Broad current sheets, current bifurcation, and collisionless reconnection—An Opinion on “Onset of fast magnetic reconnection via subcritical bifurcation” by Z. Guo and X. Wang

Citation: Treumann RA and Baumjohann W (2015) Broad current sheets, current bifurcation, and collisionless reconnection—An Opinion on " Onset of fast magnetic reconnection via subcritical bifurcation " by Z. Magnetic reconnection in current carrying plasmas, in particular when the plasma is collisionless, is an efficient channel of dissipating the energy temporarily stored in electric currents that are generated in one or the other way in the mechanical plasma flow. When collisions (including anomalous) dominate, reconnection is a slow diffusion process, proceeding on resistive timescales and properly described by fluid theory [1]. In the absence of resistive friction and Ohmic losses, the agent transforming the stored energy is the current-related magnetic field. Contacting fields of opposite direction may reorganize, locally annihilate the antiparallel components and provide the liberated energy for direct plasma heating and particle/plasma acceleration. In this case reconnection may become very fast proceeding on electron-plasma time scales. Its onset is subject to the scales of the current layer in that it goes on. Distributed currents filling the entire bulk plasma do barely support reconnection. Also field-aligned currents flowing along strong magnetic fields experience negligible reconnection even when restricted to flux tubes, unless the current is so strong that it winds the magnetic field up into spirals. This is believed to occur in stellar magnetic loops like on the sun during solar eruptions. The relevant setups for collisionless reconnection are currents flowing across moderately strong magnetic fields when two plasmas carrying fields of opposite directions encounter each other, the classical model case of reconnection. Such cases are abundant in space and all kinds of astrophysical magnetized objects. Oppositely polarized solar magnetic loops impacting on each other are but one example. The currents produced in the contact region of the two plasmas are fed by the mechanical plasma motion and are restricted to a finite range across the magnetic field only. The important parameter for collisionless reconnection to occur is the transverse scale of the currents (see Figure 1). In a broad current layer this scale exceeds the ion gyroradius. Here an mhd-fluid description is appropriate since all particles contributing to the current are magnetized. Such layers are subject to " slow reconnection " only if plasma instabilities give rise to anomalous diffusion. Narrower layers of widths in the intermediate region of scales between ion and electron gyroradii require distinguishing between electron and ion dynamics. Ions become non-magnetic while electrons maintain …