Onset of magnetic reconnection in a collisionless, high-$\unicode[STIX]{x1D6FD}$ plasma. (27th February 2019)

• Record Type: Journal Article
• Authors: Alt, Andrew; Kunz, Matthew W.
• Is Part Of: Journal of plasma physics. Volume 85:Number 1(2019)
• Publication Date: 2019-02-27
• Subjects: astrophysical plasmas, -- plasma instabilities, -- plasma nonlinear phenomena; Plasma (Ionized gases) -- Periodicals; 530.4405
• DOI: 10.1017/S0022377819000084
• Languages: English
• ISSNs: 0022-3778

Abstract

Abstract : In a magnetized, collisionless plasma, the magnetic moment of the constituent particles is an adiabatic invariant. An increase in the magnetic-field strength in such a plasma thus leads to an increase in the thermal pressure perpendicular to the field lines. Above a $\unicode[STIX]{x1D6FD}$ -dependent threshold (where $\unicode[STIX]{x1D6FD}$ is the ratio of thermal to magnetic pressure), this pressure anisotropy drives the mirror instability, producing strong distortions in the field lines on ion-Larmor scales. The impact of this instability on magnetic reconnection is investigated using a simple analytical model for the formation of a current sheet (CS) and the associated production of pressure anisotropy. The difficulty in maintaining an isotropic, Maxwellian particle distribution during the formation and subsequent thinning of a CS in a collisionless plasma, coupled with the low threshold for the mirror instability in a high- $\unicode[STIX]{x1D6FD}$ plasma, imply that the geometry of reconnecting magnetic fields can differ radically from the standard Harris-sheet profile often used in simulations of collisionless reconnection. As a result, depending on the rate of CS formation and the initial CS thickness, tearing modes whose growth rates and wavenumbers are boosted by this difference may disrupt the mirror-infested CS before standard tearing modes can develop. A quantitative theory is developed to illustrate this process, which may find application in the tearing-mediated disruption of kinetic magnetorotational 'channel' modes.