Observations Of Dynamic Processes In The High-Speed Flows In The Terrestrial Magnetosphere

The plasma often flows toward the Earth or away from the Earth rapidly in the Earth’s magnetotail.These high-speed flows promote the circulation of energy and matter in the magnetosphere.The plasma particles in the solar wind are transported from the Earth’s magnetotail to the Earth’s inner magnetosphere,being carried on the"fast train" of high-speed flows.These magnetic fluxes and particles transported to near Earth can be injected into the Earth,resulting in beautiful auroras in the polar regions and having a certain impact on satellite navigation,communication networks,and power transmission lines.In addition to being transported to the Earth,plasma mass may be transported back into interplanetary space via high-speed tailward flows produced by magnetic reconnection in the magnetotail.The voltage on the lunar’s surface will rise as it passes through the magnetotail,which will affect astronauts’ lunar exploration activities.It can be seen that the magnetotail is a very important region in the Earth’s magnetosphere.Magnetic reconnection can convert electromagnetic energy into kinetic and thermal energy of particles,generating high-speed flows in the magnetotail.High-speed flows could transport the energy to various regions in the magnetotail.In high-speed flows,the current sheet flapping occurs frequently.Various substructures of high-speed flow are closely related to particle acceleration and plasma waves.These substructures mainly include electron jets and dipolarization fronts,etc.Although previous studies have extensively investigated high-speed flows,the dynamic processes in high-speed flows are not fully understood.With the continuous improvement of satellite detection technology,the resolution of the data is increasing.These high-resolution data help us understand the dynamic processes in high-speed flows in more detail.In this paper,we use observation data from the Magnetospheric Multiscale(MMS)satellites to study the short-period current sheet flapping,electron jet,electron acceleration and plasma waves near the dipolarization front,and the electron pitch angle distribution in plasma bubble and in high-speed tailward flow.The following are the main results of this paper.1.A short-period current sheet flapping event with a short semi-period of～6 s is reported for the first time.This short time period flapping event consists of five consecutive crossings of the current sheet.By using the calculated azimuthal angle of magnetic gradient and the propagation velocity of flapping motion,it is found that the first four crossings propagated duskward and belong to kink-like flapping,and the fifth crossing belongs to steady flapping.The current sheet flapping was embedded in the diffusion region of magnetic reconnection,which was identified by the well-organized Hall electromagnetic field.The period of current sheet flapping was modulated by the reconnection electric field and perpendicular plasma flow,indicating that this flapping motion may be triggered by the periodical unsteady magnetic reconnection.2.In this paper,the electron-scale dynamical processes in the substructures of the high-speed flow including electron jets and dipolarization fronts are studied.We perform one systematic statistical study on the electron jets in the terrestrial magnetotail.It is found that electron jets belong to super-Alfvénic(ion Alfvén speed)flows.An electron jet can be observed every 4.6～5.3 hours.In addition,the relative positions of the electron jets that are often found in different structures are also identified in the present study.It is found that the electronjet mainly occurs in the center of the current sheet,at the dipolarization front,in the interior of the magnetic hole,and near the lobe region during the crossing of the plasma sheet boundary layer.The electron jets have almost the same probability at the interior and exterior of flux ropes.Broadband high-frequency electrostatic waves in the dip region ahead of the dipolarization front are reported.The frequency range of these electrostatic emissions is higher than electron cyclotron frequency but lower than electron plasma frequency.These high-frequency waves are quasi-parallel propagating,and their parallel components dominate.Based on the measured electron distribution function,the kinetic theory dispersion relationship analysis reveals that the electron distribution with a positive slope of the field-aligned distribution function is responsible for the generation of high-frequency electrostatic waves in the magnetic dip ahead of the dipolarization front.A direct quantitative analysis of the acceleration processes at the dipolarization front is performed.It is found that the fluxes of energetic electrons in the direction perpendicular to the magnetic field are enhanced at the front.Under adiabatic conditions,our quantitative analysis indicates that these electrons at the front could be locally accelerated to over 100 keV by the betatron acceleration mechanism.Our quantitative study provides,for the first time,strong direct evidence for the local electron acceleration at the dipolarization front.3.The electron pitch angle distributions in the plasma bubble and high-speed tailward flows in the Earth’s magnetotail are analyzed completely.The pitch angle distributions(PADs)of high-energy electrons exhibit three different features at four different substructures in this plasma bubble:pancake-type;isotropic type;cigar type.All these changes of electron PADs in substructures can be interpreted mainly by the capture of magnetic mirror and betatron cooling.Highfrequency waves,including whistler waves and electrostatic waves,are detected in several parts of the plasma bubble.The observations of abundant electron-scale physical processes reveal that the electrons are very dynamic in the plasma bubble.The PADs in the high-speed flows in the Earth’s magnetotail are statistically studied.We find that field-aligned anisotropy of the PADs of superthermal electrons(>2 keV)dominates in 66.3%tailward flows;the PADs of suprathermal electrons in 26.3%tailward flows are mostly isotropic;the perpendicular anisotropy of the PADs of superthermal electrons dominates in 7.4%tailward flows.For the field-aligneddominated type flows,the field-aligned anisotropy of the PADs of superthermal electrons will gradually become weaker,with the decrease of the distance from the center of the plasma sheet(CPS),or with the increase of the energy.For perpendiculardominated type flows,the perpendicular anisotropy of the PADs of superthermal electrons will become more evident,with the increase of the energy.