Study Of The Kinetic Scale Current Structure In The Solarterrestrial Space Environment

As the "Fourth Environment" of human life,Solar-Terrestrial space environment has been paid more and more attention.Intense solar activity will cause drastic changes in the Earth’s space environment and seriously affect the satellite navigation,surface communication and human life.According to the research,all space weather events are inseparable from energy transport and conversion.Therefore,it is especially important to study the energy transport and conversion for correctly understanding our living environment and carrying out space weather forecasting.Magnetic reconnection is one of the significant mechanisms of solar wind matter and energy entering the magnetosphere,and it is a multi-scale physical process in the current structure.Before the launch of Magnetospheric Multiscale(MMS)satellite,people can only study the influence of magnetic reconnection on energy transport and conversion from fluid scale or ion scale current sheet.With the electron scale regular tetrahedron structure,the MMS spacecraft not only observed the current structure of kinetic scale,but also found that the kinetic scale current structure is often accompanied by explosive energy conversion.This process often occurs in the magnetic flux rope,the reconnection diffusion region,the reconnection exhaust,Kelvin-Helmholtz vortex and other structures.What are the basic physical properties of the kinetic scale current structure?What role do they play for the generation and evolution of multi-scale coherent structures?Are their corresponding energy conversion all related to magnetic reconnection?Where does energy conversion take place preferentially?These are all issues that are important at the moment but are not fully studied.In this doctoral thesis,we mainly use the high-precision data of MMS to study the kinetic scale current structure in the Solar-Terrestrial space environment.We mainly discuss the kinetic scale current structure related to the magnetic flux rope and the kinetic scale structure characteristics with the intense current at the magnetopause boundary layer.The following are the main research results of this paper.1.Kinetic scale current structure related to magnetic flux rope:We report the first in situ observation of magnetic reconnection between an earthward propagating flux rope and the closed magnetic field lines connecting to Earth.The MMS spacecraft discovered an asymmetric magnetic flux rope at Earth’s magnetotail and an electron-kinetic scale vertical current sheet between its fore-end(negative BZ)and the Earth’s dipole field(positive BZ).Through the observation evidence such as electron outflow and Hall effect,it was found that the vertical current sheet is reconnecting.The MMS spacecraft passed through the diffusion region of the magnetic reconnection.Prior to this,scientists speculated that the earthward propagating magnetic flux rope might be reconnected with the geomagnetic field.Our observation not only confirmed this prediction for the first time,but also provided an alternative formation mechanism for the dipolarization fronts often observed in nearEarth magnetotail.We discovered an electron-only reconnection at the edge of a macroscopic magnetic flux rope.The MMS satellite observed a macroscopic-scale,highly oblique magnetic flux rope at the duskside in the Earth’s magnetotail.A kinetic-scale current sheet at the edge of the magnetic flux rope,in which a reconnection with a large guide field is taking place was also observed.Interestingly,the ions do not respond in any way during the entire reconnection process,which is similar to the electron-only reconnection observed in the magnetosheath turbulence.Through multi-event comparison,we found that the reconnection associated with macroscopic magnetic flux ropes tends to be electron-only reconnection,and these processes can occur under different plasma background conditions.This result provides the new clues to the understanding of the multiscale coupling associated with magnetic flux ropes in space plasma.The fine structure of the ion-scale magnetic flux rope corresponding to a strong parallel current is reported.The study found that there is a certain correlation between the electron density,electron temperature and electron pitch angle distribution inside the magnetic flux rope.We believe that the coexistence of different magnetic field line topologies inside the ion-scale flux rope is responsible for this correlation within the magnetic flux rope.MMS gives us the opportunity to study the electron features inside the ion-scale magnetic flux rope,and these results can be useful for understanding the reconnection process in three-dimension and the generation mechanism of the ion-scale flux rope.2.The properties of the kinetic scale current structure with intense current at the dayside magnetopause boundary layer:We statistically study the basic physical properties of the intense current structures at the dayside magnetopause boundary layer.We selected about 3,600 intense current structures(|J|>1.2 μ A/m2)at the dayside magnetopause boundary layer.The statistical results show that most of the current structures have a short duration and correspond to electron scale structures.Boundary layers closer to Earth have more intense current structures than boundary layers further away from Earth,and the occurrence rate is higher in the dusk sector near the meridian.For most intense current structures,the current is mainly carried by electrons and perpendicular current.The energy conversion(J·E)corresponding to these structures is dominated by perpendicular currents and electric fields.However,non-ideal energy conversion(J·E’)is dominated by parallel directions.These structures provide much stronger energy conversion compared to the ambient plasma in the magnetopause boundary layer.This study helps us better understand the characteristics of intense current structures and the role they play in solar wind-magnetosphere coupling.We studied more than 300 high-efficiency non-ideal energy conversion events(J·E’>4 nW/m3)observed at the dayside magnetopause boundary layer.The results show that most of the events occur in kinetic scale structures.About three-quarters of the events are reconnection events,and are mainly electron-only reconnection.The reconnection exhaust and the Kelvin-Helmholtz vortex are the two regions where these events are most commonly observed,illustrating the importance of magnetic field reconnection and Kelvin-Helmholtz instabilities for non-ideal energy conversion during solar wind-magnetosphere coupling.We analyzed the frozen-in conditions of the reconnection diffusion region at the dayside magnetopause.The results show that non-ideal electric field and the frozen-in condition have a good correlation at the ion diffusion region and the electron diffusion region,and this shows in turn that it is reliable to judge whether the frozen-in condition is destroyed by whether the non-ideal electric field is zero.In addition,the parallel components of the frozen-in conditions in the diffusion region b·(▽×(E+Vs× B)is greater than the perpendicular component b ×(▽×(E+Vs×B),indicating that in the diffusion region,magnetic compression or magnetic expansion is the main factor leading to the destruction of frozen-in conditions.These results deepen our understanding of the fundamental properties and their role of kinetic scale structures in the Solar-Terrestrial space environment,which will help us further understand the kinetic processes in Solar-Terrestrial space and the largescale space weather effects they cause.