Research On Magnetic Reconnection And Related Plasma Fluctuations

In the solar-terrestrial system,magnetic reconnection and plasma fluctuations are important physical processes in plasma.Direct or indirect observational data indicate that magnetic reconnection is related to explosive phenomena in the solar atmosphere,interplanetary solar wind,the Earth’s magnetopause,and the magnetotail.Magnetic field reconnection can convert magnetic field energy into plasma kinetic energy and heat energy quickly.In recent decades,a large number of research results on magnetic reconnection have been continuously reported.Recently,satellites on the magnetopause have observed that asymmetric magnetic reconnection has a special Hall magnetic field structure,but the magnetic structure and current system of asymmetric magnetic reconnection are still controversial.At the same time,the satellite found whistle waves in the separatrix region of reconnections.And density cavities often appear in the separatrix region.This kind of density cavity structure extends along the separatrix,which could affect the propagation of the wave.There are still unclear area in the propagation process of the whistle wave in the density duct.The author of this paper uses a two-dimensional full-particle simulation to study the reconnection of the asymmetric magnetic field of the magnetopause,and also uses a laboratory plasma device to investigate the propagation characteristics of the whistle wave.Finally,the main conclusions are as follows:1,The study Hall magnetic field in reconnection of asymmetric magnetic fieldWith two-dimensional(2-D)particle-in-cell(PIC)simulations,we study the structure of the Hall magnetic field generated during anti-parallel magnetic reconnection in an asymmetric current sheet.A quadrupolar structure of the Hall magnetic field is firstly formed,and then it evolves into a hexapolar structure with the proceeding of magnetic reconnection.In the quadrupolar structure of the Hall magnetic field,the quadrants on the side of the current sheet with the weaker magnetic field(the dominant Hall magnetic field)are much stronger that those on the other side,and they can cross the center of the current sheet.With the increase of the asymmetry of the current sheet(either the increase of the ratio of the magnetic fields or the decrease of the ratio of the number densities,here,the ratio is the ratio of the stronger parts to the weaker parts respectively),the tendency will become more salient.With the proceeding of reconnection,two ribbons with the enhanced Hall magnetic field are generated in the region below the dominant Hall magnetic field,and then a hexapolar structure of the Hall magnetic field is formed,which will become stronger with the increase of the asymmetry of the current sheet.The generation of the Hall magnetic field can be explained by the in-plane current carried mainly by the electrons.However,the increase of the ratio of the temperature would attenuate the asymmetry of the Hall magnetic field.In the asymmetric magnetic reconnection,the structure of the Hall magnetic field on the left side of the X line is different from that on the right side.In general,the Hall magnetic field on the left side has a tripolar structure,while it has a bipolar structure on the right side.On the left side of the X line,the Hall magnetic field at the upper and lower parts of the current sheet is positive,while that at the central part is negative.With the increasing guide field,the amplitude of the Hall magnetic field at the upper and center parts of the current sheet decreases,and that at the lower part increases.An increasing asymmetry(either an increase of the ratio of the magnetic field or decrease of the density ratio between the two sides of the current sheet,and the ratio is defined as the values between the side with the stronger magnetic field to that with the weaker magnetic field)leads to the decrease of the Hall magnetic field at the upper part of the current sheet and even disappearance when the asymmetry is sufficiently large,and the increase of the Hall magnetic field at the lower and central parts of the current sheet.On the right hand of the X line,the Hall magnetic field at the upper part of the current sheet is negative,while that at the central part is positive.With the increasing guide field,the Hall magnetic field at the upper current sheet increases,while that at the central current sheet decreases.The increasing asymmetry leads to the decrease of the Hall magnetic field at the upper and central parts of the current sheet,and when the asymmetry is sufficiently large there appears the Hall magnetic field below that at the central current sheet.There are also abundant plasma fluctuations in the magnetic reconnection.We have observed an asymmetric magnetic reconnection at the magnetopause using the MMS satellite.The Hall magnetic field in the asymmetric reconnection has a quadrupole structure.At the same time,we have found a whistle wave with good monochromaticity in its separatrix region.And this whistle wave propagates toward the X-line direction.2.Excitation and propagation of whistle wave in laboratory plasmaIn the Linear Magnetized Plasma Device(KLMP)of the University of Science and Technology of China,we used an antenna to excite plasma fluctuations.Through the frequency,polarization,propagation speed,propagation angle and refractive index curve of the wave,we confirmed that the whistle wave was excited in the small-scale density tube(the diameter of the tube is smaller than the wavelength of the whistle wave).In the duct with increased density,the whistle wave propagates in a quasi-parallel direction,and the energy transmitted by the whistle wave is almost undamped;while in a duct with reduced density,the whistle wave exhibits oblique propagation,and the angle between its wave vector and the axial magnetic field is close to the Gendrin angle.This result is similar to that of fluctuations in large-scale ducts.