheet triggered by interplanetary shock were observed by TC-1 on November 7,2004.It is found that the two ULF waves all contain strong toroidal standing Alfven mode through wavelet analysis.Finally,the possible trigger mechanism of ULF wave in these two cases are discussed.