[P-EM17] Space Plasma Physics: Theory and Simulation

[PEM17-P07] Microstructure of magnetic island in collisionless reconnection

*yumeng fan¹, Shuichi Matsukiyo¹, Seiji Zenitani² (1.Kyushu University, 2.Kobe University)

Keywords:collisionless reconnection, microstructure in magnetic island, weibel instability

Magnetic reconnection is a fast magnetic energy releasing process. It is widely observed in space and laboratory plasma such as solar flares, CME, substorms, tokamak devices etc. While magnetic field lines break and are reconnected in the process of reconnection, magnetic energy is converted into the thermal and kinetic energy of plasma. When there are two or more X-points, magnetic island is formed. Inside a magnetic island one can often observe nonthermal particles as well as a variety of electromagnetic fluctuations. The origin of microstructures and nonthermal particles in a magnetic island has been extensively studied by using numerical simulations. Lu et al. (2011) conducted PIC simulation of magnetic reconnection in an ion-electron plasma and observed regular fluctuations in a magnetic island. They gave a conclusion that the structure is based on the Weibel instability due to the electron temperature anisotropy inside a magnetic island. Swisdak et al. (2008) also confirmed fluctuations in a magnetic island by PIC simulation for positron-electron case. Some microstructures in a magnetic island are also visible in hybrid simulation where ions are treated as particles and electrons are as fluid (e.g., Fig.4 in Walia et al. (2022)).

We conducted 2d PIC simulation in both ion-electron and positron-electron cases. Regular structures in magnetic island are observed in ion-electron case. From the regions near the outflow to the center of magnetic island along the current sheet, we plot the distribution function of in-plane velocity components. We found that after being accelerated by reconnection, the drifted particles flow out to the magnetic island, the two streams with reversed drift velocity meeting in magnetic island can produce anisotropy of both ions and electrons (A_i > A_e > 0). According to our dispersion analysis, the electron anisotropy is not enough to give positive growth rate of Weibel instability. This conclusion is not consistent with Lu et al. (2011). The ion anisotropy is larger than electrons' and grow rapidly. In later time it gives a positive growth rate of ion Weibel instability.

We will report the results of detailed analysis on linear dispersion relation of the instabilities observed in the magnetic island.