Negative magnetic eddy diffusivity due to oscillogenic α-effect

PHYSICA D-NONLINEAR PHENOMENA

卷:399

Article

Andrievsky, A.¹  Chertovskih, R.²  Zheligovsky, V¹ 

[1] Russian Ac Sci, Inst Earthquake Predict Theory & Math Geophys, 84-32 Profsoyuznaya St, Moscow 117997, Russia

[2] Univ Porto, Fac Engn, Res Ctr Syst & Technol, Rua Dr Roberto Frias S-N, P-4200465 Porto, Portugal

关键词: Dynamo theory;    Kinematic dynamo;    Large-scale magnetic mode;    Alpha-effect;    Magnetic eddy diffusivity;   

DOI  :  10.1016/j.physd.2019.04.005

来源: Elsevier

【 摘 要 】

We study large-scale kinematic dynamo action of steady mirror-antisymmetric flows of incompressible fluid, that involve small spatial scales only, by asymptotic methods of the multiscale stability theory. It turns out that, due to the magnetic alpha-effect in such flows, the large-scale mean field experiences harmonic oscillations in time on the scale O(epsilon t) without growth or decay. Here epsilon is the spatial scale ratio and t is the fast time of the order of the flow turnover time. The interaction of the accompanying fluctuating magnetic field with the flow gives rise to an anisotropic magnetic eddy diffusivity, whose dependence on the direction of the large-scale wave vector generically exhibits a singular behaviour, and thus to negative eddy diffusivity for whichever molecular magnetic diffusivity. Consequently, such flows always act as kinematic dynamos on the time scale O(epsilon(2)t); for the directions at which eddy diffusivity is infinite, the large-scale mean-field growth rate is finite on the scale O(epsilon(3/2)t). We investigate numerically this dynamo mechanism for two sample flows. (C) 2019 Elsevier B.V. All rights reserved.

【 授权许可】

Free