3D kinetic plasma simulations reveal the Fermi mechanism for magnetic reconnection

In magnetic reconnection – intersecting magnetic fields induce magnetic energy converting to kinetic energy, and particles accelerate, spawning energetic events, such as solar flares. However, convincing explication for reconnection particle acceleration has eluded physicists. A recent University of Maryland physics investigation employed 3D particle-in-cell (PIC) modelling to demonstrate the dominate Fermi mechanism for acceleration due to a sequence of random impulses within reconnection regions. They report on their research in Physics of Plasmas.

Lead author Joel Dahlin says that coauthor James Drake’s earlier work explains how magnetic field lines accelerate particles that reflect from contracting magnetic fields (the Fermi mechanism). However, the dominant reconnection 2D modelling, highlighted the formation of “islands,” which are observed, but in 2D, particles do not rejoin acceleration streams, limiting energy gain. The University of Maryland team used a 3D PIC simulation, which Dahlin says better represents the turbulent systems in the universe. In these 3D situations, particles leak out of the islands and rejoin acceleration sites.

The guide field, which is the component of the magnetic field parallel to the reconnection axis, is key for inducing stochastic orbits by which particles can leave magnetic islands. The 3D aspect enabled this understanding, Dahlin says. The researchers also conducted simulations varying guide field strength, which demonstrated that the guide field cannot be too strong or else it will throttle the Fermi mechanism.

The researchers’ simulations also supported earlier understandings of why electrons and not ions and protons are accelerated, with the electron having higher velocities to return upstream of the accelerating outflows. Dahlin says, however, the paper’s central offering is highlighting “the neglected guide field, which does not contribute energy, but shapes both the angle of the particle reflections of the system and the transport of electrons.”

Source: “The role of three-dimensional transport in driving enhanced electron acceleration during magnetic reconnection,” by J. T. Dahlin, J. F. Drake, and M. Swisdak, Physics of Plasmas (2017). The article can be accessed at https://doi.org/10.1063/1.4986211 .