Observations Of Kinetic Scale Magnetic Structures In Terrestrial Space

Electromagnetic environment in terrestrial space is important,inherently complex,and rich,exhibiting various kinds of energy conversion,particle acceleration,and mass and momentum transportation.Particularly,the perturbations of magnetic field are significant components in these processes.Due to the low resolution of available spacecraft data,previous studies have mainly focus on the large(magnetohydrodynamics,MHD)scale magnetic perturbations,and seldom go deep into small size(kinetic-scale).However,understanding kinetic-scale magnetic perturbations with charged particles is critical to predicting and unraveling the relevant physics of these fundamental processes.The launch of Magnetospheric MultiScale(MMS)satellites,which on board unprecedented high-temporal cadence particle and electromagnetic field instruments,offers an unsurpassed opportunity to investigate microphysics at the ion gyro-scale or even the electron dynamics-scale in space.We hereby study the magnetic variations in kinetic-scale in terrestrial solar wind and magnetosheath,including their electromagnetic and plasmas features,propagations,dynamics,current systems,particle distributions and acceleration,and relation to various kinds of plasma waves.1.Magnetic holeKinetic-scale magnetic holes(KSMHs),with a significant depression in magnetic field strength,and scale length close to and less than one proton gyroradius,were reported in the turbulent plasmas both in recent observation and numerical simulation studies.These KSMHs play important roles in energy conversion and dissipation.In Chapter 2,we present observations of the KSMHs which are labeled whistler mode waves,electrostatic solitary waves,and electron cyclotron waves in the magnetosheath.The observations suggest that electron temperature anisotropy or beams within KSMH structures provide free energy to generate these waves.In addition,the occurrence rates of the waves are higher in the center of the magnetic holes than at their edges,implying that the KSMHs might be the origin of various kinds of waves.We suggest that the KSMHs could provide favorable conditions for the generation of waves and transfer energy to the waves in turbulent magnetosheath plasmas.To date,there are still three major issues remain unresolved for the KSMHs:1.the source(locally generated in near-Earth space,or carried by the solar wind);2.their generation mechanism and environmental conditions leading to their generation;3.their spatial and temporal evolution.In Chapter 3,KSMHs in near-Earth space are statistically investigated using data from MMS mission.Approximately two hundred thousand events are observed from September 2015 to March 2020.Occurrence rates of such structures in the solar wind,magnetosheath,and magnetotail are obtained.It is found that the occurrence rate of the KSMHs in the magnetosheath is far above that in the solar wind.This indicates that most of the structures are locally generated in the magnetosheath,rather than convected from the solar wind.Moreover,the occurrence rate of the KSMHs in the downstream region of the quasi-parallel shock is significantly higher than in the downstream region of the quasi-perpendicular shock,indicating a relationship with the turbulent plasma environment.When close to the magnetopause,we find that the depths of KSMHs decrease as their temporal-scale increases.We also find that the spatial-scales of the KSMHs near the subsolar magnetosheath are smaller than those in the flanks.Furthermore,the global distribution shows a significant dawn-dusk asymmetry(duskside dominating)in the magnetotail.As far as we are aware,there have been no previous studies concerning the evolution(contracting or expanding)of these types of structures,and their propagation properties cannot be unambiguously determined.In Chapter 4,using MMS high-temporal resolution data and multi-spacecraft analysis methods,we obtain the propagation and dynamic features of a set of magnetic holes.Four different types of MHs are identified;"frozen-in","expanding","contracting" and"stable-propagating".Significantly,a stable-propagation event is observed with a sunward propagation component.This indicates that the source of the structure in this case is closely associated with the magnetopause,which provides strong support to the contention in earlier research.We further reveal the mechanism leading to the magnetic hole contraction or expansion.The motion of the magnetic hole boundary is found closely related with the dynamic pressure.The scale of the contracting and expanding events are typically-5-20 ?i(ion gyroradius),significantly smaller than that of frozen-in events(?40 ?i).The observations could relate large-scale(more than several tens ?i)and kinetic-scale(less than ?i)magnetic holes,by revealing an evolution that spans these different scales,and help us better understand the variation and dynamics of magnetosheath structures and plasmas.2.Magnetic mirror mode.Mirror mode is typical magnetic perturbation structure,and is widely observed in space plasma environments.Although plasma features within the structures