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Title:
Onset of magnetic reconnection in a collisionless, high-> beta > plasma
Authors:
Alt, Andrew; Kunz, Matthew W.
Affiliation:
AA(Department of Astrophysical Sciences, Princeton University, Peyton Hall, Princeton, NJ 08544, USA), AB(Department of Astrophysical Sciences, Princeton University, Peyton Hall, Princeton, NJ 08544, USA)
Publication:
Journal of Plasma Physics, Volume 85, Issue 1, article id. 764850101, 17 pp.
Publication Date:
02/2019
Origin:
CUP
Keywords:
astrophysical plasmas, plasma instabilities, plasma nonlinear phenomena
Abstract Copyright:
(c) 2019: © Cambridge University Press 2019
DOI:
10.1017/S0022377819000084
Bibliographic Code:
2019JPlPh..85a7601A

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 > beta >-dependent threshold (where > beta > 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-> beta > 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.
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