Plasma and Fusion Research

Volume 12, 1401010 (2017)

Regular Articles

An Explosive Scaling Law for Nonlinear Magnetic Reconnection and Its Insensitivity to Microscopic Scales
Makoto HIROTA
Institute of Fluid Science, Tohoku University, Sendai, Miyagi 980-8577, Japan
(Received 11 January 2017 / Accepted 16 February 2017 / Published 24 March 2017)

Abstract

The nonlinear phase of magnetic reconnection is investigated by numerically solving a gyrofluid model. The scaling law for the explosive reconnection rate, which has been recently derived for an ideal two-fluid model [Hirota et al., Phys. Plasmas 22, 052114 (2015)], is found to consistently hold when either the ion-sound gyroradius ρS, or the ion gyroradius ρi is comparable to the electron skin depth de, even in the presence of finite resistivity η. In this explosive phase, a local X-shaped current layer is spontaneously generated, in which the reconnection speed is closely related to the macroscopic shape of the layer and is almost independent of the layer width. The reconnection speed is therefore insensitive to the size of the microscopic scales, ρS, ρi, de and η. On the other hand, in the cold plasma limit, where ρS = ρi = 0, the intermittent acceleration of the reconnection speed is caused by the plasmoid instability. This also seems to be explosive on average, but the rate always falls below the explosive scaling law. The reconnection time extrapolated from this scaling law is shown to be fast enough to explain the time scale of solar flares.

Keywords

magnetic reconnection, explosive instability, gyrofluid model, plasmoid instability, solar flare

DOI: 10.1585/pfr.12.1401010

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