Observations Of Kinetic-size Magnetic Holes And Dipolarization Fronts In The Terrestrial Magnetotail

The magnetotail is an important ingredient of the Earth’s magnetosphere and it is also a natural laboratory for human research on plasma physics.For macroscopically systems such as plasma,there are differences in parameters among particles at different time and space scales.In the past,due to accuracy constraints of satellite data,there are a lot of researches focus on large（i.e.,MHD,magnetohydrodynamics）scale magnetic structures.With the rapid development of satellite technology and information processing technology,more and more attention has been paid on kinetic-size magnetic structures.Magnetic holes（MHs）and dipolarization fronts（DFs）are widely exist in magnetotail.Kinetic-size magnetic holes（KSMHs）are considered to effectively affect the electron dynamic and energy exchange between the plasma and the field.DFs are crucial ingredients of substorms as they play an important role in energy exchange and electron acceleration.Takes advantage of high-time resolution field and plasma measurements from the Magnetospheric Multiscale（MMS）mission,we present observational studies of the DFs and MHs,including the occurrence rate distributions,electron properties and waves of KSMHs in the terrestrial magnetotail plasma sheet,as well as distribution and motion characteristics of DFs,and the relationship of velocity between the DFs and the plasma flow.Studying the KSMHs and the DFs in the Earth’s magnetotail and figuring out their dynamics are of great significance for us to understand energy transmission and particle motion.The KSMHs in the terrestrial magnetotail plasma sheet are statistically investigated using the data from the MMS.The occurrence distributions of KSMHs have dawn–dusk asymmetry（higher occurrence on the duskside）,which may be caused by the Hall effect.Most events of KSMHs（71.7%）are accompanied by the substorm,implying that the substorm may provide favorable conditions for the excitation of KSMHs.However,there is a weak correlation between the KSMHs and the reconnection.The statistical results reveal that for most of the events,the electron total temperature and perpendicular temperature increase while the electron parallel temperature decreases inside the KSMHs.The electron temperature anisotropy(T_e⊥/T_e||>1)is observed in 72%of KSMHs.Whistler-mode waves are frequently observed inside the KSMHs,and most（92%）KSMHs associated with whistler waves have enhancements of electron perpendicular distributions and satisfy the unstable condition of whistler instability.This suggests that the observed electron-scale whistler waves,locally generated by the electron temperature anisotropy,could couple with the electron-scale KSMHs.The observed features of KSMHs and their coupling to electron-scale whistlers are similar to the ones in the turbulent magnetosheath,implying that they are ubiquitous in the space plasmas.The generation of KSMHs in the plasma sheet could be explained by an electron vortex magnetic hole,magnetosonic solitons,and/or interchange instabilities.As for the observation of the DFs in the magnetotail,based on the observation data from the MMS satellite in the magnetotail（May 2017-September 2017）,the statistical research on the relationship between the plasma flow and the DF in the magnetotail plasma sheet is performed,and a total of 48 earthward propagating DF events is selected.The normal velocity（V_n）and direction of the DFs are calculated by Timing analysis method,and the local plasma flow velocity（V_i）and direction are measured at the same time by MMS.The results indicate that in most cases,the normal velocity of the DF is greater than the velocity of the plasma flow,and the overall velocity of the plasma flow is opposite to the overall motion of the DF in the dawn-dusk direction and the north-south direction,which indicates that the plasma flow pressure has a certain effect on the deflection of the propagation direction of the DF.We defined the events that meet direction angle between V_n and V_i at 0–35°and V_n/V_i between 0.6 and 1.5 as co-movement events where the DF and the plasma flow are frozen.According to the statistical results,a total of 14 events in which the DFs and the plasma flow moved together were selected,accounting for 29.2%of the total events.The DF events are classified into three categories:growing,steady,and decaying for statistical analysis.It was found that co-moving events accounted for 30%in growing events,29.4%in steady events,and 27.3%in decaying events.At the same time,it was found that the velocity of the plasma flow and the propagation velocity of the DF during the steady event were smaller than those of the other two types.