Spontaneous Formation of Magnetic Flux Concentrations in Stratified Turbulence

被引:0
|
作者
Koen Kemel
Axel Brandenburg
Nathan Kleeorin
Dhrubaditya Mitra
Igor Rogachevskii
机构
[1] AlbaNova University Center,Nordita
[2] Stockholm University,Department of Astronomy
[3] Ben-Gurion University of the Negev,Department of Mechanical Engineering
来源
Solar Physics | 2012年 / 280卷
关键词
Magnetohydrodynamics (MHD); Sun: dynamo; Sunspots; Turbulence;
D O I
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中图分类号
学科分类号
摘要
The negative effective magnetic pressure instability discovered recently in direct numerical simulations (DNSs) may play a crucial role in the formation of sunspots and active regions in the Sun and stars. This instability is caused by a negative contribution of turbulence to the effective mean Lorentz force (the sum of turbulent and non-turbulent contributions) and results in the formation of large-scale inhomogeneous magnetic structures from an initially uniform magnetic field. Earlier investigations of this instability in DNSs of stably stratified, externally forced, isothermal hydromagnetic turbulence in the regime of large plasma β are now extended into the regime of larger scale separation ratios where the number of turbulent eddies in the computational domain is about 30. Strong spontaneous formation of large-scale magnetic structures is seen even without performing any spatial averaging. These structures encompass many turbulent eddies. The characteristic time of the instability is comparable to the turbulent diffusion time, L2/ηt, where ηt is the turbulent diffusivity and L is the scale of the domain. DNSs are used to confirm that the effective magnetic pressure does indeed become negative for magnetic field strengths below the equipartition field. The dependence of the effective magnetic pressure on the field strength is characterized by fit parameters that seem to show convergence for larger values of the magnetic Reynolds number.
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页码:321 / 333
页数:12
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