Characteristics Of Magnetic Structures In Terrestrial Space

Plasma is a macroscopically neutral system which contains lots of interacting ionized particles.Because of the much higher ion mass comparing to electron,plasma is a complicated multiple characteristic scales system.Thus it is necessary to carefully choose different models corresponding to the relevant scale size when analyzing magnetic structures.Although there are many studies for the large(i.e.MHD,magnetohydrodynamics)scale magnetic structures,few of them are for the small(i.e.kinetic)scale.Meanwhile,many deficiencies still exist in the previous studies.Hence the parameters and generation mechanisms of different scales magnetic structures(e.g.magnetic holes,magnetic peaks,foreshock caviton,and hot flow anomalies)in the terrestrial space are investigated in this study.In the first study of Cluster observations,by the timing method,the minimum directional difference(MDD)method,and the spatiotemporal difference(STD)method,we obtain the propagation velocity of plasma sheet kinetic-size magnetic holes(KSMHs)in the plasma flow frame.Furthermore,based on electronmagnetohydrodynamics(EMHD)theory we calculate the velocity,width,and depth of the electron solitary wave and compare it to KSMH observations.The result shows a good accord between the theory and the observation.In the second study we report such KSMHs in the magnetosheath whereby we use measurements from the Magnetospheric Multiscale mission,which provides three-dimensional(3-D)particle distribution measurements with a resolution much higher than previous missions.The MHs have been observed in a scale of 10-20 ρe(electron gyroradii)and lasted 0.1-0.3 s.Distinctive electron dynamics features are observed,while no substantial deviations in ion data are seen.It is found that at the 90° pitch angle,the flux of electrons with energy 34-66 eV decreased,while for electrons of energy 109-1024 eV increased inside the MHs.We also find the electron flow vortex perpendicular to the magnetic field,a feature self-consistent with the magnetic depression.Moreover,the calculated current density is mainly contributed by the electron diamagnetic drift,and the electron vortex flow is the diamagnetic drift flow.Test particle is used to simulate the electron acceleration of the structure.The results fit well with the observations.The electron magnetohydrodynamics soliton is considered as a possible generation mechanism for the KSMHs with the scale size of 10-20 pe.In the third study,small scale magnetic flux rope is found in the magnetosheath boundary layer with the use of high time resolution MMS data.For this flux rope,the scale size is closed to ρi(ion gyroradii);the axis is along the background magnetic field orientation;the field-aligned current is contributed by the electron beams and the perpendicular current is partly contributed by the diamagnetic drift.In the last two works,the propagation velocity of large scale magnetic holes and hot flow anomalies are calculated.Propagating,contracting and expanding magnetic holes are found in the magnetosheath.Clearly electron pitch angle distributions of magnetic mirror are firstly found in this work.Furthermore,we find the propagation velocity of hot flow anomalies are closed to the fast mode magnetosonic wave.Taking the plasma parameters into consideration,we infer that these structures may have the characteristic of the MHD fast mode soliton.