Observation-based Statistical Study Of KH Instability In The Earth's Magnetopause

The solar wind-magnetosphere coupling is an important part of the solar-terrestrial environment.The magnetopause,as the interface between the thin,weakly magnetized solar wind and the Earth's magnetosphere,plays an important role in the process of solar wind particles,momentum,and energy transfer into the magnetosphere.The study of the energy transport mechanism between the solar wind and the magnetosphere as a significant part of solar windmagnetosphere coupling of solar has attracted much attention.During the southward(IMF),the interplanetary magnetic field antiparallel to the Earth's magnetic field at the subsolar magnetopause,the magnetic reconnection is the main mechanism of plasma transfer.While,during the northward IMF,high-latitude reconnection,viscous transport mechanism,and Kelvin-Helmholtz(KH)instability are the possible mechanisms of plasma transfer.KH instability is a process that occurs in the boundary layer of two different fluids with large velocity shear.The Earth's magnetopause,which has large velocity shear,is an ideal place for KH instability to arise and develop.Previous research on KH waves focused on the sun side and flank magnetopause,in this paper we have investigated the characteristics of KH waves and velocity shear layers at the Earth's magnetosphere using the observations from the THEMIS satellite at lunar distances.The spatial distribution and the effects of solar wind conditions and magnetosheath conditions have also been investigated.In addition,we have also reported the KH vortex event observed at lunar distances during the southward IMF.This study can reveal the evolutionary process as well as the physical characteristics of the KH instability.The main research contents and conclusions of this paper are as follows.(1)Using the observations from the THEMIS satellite between 2007 and 2009 and the ARTEMIS satellite between 2015 and 2020,we have obtained 72 KH wave events at the dayside,flanking,and tail magnetopause.The wavelet analysis is used to determine the dominant period of KH waves,the phase velocity of KH waves is the center-of-mass velocity,and the wavelength of KH waves is calculated according to the dominant period and phase velocity.We have analyzed the influence of solar wind conditions on the KH wave characteristics,and the corresponding upstream solar wind observations are obtained from the WIND satellite.The results show that with the increasing solar wind velocity,the phase velocity and dominant period of KH waves increase.There also exists a correlation between the IMF cone angle and the phase velocity and dominant period of KH waves.In addition,the analysis also shows that the magnetic field and thermal pressures in the magnetosheath also affect the wavelength of KH waves.(2)Using the observations from the ARTEMIS satellite during 05:40 UT to 06: 15 UT on 18 September 2016,we have reported a KH wave event observed at lunar distances under southward IMF.Previous studies have shown that KH wave transport events have similar quasi-periodic fluctuations with the flux transport events(FTEs)during southward IMF.Our detailed analysis of the velocity,magnetic field,and pressure during the interval of interest confirms that the event has both FTE and KH wave observations.The results show that the normal directions on both sides of the KH wave change largely,and there has an interval between two wave structures during southward IMF.Compared with the KH wave under northward IMF,the KH waves under southward are characterized by irregularity and intermittence.(3)We investigate the effects of the magnetic field,density,total pressure,and Alfvén speed in the magnetic sheath on the thickness of the velocity shear layer.The velocity shear layer is the interface where the KH instability is excited and its thickness is an important parameter of KH instability,so the research on the velocity shear layer is helpful to understand the excitation mechanism and growth mode of KH instability.In this paper,the criteria for judging velocity shear layer crossing events have been formulated.Using the observations from the THEMIS satellite from 2007 to 2008 and the ARTEMIS satellite from 2011 to2020,we have obtained 65 velocity shear layer crossing events at the dayside,flank,and tail magnetopause.In order to ensure that the KH instability can be satisfied for all these events,we have examined these events using the KH instability criterion,and the events that meet the instability criterion have been retained.The thickness of the velocity shear layer is calculated in LMN coordinate,and the LMN coordinate has been determined using Minimum Variation Analysis(MVA)and Faraday Residual Minimum Variance Analysis(MFR).We have investigated the effects of the magnetic field,density,total pressure,and Alfvén speed in the magnetic sheath on the thickness of the velocity shear layer.The results show that the average thickness of the velocity shear layer is 2102 km and 21.53 in ion cyclotron radius at the near-Earth distance,and 3377 km and 22.71 in ion cyclotron radius at the lunar distance.The thickness of the shear layer has dawn-dusk asymmetry,and the average thickness on the dusk side is 2803 km,which is greater than that on the dawn side,which is 2166 km.In addition,the velocity shear layer becomes thinner as the ion density,magnetic field,and total pressure in the magnetic sheath decrease.(4)Based on the observations,the spatial distribution of velocity shear layer and wavelength have been analyzed.The results show that the dominant period and wavelength of KH waves increase when the KH waves propagate to the magnetotail,and the phase velocity changes a little.The wavelength of KH waves and the thickness of velocity shear layer at the Lunar distance are greater than those at near-Earth magnetopause.In addition,the ratio of KH wavelength to velocity shear layer thickness at the lunar distance is about four times that at the near magnetopause,indicating that KH wave at the lunar distance may not of the fastest growing mode.The KH waves at the lunar distance may not be excited locally,but at the dayside magnetopause and propagate along the magnetopause to the magnetotail.The process of propagation is accompanied by the vortex merging...