Electric And Magnetic Field Structures And Energy Conversion In Magnetic Flux Ropes During Magnetic Reconnection

Magnetic reconnection is an important physical process in space,astronomy,and laboratory plasmas,during which magnetic topology is changed and magnetic energy is rapidly converted into plasma kinetic and thermal energy.Magnetic reconnection is the main cause of many explosive phenomena in the Solar-Terrestrial system,such as solar flare and magnetospheric substorm.Understanding magnetic reconnection can help us reduce the damages and losses caused by these explosive phenomena.Magnetic flux rope is an important product of magnetic reconnection,and in particular,ion-scale flux rope can significantly affect the reconnection rate,electron acceleration,and energy dissipation during magnetic reconnection.Therefore,ion-scale flux rope is one important subject of magnetic reconnection study.Before the Magnetospheric Multiscale(MMS)mission,spacecraft observations of ion-scale flux ropes and their internal structure have been limited by the time resolution of the available measurements.Thus numerical simulations were the main method to study them.In this thesis,we use the high-resolution data from the MMS mission to study the electric and magnetic field structures and associated plasma processes inside the ion-scale flux ropes in the Earth’s magnetosphere.The main results are given as follows:1.Electron distributions and whistler waves in magnetic flux ropesWe report a series of six ion-scale magnetic flux ropes observed in the continuous southward ion outflows at the magnetopause.Their properties,such as current density,frozen conditions for ions and electrons,electron pitch angle distributions,magnetic field topology,and waves are studied in detail.Their similarity and differences are discussed.The first two flux ropes are adjacent to the ion diffusion region,and the other four flux ropes are away from the ion diffusion region.The enhancements of the heated electron fluxes in the perpendicular direction are observed in the trailing parts of the flux ropes,which have never been reported previously.These electrons may be generated by the betatron acceleration caused by the compression of the localized magnetic fields.The whistler waves associated with the flux ropes are observed and categorized into the lower and upper bands according to their frequency ranges.For the lower band whistler waves,they propagate in variable directions along the magnetic field lines and therefore could be generated locally.The trailing parts of the flux ropes with perpendicularly heated electrons are considered to be one possible source region.The upper band whistler waves are all found in the core region of the flux ropes and propagate antiparallel to the magnetic field and therefore originate from the same source region.In addition,we find that in the flux rope,high-frequency whistler waves close to the electron gyrofrequency have strong interactions with electrons in some energy bands.According to our analysis,the flux ropes are important channels for the mass and wave transport between the magnetosheath and the inner magnetosphere,and the electron dynamics can be modified during the evolution of the flux ropes.2.Secondary reconnection inside magnetic flux ropesWe report for the first time direct evidence of secondary reconnection in the filamentary currents within the flux ropes.Using MMS mission data,we find clear filamentary current structures inside the flux ropes.Our analysis shows that these filamentary currents are all located within the varied magnetic field pulses.In the magnetic field pulse,the electron vorticity is enhanced,and the electron flow is reversed,which indicates that there are electron vortices.The statistics on the current density and energy conversion in the flux ropes show that the energy conversion mainly occurs in the region with strong current density.The sums of the energy conversion in the flux ropes suggest that magnetic free energy is converted into the plasma energy inside the flux ropes.We examined carefully each of the filamentary currents and find that the signatures associated with secondary reconnection are observed in all of the magnetic field pulses.There was no ion-couple detected in these reconnection events,indicating that they are electron-only reconnection events.According to our observation,it is undoubted that magnetic energy can be released inside flux ropes via secondary reconnection.This is an important complement to the energy dissipation in the electron diffusion region.3.Magnetic flux ropes and thin current sheets in the magnetosheathUsing the MMS mission data,we find three magnetic peak structures in the magnetosheath.Based on the magnetic field signals,magnetic curvatures,and the toroidal magnetic field lines in the local coordinates,three peaks are identified as magnetic flux ropes.There are thin current sheets in the trailing parts of the flux ropes,and their current densities are significantly larger than the background current densities in the flux ropes.This means that the current density shows a double-layered structure in each flux rope characterized by the large-scale current layer and an embedded thin electron current layer with a strong current density.The double-layered current structure is reported for the first time in magnetosheath flux ropes,which suggests the multiscale characteristics of the magnetosheath flux ropes.Direct evidence of magnetic reconnection is found in the thin current layer with the strongest current density.By calculating the electron energy gain rates by the parallel electric field,Fermi mechanism,and betatron mechanism in the flux ropes,we investigate the contributions of these three mechanisms to the electron acceleration quantitatively.The electrons are heated mainly in two regions of flux ropes,that is,the reconnecting current layer by the parallel electric field and the trailing edges by Fermi and betatron mechanisms.Our results suggest that the ionscale flux ropes are directly associated with energy dissipation and electron acceleration in the magnetosheath.Thin current sheets can be formed in flux ropes,and magnetic reconnection can play a significant role in the energy dissipation within flux ropes.4.Parallel electric field in the electron diffusion region of magnetosheath reconnectionWe report the large-amplitude and unipolar parallel electric field structure in the electron diffusion region of magnetosheath reconnection,and quantitatively calculate the acceleration of the parallel electric field to electrons.The four MMS satellites crossed a high-speed electron jet embedded in a broad reconnecting current sheet.The characteristics of the electron jet are consistent with the signatures of the electron diffusion region.It is found that the electron diffusion region extends from the X-line to the outflow region.The electron pressure anisotropy inside the reconnecting current sheet suggests that the electron diffusion region could be generated by the firehose instability.The large-amplitude unipolar parallel electric field is observed by four satellites during the electron diffusion region crossing,indicating that the parallel electric field can fill the entire electron diffusion region.Based on two different methods,the potential drop caused by the parallel electric field is calculated to be 120V.The parallel electric field can accelerate the electrons passing through the electron diffusion region and generates electron beams with higher energy,which can cause electron holes.Our study suggests that the parallel electric field can dominate the electron dynamics inside the electron diffusion region of magnetosheath reconnection due to its large spatial distribution.5.Electron vortices and electron acceleration in the magnetic flux ropeWe report for the first time the electron acceleration by the electron vortices within magnetic flux rope and discuss the electron acceleration mechanisms and the secondary reconnection in the electron vortices.Electron vortices are formed by the electron Kelvin-Helmholtz instability excited by the electron shear flow in the center of the flux rope.The vortices are not stable and are expanding according to the different propagating velocities inside the vortices.The estimated magnitude of the induced parallel electric field generated by the vortices’ expansion is almost the same as the measurement.In addition,we find an anomalous bipolar magnetic field variation at the vortex boundary.corresponding to a thin reconnecting current sheet.The current sheet is possibly generated by the vortical flows which can roll up the magnetic field lines.The vortices’expansion and magnetic reconnection both produce strong parallel electric fields,which can directly accelerate electrons within vortices.The accelerated electrons are mainly in the parallel or antiparallel direction.On the other hand,between two vortices,the energetic electrons up to 200 keV in the perpendicular direction are observed.As the magnetic field between vortices is highly compressed due to the interaction of two vortices,these energetic electrons may be accelerated by the betatron mechanism as in the reconnection front.Our results shed new light on the study of electron acceleration in magnetic reconnection and demonstrate that vortices at the center of flux ropes can also accelerate electrons.These results based on the spacecraft observations demonstrate the important role of the magnetic flux ropes during magnetic reconnection and deepen our understanding of the electric and magnetic field structures and energy conversion in the flux ropes.In addition,our studies also raise some new questions,which can promote associated spacecraft observations and numerical simulations.