Magnetic reconnection: dynamics and particle acceleration

A significant fraction of the magnetic energy released during magnetic reconnection in solar flares appears as energetic electrons and protons and the observations suggest a common acceleration mechanism. How this conversion of magnetic energy takes place so efficiently has been a scientific topic of great interest. Recent developments in our understanding of reconnection have important implications for understanding energetic particle production. The onset of fast reconnection occurs only after sufficiently narrow current sheets develop. It has been suggested that the coronae of stars and accretions discs naturally evolve to a critical state close to fast reconnection onset. In flares the classical picture of the formation of a single large x-line does not seem to be viable: the narrow current layers that develop near the reconnection sites break up into secondary magnetic islands whose dynamics and size spectrum are likely to control particle acceleration. Energetic electrons are produced though the Fermi-like reflection in contracting magnetic islands rather than by parallel electric fields. Their energy gain is linked to the release of magnetic energy. The Fermi mechanism is viable only for super-Alfvenic ions so a seed mechanism for ions is required. Ion pickup in reconnection outflow exhausts is proposed as the seed mechanism. This seed model has the property that low Q/M ions gain energy faster than protons, which is consistent with observed enhancements of low Q/M ions in impulsive flares. Ion and electron acceleration in a multi-island environment remains poorly understood. (author)