Spontaneous magnetic reconnection: Collisionless reconnection and its potential astrophysical relevance

• Published in: The Astronomy and astrophysics review 2015-12, Vol.23 (1), p.1-91, Article 4
• Main Authors: Treumann, R. A., Baumjohann, W.
• Format: Article
• Language: English
• Subjects: Acceleration; Anisotropy; Astronomy; Astrophysics; Astrophysics and Astroparticles; Astrophysics and Cosmology; Current sheets; Electric currents; Filaments; Fluid dynamics; Fluid flow; Gamma rays; Leptons; Magnetic fields; Magnetism; Observations and Techniques; Physics; Physics and Astronomy; Planetology; Radio waves; Review Article; Space Exploration and Astronautics; Space Sciences (including Extraterrestrial Physics; Spontaneous; Thermal radiation; Turbulence; Turbulent flow
• ISSN: 0935-4956; 1432-0754
• DOI: 10.1007/s00159-015-0087-1

The present review concerns the relevance of collisionless reconnection in the astrophysical context. Emphasis is put on recent developments in theory obtained from collisionless numerical simulations in two and three dimensions. It is stressed that magnetic reconnection is a universal process of particular importance under collisionless conditions, when both collisional and anomalous dissipation are irrelevant. While collisional (resistive) reconnection is a slow, diffusive process, collisionless reconnection is spontaneous. On any astrophysical time scale, it is explosive. It sets on when electric current widths become comparable to the leptonic inertial length in the so-called lepton (electron/positron) “diffusion region”, where leptons de-magnetise. Here, the magnetic field contacts its oppositely directed partner and annihilates. Spontaneous reconnection breaks the original magnetic symmetry, violently releases the stored free energy of the electric current, and causes plasma heating and particle acceleration. Ultimately, the released energy is provided by mechanical motion of either the two colliding magnetised plasmas that generate the current sheet or the internal turbulence cascading down to lepton-scale current filaments. Spontaneous reconnection in such extended current sheets that separate two colliding plasmas results in the generation of many reconnection sites (tearing modes) distributed over the current surface, each consisting of lepton exhausts and jets which are separated by plasmoids. Volume-filling factors of reconnection sites are estimated to be as large as < 10 - 5 per current sheet. Lepton currents inside exhausts may be strong enough to excite Buneman and, for large thermal pressure anisotropy, also Weibel instabilities. They bifurcate and break off into many small-scale current filaments and magnetic flux ropes exhibiting turbulent magnetic power spectra of very flat power-law shape W b ∝ k - α in wavenumber k with power becoming as low as α ≈ 2 . Spontaneous reconnection generates small-scale turbulence. Imposed external turbulence tends to temporarily increase the reconnection rate. Reconnecting ultra-relativistic current sheets decay into large numbers of magnetic flux ropes composed of chains of plasmoids and lepton exhausts. They form highly structured current surfaces, “current carpets”. By including synchrotron radiation losses, one favours tearing-mode reconnection over the drift-kink deformation of the current sheet. Lep