Onset of collisionless magnetic reconnection in sheared field reversals

Abstract

An extended analysis of the kinetic stability theory of a current sheet in a curved and sheared magnetic field reversal with respect to a tearing mode perturbation is presented. The coupling between quasi‐parallel and quasi‐perpendicular mode components is newly taken into account. We obtain that an unstable branch of tearing mode instability with a growth rate less than the electron bounce frequency can be excited. The dependencies of the growth rate of this instability on the shear (Bs) and normal (Bn) field components as far as on the wave length of the mode are illustrated. The resulting tearing mode destabilization takes place favorably for an inclination of the magnetic field near the quasi‐neutral plane at about 45° with respect to the neutral plane. Our theory reveals that for proper geometry of magnetic field lines in field reversal region a collisionless ion tearing mode can be triggered and magnetic flux ropes are formed. The growth of the unstable tearing mode can be accompanied by the generation of magnetic field aligned electric fields and ion acceleration.