Abstract
Magnetic reconnection caused by turbulence in a current sheet is studied by means of numerical simulations of fluid equations. It is found that turbulence produces long-wavelength magnetic islands even if the current sheet is so thick that spontaneous magnetic reconnection does not occur. Thus, turbulence modifies the threshold of magnetic island formation predicted by the conventional theory of spontaneous magnetic reconnection in a current sheet. In spite of the fact that the turbulence is driven by a short-wavelength instability due to a pressure gradient, the length of the magnetic island is the same order as the system size. The width of the island is several times the ion Larmor radius, and stronger turbulence causes wider magnetic islands. This suggests that the turbulence can trigger neoclassical tearing modes, which are the main nonlinear instability that limits the plasma pressure in magnetically confined plasmas. The long-wavelength magnetic island is formed by merging of small-scale magnetic islands.
Highlights
Magnetic reconnection is thought to be a mechanism of explosive phenomena in space plasmas, such as coronal mass ejections and magnetospheric substorms and a mechanism of confinement degradation due to violation of magnetic surfaces in magnetically confined plasmas.1–3 These magnetic reconnection phenomena cause magnetic islands or plasmoids, and they can strongly affect the dynamics of a plasma.1Tearing instabilities are spontaneous magnetic reconnection that may occur in sheared magnetic configuration, such as a current sheet and a rational surface in a magnetic confinement.4 The conventional theory based on resistive magnetohydrodynamicsMHD ͑Ref. 4͒ shows that the instability is driven by the current density gradient of a current sheet and is caused by resistivity
In order to understand the effect of turbulence on the threshold for magnetic island formation, we investigate turbulence in a current sheet, which is so thick that there is no spontaneous magnetic reconnection by numerical simulations based on an MHD model and on a twofluid model
We have found that turbulence driven by shortwavelength instability causes magnetic reconnection and produces long-wavelength magnetic islands in a current sheet even when the sheet is so thick that there is no spontaneous magnetic reconnection
Summary
Magnetic reconnection is thought to be a mechanism of explosive phenomena in space plasmas, such as coronal mass ejections and magnetospheric substorms and a mechanism of confinement degradation due to violation of magnetic surfaces in magnetically confined plasmas. These magnetic reconnection phenomena cause magnetic islands or plasmoids, and they can strongly affect the dynamics of a plasma.. In order to understand the effect of turbulence on the threshold for magnetic island formation, we investigate turbulence in a current sheet, which is so thick that there is no spontaneous magnetic reconnection by numerical simulations based on an MHD model and on a twofluid model. In order to include the shortwavelength instability we consider two models: one is a twodimensional MHD model including the interchange term; the other is a two-dimensional reduced two-fluid model These two models describe typical short-wavelength instabilities. Numerical simulations based on the two-fluid model show that turbulence driven by drift-wave instabilities causes longwavelength islands. Numerical simulations based on the MHD model show that long-wavelength magnetic islands are produced through energy transfer from small-scale turbulence driven by short-wavelength instability due to a pressure gradient.
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