Electron Dynamics Processes In The Separatrix Region Of Magnetic Reconnection

Magnetic reconnection is an important physical process,which is universal in space,astrophysical,and laboratory plasmas.It can efficiently convert magnetic energy into plasma energy,and change the macroscopic topology of the magnetic field.Magnetic reconnection is the main cause of various explosive phenomena in the plasma environment.There are various complex plasma dynamics processes near X-line of magnetic reconnection,such as the excitation of various plasma waves,the interaction between plasma wave with charged particles,and the generation of high-energy particles,etc.These plasma dynamics processes may in turn affect the magnetic reconnection.Therefore,studying the dynamic processes in collisionless magnetic reconnection is crucial for understanding magnetic reconnection.The separatrix region is the boundary region between the ion inflow and the ion outflow,and it is also the channel for the electron inflow.Therefore,the electron dynamics processes inside the separatrix region are key to understanding the wave excitation and electron acceleration during magnetic reconnection.In this dissertation,we mainly use the high-resolution MMS data to study the physical processes related to electron dynamics in the separatrix region of magnetic reconnection,which includes the observations of the nonideal electric field in the separatrix region,the electron acceleration in the separatrix region,and the excitation of various plasma waves in magnetic reconnection.The main results are as follows:1.The observations of the nonideal electric fields in the separatrix regionBased on the MMS data,we have observed the nonideal electric field in the separatrix region of magnetotail reconnection.According to the generalized Ohm’s law,we estimate the contributions of the electron pressure gradient term,the electron inertia term and the anomalous resistivity term to the nonideal electric field.The contributions of the electron inertia term and the anomalous resistivity term are negligible.In the same region,the nongyrotropic electron velocity distribution and significant energy dissipation are observed.Therefore,it can be inferred that the nonideal electric field in the separatrix region is supplied by the electron pressure gradient term,even though the estimated electron pressure gradient term has a large perturbation.This conclusion is consistent with the previous simulation results.At the same time,the observed features of the separatrix region are consistent with the typical features of the electron diffusion region,indicating that the internal electron diffusion region may extend along the separatrices,or some electron-scale instability can be destabilized in the separatrix region.2.Study of the electron acceleration in the separatrix regionUsing the MMS data,we investigate the electron aceeleration in the separatrix region for two reconnection events respectively.In the observation event on June 19,2017,the field-aligned inflowing electrons are observed in the separatrix region,and their energy can be extended to l0keV,which indicates that the inflowing electrons have been accelerated before entering the electron diffusion region.Along the separatrix region,a net parallel potential is observed,and it may be the reason for the acceleration of the inflowing electrons.For the reconnection event on July 11,2017,the flux of high-energy electrons is significantly enhanced in the separatrix region,and the electron energy can be up to 150keV.In this region,the electrons at the low-energy band(2-10keV)exhibit field-aligned distribution and move mainly toward the X-line along the separatrix.The electrons at the high-energy band(39-149keV)are mainly distributed in the field-aligned and perpendicular directions,and the electrons move away from the X-line.Meanwhile,the energy distribution of the high-energy electrons in the separatrix region shows a power-law spectral distribution,indicating that the electrons are accelerated.For the high-energy electrons,we propose a possible acceleration mechanism:the electrons with smaller parallel velocity can be trapped by the parallel potential and the magnetic mirror effect in the separatrix region and enter the electron diffusion region many times,thereby being accelerated to a high energy by the reconnection electric field.3.Excitation of Plasma Fluctuations in magnetic ReconnectionBased on the MMS data,whistler waves and Electron Bernstein waves are observed simultaneously in the separatrix region of magnetopause reconnection.The frequency of whistle wave is near 0.6 fce,and their wave normal angle is about 20°-40°.The Poynting vector of whistler waves in the parallel direction is positive,indicating that these waves propagate parallel to the background magnetic field.Combined with the local magnetic field topology,it can be judged that whistler waves propagate away from the X-line.For Electron Bernstein waves,the peaks of wave power density are observed in a range between the first and fifth harmonic of the electron cyclotron frequency,and its wave vector direction k is almost perpendicular to the background magnetic field by minimum variance analysis.Furthermore,the electrons have the losscone distribution at the power density peaks of these two waves.Based on the electron velocity distributions and linear theory,we infer that whistler waves and Electron Bernstein waves in the separatrix region are generated by the electron loss-cone instability.Based on.MMS data,the broadband electrostatic waves are observed in the electron diffusion region of magnetotail reconnection for the first time.Through the analysis of the electric field waveform,it is found that the parallel electric field of these broadband electrostatic waves exhibits two different types of waveforms:one is a series of isolated bipolar structures,which is the typical characteristics of electron holes,the other is continuous bipolar structure.The parallel spatial scales l‖ of these electron holes are～3-5 λD(λD is the Debye length),and the perpendicular spatial scales l⊥ of these electron holes are larger than～18.6λD.Therefore,the length ratios l⊥/l‖ of these electron holes are larger than～3.7-6.2,which means that these electron holes have oblate shapes.Associated with these electrostatic waves,electron beams propagating away from the X-line have been observed.Based on the electron velocity distribution and the linear theory analysis,we think that these electron holes are generated by the electron bump-on-tail instability.Combining to the 1-D electrostatic PIC,it can be inferred that these different waveforms of the broadband electrostatic waves should be the products at different stages during the time evolution of the bump-on-tail instability.Our observations also suggest that the electron diffusion region may be the source of high-frequency electrostatic waves.In summary,various complex electron dynamics processes are observed in the separatrix region of magnetic reconnection,and these physical processes have important contributions to energy dissipation and transfer in the reconnection.These results can help us to understand magnetic reconnection,and also stimulate our new thinking on some issues in the reconnection.These results have certain reference value for the future research on magnetic reconnection.