Study Of Dayside Magnetic Reconnection And Its Mass Transport Process

Magnetic reconnection is a fundamental process for field line topological change and energy conversion mechanism in space physics.During the magnetic reconnection,the topological configuration of the magnetic line is changed and the plasma on both sides of the current sheet is mixed,accelerated and transported close to the current sheet.The stored magnetic free energy is released and converted into the kinetic energy and heating of the plasma.During the solar wind-magnetosphere coupling process,magnetic reconnection plays an important role in transporting energy and particles from solar wind to the magnetosphere.The aurora in the ionosphere,the magnetic storms,substorms and the cold and dense plasma sheet in the magnetotail are closely related to the solar wind entry driven by magnetic reconnection.Recent studies have found that the bow shock transient will produce ULF wave and field-aligned current in the magnetosphere which will accelerate particles and generate aurora in the ionosphere,affecting the plasma transport in the magnetosphere.In the hybrid simulation,the magnetic reconnection occurs at the early stage of the bow shock transient,but there is no relevant in-situ observations.Based on the analysis of the multi-satellite data observation,the following subjects are investigated in detail.1.In-situ observation of signature of reconnection inside the bow shock transientRecently,magnetic reconnection near the bow shock region has been reported by MMS observation.High temperature and weak magnetic field inside the bow shock transient would favor for the occurrence of magnetic reconnection,which has been seen in hybrid simulation,but never reported with in-situ observation.Based on a small-scale flux rope event observed by MMS satellite at the trailing edge of HFA,we suggest that magnetic reconnection could occur inside the bow shock transient.Further analysis of the current and ion energy spectrum inside the small-scale flux rope eliminates other possibilities,such as compression boundary caused by the expansion of HFA,which can also create bipolar variation of the magnetic field.Furthermore,the energy transformation,magnetic force and pressure tensor force caused by electrons and ions inside the ion-scale flux rope are analyzed.2.Plasma transport from diffusion region to reconnection outflow region based on two types of electron/ion dispersion close to the separatrixThe rapid and gradual dispersions of electrons and ions are observed by MMS during a separatrix crossing.By analyzing the formations of rapid and gradual dispersions,we consider that two types of dispersions caused by different parallel velocity,and thus different ExB drift time,which leads to different time scales of dispersion.We suggest that the electrons/ions are not magnetized along the separatrix away from the X-line.Therefore,the source region of the rapid electron/ion dispersion could trace back to the electron/ion diffusion region.The process of plasma transport from the diffusion region to the downstream outflow of the reconnection is determined.3.Semi-annual variation of plasma transport driven by magnetic reconnection undernorthward Interplanetary Magnetic Field(IMF)conditionWhen the IMF is southward,strong geomagnetic activities,such as magnetic storms and substorms,are more likely to occur around the spring equinox and autumn equinox every year,and there is a semi-annual variation.The diurnal and annual variations of the Bz amplitude of the southward IMF predicted by Russell-McPherron effect can well explain this variation.The intensity of the magnetic storm and substorm is related to the parameters of the plasma sheet,such as number density and temperature,in the magnetotail during the quite time.In simulation,ring current would be strong using the cold and dense plasma sheet(instead of hot and tenuous plasma sheet)as the boundary condition.The number density and temperature of the magnetotail plasma sheet during the quite time is affected by the solar wind plasma entry under the northward IMF condition,and the magnitude of the northward IMF Bz also has the similar diurnal and annual variations as southward IMF,which can be well predicted by the Russell-McPherron effect.Based on statistical analysis of years of Geotail and Image satellite data,we find semiannual variation of the occurrence of the CDPS and Transpolar aurora,both of which are related to the high-latitude magnetic reconnection.Therefore,we suggest that the solar wind entry driven by the high latitude reconnection also has a semi-annual variation.Our statistical analysis provides new evidences for the R-M effect.