Study Of Multiscale Structures Associated With Magnetic Reconnection

Magnetic reconnection is a fundamental physical process in nature and laboratory plasma systems.It can quickly convert magnetic energy to plasma thermal and kinetic energy during the macroscopic changes in magnetic field topology.Magnetic reconnection can result in many explosive space weather in the Solar-Terrestrial space environment,such as magnetospheric substorm and solar flare.Magnetic reconnection involves the coupling between the large fluid scale and the kinetic scale of the system.Magnetic reconnection frequently occurs in multiscale structures and can generate some multiscale structures,such as reconnection diffusion region,magnetic flux rope,dipolarization front,magnetic hole and turbulence,etc.How are these multiscale structures generated? What role do they play in the onset and evolution of reconnection? Will they affect the energy conversion and dissipation,magnetic flux and matter transfer,or reconnection rate in magnetic reconnection? These questions are important but have not yet been fully answered.The Earth's magnetosphere is an ideal natural plasma laboratory to study collisionless magnetic reconnection.Investigating magnetic reconnection in the Earth's magnetosphere is not only helpful for understanding the physical mechanism of explosive space weather,but also for understanding the mechanism of fast release of magnetic energy in astrophysical and laboratory plasmas.There have been lots of satellite exploration programs aimed at the Earth's magnetosphere in history.The satellite exploration programs before the Magnetospheric Multiscale(MMS)mission studied magnetic reconnection at the fluid and ion scales mainly,while the MMS mission ushered in a new era of studying magnetic reconnection at the electron scale.In this dissertation,we use the high-precision data detected by MMS to study the multiscale structures associated with magnetic reconnection,and reveal a series of new physical images and processes.The main results are as follows:1.The properties and formation mechanisms of small-scale structures in magnetic reconnection:We prove for the first time that the secondary magnetic flux rope can be generated by the electron Kelvin-Helmholtz instability.The MMS spacecraft observed a sub-ion scale magnetic flux rope embedded in an electron vortex in the magnetic reconnection diffusion region at the magnetopause.An electron diffusion region at the edge of the flux rope.The electron vortex is generated by the electron Kelvin-Helmholtz instability excited by the electron flow shear formed by the primary reconnection.The generated electron vortex distorts the magnetic field and triggers the secondary magnetic reconnection,thereby creating the secondary flux rope.We reproduce this process through Particle-in-Cell(PIC)numerical simulation.We report,for the first time,the kinetic-scale electron vortex magnetic hole in the reconnection diffusion region.Intense currents and non-ideal electric fields were observed in the magnetic hole,which provided an additional energy dissipation channel besides the electron diffusion region.The magnetic hole may be excited by the electron solitary wave generated by the Biermann battery effect and formed by the evolution of the diamagnetic drift current of the trapped electrons.We discovered a long electron diffusion region that extended at least 20 ion inertial lengths downstream of an X line.This electron diffusion region was detected in the exhaust of an asymmetric magnetic reconnection with a moderate guide field.This long electron diffusion region does not decrease the reconnection rate.It is still a fast reconnection with a reconnection rate ?0.1.The present theoretical and numerical results can not explain the formation of this extending electron diffusion region.2.The magnetic reconnection in multiscale structures:We present the first observation of secondary reconnection occursing in the separatrix region of primary reconnection.This secondary reconnection occurs between the magnetospheric field and the axial magnetic field of the magnetic flux rope.Thus,this secondary reconnection is essentially a three-dimensional reconnection and cannot be accommodated in two-dimensional magnetopause reconnection.The scale size of this reconnecting current sheet is limited in all three directions and ions do not respond to this electron-scale current sheet,indicating an electron patchy reconnection.This three-dimensional patchy magnetic reconnection sheds new light on the destiny of magnetic flux rope from a three-dimensional perspective which may play a critical role on the reconnection-driven turbulence.We report a magnetic reconnection occurring in the magnetosheath downstream of a quasi-perpendicular bow shock.This is a magnetic reconnection from mutiscal instability cascade.The ion mirror instability modulates the reconnection at the fluid scale and changes the ambient environment of the current sheet.The oblique tearing instability modulates reconnection at the ion scale and produces the bifurcated structure of the electron diffusion region consistent with the intense electron shear flow.The electron Kelvin-Helmholtz instability produces the electron vortices,magnetic holes,and magnetic peaks.These observations provide a new mode of multiscale instability cascade in the magnetic reconnection and a new electron dynamic of reconnection diffusion region.3.On the role of multiscale structures in energy conversion of magnetic reconnection:We quantitatively study the electron adiabatic acceleration within a magnetic flux rope.It is directly proved that the flux rope can generate energetic electrons through local adiabatic acceleration.The electrons are accelerated by betatron acceleration and Fermi acceleration within the magnetic flux rope.The highest energy electrons(>100 ke V)are produced by betatron acceleration in a very short period,while the Fermi acceleration produces the thermal electrons rather than energetic electrons.We statistically study the energy conversion and dissipation of 122 dipolarization fronts.The result indicates that electromagnetic energy transfers to plasma at dipolarization fronts.The released energy is mainly transferred to ions rather than electrons.Ions gain energy across the whole dipolarization front,while electrons gain energy at the leading part but lose energy at the trailing part of dipolarization front.Joule dissipation is very small at dipolarization front.The kinetic energy dissipation parameter Pi-D,which describes the energy exchange between the fluid flow and the random kinetic energy,is not suitable for quantifying the energy dissipation at dipolarization front.We investigate whistler waves and broadband electrostatic waves within an ion diffusion region at the magnetopause.The whistler waves are observed in the separatrix region,while the electrostatic waves are observed in the center of the current sheet.These waves are locally produced by the unstable structures in electron or ion velocity distribution functions.This indicates that energy of plasma transfers to plasma waves.The whistlers are linked with Buneman-type waves by the electron Pacman distribution rather than by wave-wave process.The oblique electrostatic waves are associated with the ion beams produced by multiple X line reconnection.These results update or deepen the understanding of the electron dynamics in magnetic reconnection,the triggering and evolution of magnetic reconnection,the energy conversion,the electron acceleration,and the plasma waves in magnetic reconnection.Meanwhile,some new problems and challenges are put forward,which will be helpful to the future research of magnetic reconnection.