Electron Acceleration And Formation Of Power-law Spectra Of Energetic Electrons During The Merging Process Of Multiple Magnetic Islands

Energetic electrons are closely related to physical phenomena in astrophysical and space environments,such as gamma ray bursts,solar flares,substorms in Earth's magnetosphere,and these energetic electrons usually develop power-law spectra.Magnetic reconnection is a fundamental physical process,which dissipates magnetic energy into plasma kinetic and thermal energy by changing the topology of magnetic field lines.The production of energetic electrons is also a critical signature in magnetic reconnection,and thus it may provide a possible mechanism to explain the ubiquitously observed energetic electrons in these explosive phenomena.In this paper,two dimensional(2D)particle in cell(PIC)simulations are deployed to investigate electron acceleration during the merging process of multiple magnetic islands in a current sheet with a strong guide field.Due to the existence of the strong guide field,we can analyze the contributions of the parallel electric field,Fermi,and betatron mechanisms to the electron acceleration with the guiding-center theory.The results are shown as follows:1.The merging process of multiple magnetic islands and the mechanisms of electron acceleration with a strong guide fieldDuring the coalescence process of magnetic islands,the islands merge each other continuously until only one big island remains.The electrons can be accelerated both by the parallel electric field mechanism in the vincinity of the X point and the merging point and the Fermi mechanism at the two ends of contracting magnetic islands.The contribution of the parallel electric field mechanism to electron acceleration is more important than that of the Fermi mechanism with the existence of a strong guide field,while the betatron mechanism to electron acceleration is negligible compared to that of the parallel electric field and Fermi mechanisms.Energetic electrons are generated during such process,and the electrons with sufficient high energy develop a power law spectrum.2.The influence of different parameters(shape of magnetic islands,electron plasma beta,the number of magnetic islands,and the guide field)on the merging process and the electron accelerationWith a larger ?,more magnetic islands in the simulation domain and smaller guide field,the merging process of magnetic islands will be faster.The contribution of Fermi mechanism to the electron acceleration becomes more important with a larger electron plasma beta and less and less important with the increase of the guide field.Among all the multi-islands merging simulations,the energetic electrons form power law spectra,and the spectrum index increases with the increase of the number of magnetic islands and the guide field,while it is not clearly relevant with the shape of magnetic islands(?)and the electron plasma beta.