Particle Acceleration during Magnetic Reconnection in a Low-beta Plasma

Abstract

Magnetic reconnection is a primary mechanism for particle energization in space and astrophysical plasmas. By carrying out two-dimensional (2D) fully kinetic simulations, we study particle acceleration during magnetic reconnection in plasmas with different plasma β (the ratio between the thermal pressure and the magnetic pressure). For the high-β cases, we do not observe significant particle acceleration. In the low-β regime ( β < 0.1 ), we find that reconnection is efficient at energizing both electrons and ions. While the distribution of accelerated particles integrated over the whole simulation box appears highly non-thermal, it is actually the superposition of a series of distributions in different sectors of a 2D magnetic island. Each of those distributions has only a small non-thermal component compared with its thermal core. By tracking a large number of particles, we show that particles get energized in X-line regions, contracting magnetic islands, and magnetic island coalescence regions. We obtain the particle energization rate j · E by averaging over particle drift motions and find that it agrees well with the particle kinetic energy change. We quantify the contribution of curvature drift, gradient drift, polarization drift, magnetization, non-gyrotropic effect, and parallel electric field in different acceleration regions. We find that the major energization is due to particle curvature drift along the motional electric field. The other particle motions contribute less but may become important in different acceleration regions. The highly efficient particle energization in low-β plasmas may help us understand the strong particle energization in solar flares and accretion disk coronae.

MMS Examination of FTEs at the Earth's Subsolar Magnetopause

“…Plasma beta (the ratio of the plasma pressure to the magnetic pressure) is suggested to play a role in determining the extent of plasma energization during flux rope formation (e.g., Phan et al, ). Magnetic reconnection is credited for spawning significant changes in the particle distribution function for β < 0.1 (Li et al, ) and is shown to be less proficient in plasmas with relatively large β (β < 0.1) (Drake et al, , ; Hoshino et al, ; Oka et al, ). Simulations have also attributed flux rope's shape, core magnetic field strengths, and evolution to plasma beta (Karimabadi et al, ; Schoeffler et al, ).…”

The Roles of Fluid Compression and Shear in Electron Energization during Magnetic Reconnection

“…Note that the peak at the highest energy bin is due to statistical error generated by merely a few electrons. studied energization term · j E because a flux term · ( ) v p s E was not considered (Li et al , 2017. We find that the inertial energization term is small compared with other terms for electrons because of small electron mass but can contribute over 20% of energization for ions (Li et al 2017).…”

“…This description is not completely accurate in regions with weak magnetic fields and in the diffusion region because particles are not well magnetized. However, for sufficiently large systems, the effect of the asymmetric pressure tensor has a minor role in the energization during reconnection (Li et al 2017). Then, the pressure gradient effect can be broken into where the first term is due to diamagnetic drift, and the second term is due to magnetic field curvature and is proportional to the pressure anisotropy.…”

“…In the anti-reconnection layer (x∼18d i ), where these two islands merge, the overall compressional energization is small compared with other regions due to two reasons: the convergence of v Ex is accompanied by the divergence of the outflow in the anti-reconnection region along the z-direction; the compression in the island on the right is accompanied by the expansion in the one on the left. More detailed trajectory analyses (e.g., Li et al 2017) have found that some particles can get efficiently accelerated by accessing those compression regions. Figure 5 suggests that shear energization is much weaker than compressional energization in those regions.…”

“…To quantify the energization due to compression and shear effects, we calculate energization due to different fluid-motion terms such as compression, shear, and fluid inertia discussed in Section 2 (Equation (9)), as well as the contribution from agyrotropic particle distribution (Li et al 2017). In Figure 3, we show the time evolution of each energization effect for runs G1 (β e =0.02, B g =0) and G3 (β e =0.02, B g =0.5B 0 ).…”