Structure Of Magnetic Reconnection And Magnetic Energy Conversion

Magnetic field reconnection is a universal physical phenomenon in space plasma.It is also the focus of laboratory plasma and simulation research.The special environment of near-earth space provides good conditions for the generation of magnetic field reconnection.Satellite observation is an important method to study magnetic field reconnection.In this paper,whistler mode waves coupled with magnetic field reconnection,magnetic hole,as well as structure and energy dissipation of the diffusion region,while magnetic field reconnection on a small scale are studied by using the high-precision data observed by MMS satellite.Through analysis,some important results are obtained,which are as follows:1.Whistle sound wave and magnetic hole observed in magnetic field reconnection of magnetopauseThe data observed by the MMS satellite on November 12,2015 were used to analyze the related physical processes.During observation,whistling waves appear near the separatrix region of magnetic reconnection.Whistler mode waves in the magnetosphere have higher emission frequency(>0.5 fce)and higher resonance energy.Meanwhile,Poynting vector indicates that waves propagate along the magnetic field line toward the X-line.On the contrary,whistler mode waves observed on the magnetosheath side have a lower emission frequency and resonance energy,and are propagated back along the magnetic field line,i.e.away from the X-line.Although the properties of both whistler waves are different,they axe all related to the enhancement of electron flux in the perpendicular direction.The strong interaction between whistler waves and electrons can affect the dynamic process of plasma.At the same time,a small-scale magnetic hole was observed in the downstream region away from the X-line.Because the electron temperature inside the magnetic hole increases,it is considered to be an important structure for electron heating.This structure is rarely observed in the case of magnetic reconnection in near-earth space,and it turns out that the downstream region of the X-line is also an important channel for energy generation.Further studies show that there is a microscopic physical process at the boundary of the magnetic hole,corresponding to the obvious energy conversion process.The boundary of the magnetic hole is a possible way for the hole to heat electrons.In summary,the physical process associated with magnetic reconnection plays an important role in the quantization of energetic particles.2.Small-scale magnetic field reconnection observed in the magnetosheathThe strong interaction between the solar wind and the earth’s magnetic field leads to the complex and changeable environment in near-earth space.The magnetosheath region contains a large amount of turbulent plasma,which is expected to produce many small-scale current sheets.Here,in the magnetosheath environment,the MMS satellite detected electron-scale magnetic field reconnection at the center of a flux rope.The analysis shows that each reconnecting current sheet corresponds to the distinct energy conversion.Shear flow in the center of the flux rope is a possible mechanism for reconnection generation.All electron-scale reconnections have no corresponding ion current response.Further data show that such reconnection heating of electrons is limited,with all heated electrons located in a lower energy range of 100-200 eV.As a ubiquitous structure in magnetosheath plasma,small-scale reconnection cannot produce high-energy electrons.In another observed reconnection case with strong guide field,the boundaries of recon?nection have important structural features.At the same time,corresponding to the unipolar parallel electric field and obvious energy dissipation,the accelerated electrons flow along the separatrix toward the X-line,mixed with other incoming electrons,resulting in the sharp increase of the electron temperature in the parallel direction.Since electron acceleration exists only in one of the separatrix,electron heating is more obvious only on one side of the current plate,resulting in temperature asymmetry on sides of the current sheet.The shear flow,is conducive to the formation of parallel electric field.3.Structure of asymmetric magnetic reconnectionThe magnetic reconnection at the magnetopause is mostly asymmetric reconnection,that is,the magnetic field intensity and density on both sides of the plasma sheet are very different.In the asymmetric reconnection observed on January 1,2016,quadrupole Hall magnetic field and bipolar Hall electric field can be observed at the distance of 9 ion inertia far from the X-line.This structure is generally believed to exist in ideal reconnection.Further analysis shows that the Hall electric field on the sheath side(high density side)is much smaller than that on the magnetosphere side.The results of the generalized Ohm’s law show that Hall effect is the main cause of electric field.At the same time,the current density in the dissipation region is highly filamentary,and the corresponding energy dissipation is also discontinuous.The electron heating in the exhaust region mainly comes from the parallel direction,because the existence of strong guidance field inhibits the electron temperature heating in the vertical direction.The separatrix region of magnetic reconnection has strong energy dissipation and electron heating.At the separatrix,the unipolar parallel electric field effectively accelerates the electron,and the betatron mechanism leads to the increase of the electron temperature in the perpendicualr direction.Therefore,various micro-physical processes in the separatrix region play a vital role in the heating and acceleration of electrons.