Two-fluid regimes of the resistive and collisionless tearing instability

The two-fluid (electrons and ions) plasma description is used to investigate spontaneous magnetic reconnection (tearing instability) in a sheared force-free magnetic configuration. The study includes both collisional (resistive) and collisionless (electron inertia) reconnection mechanisms. Together with the Hall effect, this yields various instability regimes, which can be identified by the magnitude of the four nondimensional parameters: Lundquist number $S⪢1$⁠, electron/ion mass ratio $μ≡me∕mi⪡1$⁠, plasma $β$⁠, and the scaled ion inertial skin-depth $di=c∕(ωpil)$⁠, with $l$ being the shear length of the magnetic field. The role of the electron gyroviscosity in the collisionless case is also clarified.