Chaos-induced resistivity of collisionless magnetic reconnection in the presence of a guide field

It is one of the most puzzling problems in astrophysics to understand the anomalous resistivity in collisionless magnetic reconnection that is believed extensively to be responsible for the energy release in various eruptive phenomena. The magnetic null point in the reconnect current sheet, acting as a scattering center, can lead to chaotic motions of particles in the current sheet, which is one of possible mechanisms for the anomalous resistivity, called the chaos-induced resistivity. In many interesting cases, however, instead of the magnetic null point, there is a nonzero magnetic field perpendicular to the merging field lines, usually called the guide field, whose effect on the chaos-induced resistivity has been an open problem. By use of test particle simulation method and statistical analysis, we investigate the chaos-induced resistivity in the presence of a constant guide field. The characteristic of particle motion in the reconnecting region, in particular, the chaotic behavior of particle orbits and the statistically evolving feature are analyzed. The results show that as the guide field increases the radius of the chaos region increases and the Lyapunov index decreases. However, the effective collision frequency, and hence the chaos-induced resistivity, reach their peak values when the guide field approaches half of the characteristic strength of the reconnection magnetic field. The presence of a guide field can significantly influence the chaos of the particle orbits and hence the chaos-induced resistivity in the reconnection sheet, which decides the collisionless reconnection rate. The present result is helpful for us to understand the microphysics of the anomalous resistivity in the collisionless reconnection with a guide field.