have been extensively investigated in theoretical models and numerical simulations,relatively few observational studies have been made,due to a lack of high-cadence measurements of particle distributions in previous space missions.In Chapter 5,electron dynamics associated with mirror mode structures are studied based on MMS observations of electron pitch angle distributions(PADs).We define mirror mode peaks/troughs as the region where the magnetic field strength is greater/smaller than the mean field.The observations show that most electrons are trapped inside the mirror mode troughs,and display a donut-like PAD configuration.Besides the trapped electrons in mirror mode troughs,we find that electrons are also trapped between ambient mirror mode peaks,and co-existing un-trapped electrons within the mirror mode structure.Analysis shows that the observed donut-like electron distributions are the result of betatron cooling and the spatial dependence of electron pitch angles within the structure.The small-scale mirror mode excited by electron dynamics is also a fundamental physical process,attracting research interest in space,laboratory,and astrophysical plasma physics over the past half century.However,the investigations of this process were mostly limited to theories and numerical simulations,with no direct observational evidence for their existence.In Chapter 6 we present clear observations of electron mirror-mode using MMS data at unprecedented high-temporal cadence.These structures are train-like,compressible,non-propagating and satisfy the theoretical excitation and electron trapping conditions.They were observed near the Earth's foreshock and its downstream turbulence during the Corotating Interaction Region events(CIRs),which could involve with the interaction between solar wind and Earth.3.Magnetic peakThe sudden enhancements of magnetic strength,named Magnetic peaks(MPs),are often observed in the magnetosheath and solar wind of magnetized planets.They are usually identified as flux ropes(FRs)or magnetic mirror mode structures in the magnetosheath.Previous studies of MPs are mostly on the MHD-scale.In Chapter 7,an electron-scale MP is reported in the Earth magnetosheath.We present a typical case with a scale of?7 electron gyroradii and a duration of?0.18 s.A strong magnetic disturbance and associated electrical current are detected.Electron vortex is found perpendicular to the magnetic field line and is self-consist with the peak.We use multi-point spacecraft techniques to determine the propagation velocity of the MP structure,and find that the MP does propagate relative to the plasma(ion)flow.This is very different from the magnetic mirror mode that does not propagate relative to the plasma flow.Furthermore,we developed an efficient method that can effectively distinguish "magnetic bottle like" and "FRs like" structures.The MP presented in this study is identified as "magnetic bottle like" type.The mechanism to generate the electron-scale "magnetic bottle like" structure is still unclear,suggesting that new theory needs to be developed to understand such small-scale phenomena.The low temporal resolution of spacecraft data prevent previous studies on the kinetic-scale flux rope(KFR).In Chapter 8,we present a new type of KFR(size?1 ion gyroradius)in the Earth's dayside magnetosheath boundary layer with MMS high-temporal cadence data.This structure exhibits a slight twist of magnetic field that is possibly generated by a field-aligned current,which differs from typical dayside flux ropes usually observed within the current sheet where magnetic reconnection can occur.The perpendicular electron fluxes within 19-52 eV is increased?10%inside the KFR.Detailed analysis shows that these perpendicular electrons may encounter their mirror point(at the position of the KFR,strong field region)when traveling from the magnetosheath toward the ionosphere,and will be reflected to the magnetosheath.A possible scenario is that this KFR is different from previous flux ropes that transfer electron flux to the magnetosphere,but could intercept magnetosheath large pitch angle electron flux to the magnetosphere.The short large-amplitude magnetic structure(SLAMS)is also an observably magnetic enhancement near the terrestrial bow shock.In Chapter 9,the ultralow frequency whistler waves(U-WHs,-2 Hz)were found associated with the SLAMS and strongly modulated the electron bulk flows,resulting in a relative motion between electrons and ions to cause intense current.Meanwhile,low band whistler waves(L-WHs,?0.2 kHz)were observed embed in the front of the SLAMS and the troughs of the U-WHs,while the high band whistler waves(H-WHs,?1 kHz)were hanged on the SLAMS hump.The whole structure was filled with high frequency electrostatic waves(H-EWs,1-3 kHz).Multi-spacecraft analysis methods were applied to investigate the evolution of the SLAMS.The results indicated that the structure was still rapidly developing to become a stronger pulse.The observations strongly suggest that the SLAMS are complex in structure and play an important role in energy conversion,wave-particle,and wave-wave interaction in the space environment.