The Physics of Magnetic Reconnection: a tutorial

Abstract:

Magnetic reconnection is the dominant mechanism for dissipating magnetic energy in the universe. It is the driver of solar and stellar flares, and possibly energy release in magnetars, pulsar magnetospheres, jets and GRBs. During reconnection, oppositely directed magnetic fields break and cross-connect. The resulting magnetic slingshots convert magnetic energy into high velocity flows, thermal energy and energetic particles. Significant progress has been made on fundamental questions such as how magnetic fields lines reconnect and why energy is released so quickly. Two fundamental routes to fast reconnection have been identified: through the formation of a Petschek-like open outflow exhaust as in Hall reconnection; and through the formation of a multi-island current layer and exhaust. Anomalous resistivity, which has often been invoked to boost the rate of reconnection, has yet to be convincingly documented with observations. The mechanisms for particle heating and particularly the production of the highest energy particles during reconnection are not as well understood. Since energy release during reconnection takes place primarily in the reconnection outflow where newly reconnected field lines relax their tension, mechanisms within the exhaust are the most promising. In the magnetosphere counter-streaming ions associated with Fermi reflection in the exhaust carry most of the released magnetic energy although the measured heating is less than that predicted. Explaining electron heating remains a challenge for a single x-line model. Models based on a multi-x-line, multi-island environment are more promising. The relative roles of parallel electric fields versus Fermi acceleration in contracting and merging islands continue to be explored.