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Effect of Magnetic Helicity on Non-Helical Turbulent Dynamos

This study explores the impact of magnetic helicity on non-helical turbulent dynamos, specifically focusing on the shear-current effect and its comparison with the alpha effect. It investigates the generation of large-scale magnetic fields and discusses the physics of the alpha effect. Direct numerical simulations and mean-field approaches are utilized to understand the dynamics of magnetic helicity. The results shed light on the generation of mean magnetic fields in non-helical and non-rotating homogeneous turbulence.

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Effect of Magnetic Helicity on Non-Helical Turbulent Dynamos

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  1. Effect of Magnetic Helicity on Non-Helical Turbulent Dynamos N. KLEEORINand I. ROGACHEVSKII Ben-Gurion University of the Negev, Beer Sheva, ISRAEL

  2. Outline • Introduction • Physics of “shear-current” effect and comparison with alpha effect (hydrodynamic helicity) • Generation of large-scale magnetic field due to the “shear-current” effect (kinematic and nonlinear dynamos with magnetic helicity)

  3. Alpha-Omega Dynamo(Mean-Field Approach) Induction equation for mean magnetic field: Electromotive force:

  4. Alpha-Omega Dynamo(Mean-Field Approach) • Induction equation for mean magnetic field: • Electromotive force:

  5. Alpha-Omega Dynamo(Mean-Field Approach) Induction equation for mean magnetic field: Electromotive force:

  6. Generation of the mean magnetic field due to the dynamo Mean magnetic field: Dynamo number:

  7. Physics of the alpha-effect • The -effect is related to the hydrodynamic helicity in an inhomogeneous turbulence. • The deformations of the magnetic field lines are caused by upward and downward rotating turbulent eddies. • The inhomogeneity of turbulence breaks a symmetry between the upward and downward eddies. • Therefore, the total effect of the upward and downward eddies on the mean magnetic field does not vanish and it creates the mean electric current parallel to the original mean magnetic field.

  8. Mean-Field Dynamo Is it possible to generate a large-scale magnetic field in a non-helical and non-rotatinghomogeneous turbulence ? turbulence The answer is YES (I. Rogachevskii and N. Kleeorin, Phys. Rev. E68, 036301 (2003).)

  9. Direct Numerical Simulations • A. Brandenburg, Astrophys. J. 625, 539-547 (2005). • A. Brandenburg, N.E.L. Haugen, P.J. Käpylä, C. Sandin, Astron. Nachr. 326, 174-185 (2005). 1. Non-helical forcing 2. Imposed mean velocity shear 3. Open boundary conditions (non-zero flux of magnetic helicity)

  10. Direct Numerical Simulations (linear shear velocity) T. A. Yousef, T. Heinemann, A.A. Schekochihin, N. Kleeorin, I. Rogachevskii, A.B. Iskakov, S.C. Cowley, J.C. McWilliams,Phys. Rev. Lett., v.100, 184501 (2008) 1. A white noise non-helical homogeneous and isotropic random forcing 2. Imposed mean linear shear flow 3. Sheared box (shear-periodic boundary conditions)

  11. Numerical set up Incompressible MHD equations with background shear (Units: ) Parameters Turbulence: Weak shear:

  12. Magnetic field grows

  13. Generated field is large scale

  14. Growth rate  S

  15. Mean-Field Approach • Induction equation for mean magnetic field: • Electromotive force:

  16. Generation of the mean magnetic field due to the shear-current effect Mean velocity shear: The growth rate of B

  17. Comparison of the alpha-effect with the ''shear-current" effect • The effect is caused by a uniform rotation and inhomogeneity of turbulence: , where • The ''shear-current" effect is related to the term and is caused by mean shear and nonuniform mean magnetic field, , where • Therefore,

  18. The effect of large-scale shear • The large-scale shear motions cause the stretching of the magnetic field generating the field component • The interaction of the non-uniform magnetic field with the background vorticity produces electric current along the field

  19. Physics of ''shear-current" effect • In a turbulent flow with the mean velocity shear, the inhomogeneity of the original mean magnetic fieldbreaks a symmetry between the influence of the upwardand downward turbulent eddies on the mean magnetic field. • The deformations of the magnetic field lines in the ''shear-current" dynamo are caused by the upward and downward turbulent eddies which result in the mean electric current parallel to the mean magnetic field and produce the magnetic dynamo.

  20. Generation of the mean magnetic field (kinematic dynamo) The growth rate of B: Critical dynamo number: Solution for the symmetricmode:

  21. The shear-current nonlinear dynamo (algebraic nonlinearity) Dynamo number: Shear number: Mean magnetic field: Nonlinear shear-current effect:

  22. Nonlinear shear-current effect Weak magnetic field: Strong mean magnetic field: There is no quenching of the nonlinear "shear-current" effect contrary to the quenching of the nonlinear alpha effect, the nonlinearturbulent magnetic diffusion, etc.

  23. Nonlinear “shear-current” dynamo(algebraic nonlinearity)

  24. Magnetic Helicity Total magnetic helicityis conserved for very large magnetic Reynolds numbers Magnetic part of alpha effect: The nonlinear function: Dynamics of small-scale magnetic helicity:

  25. Dynamics of magnetic helicity Kleeorin and Ruzmaikin (1982); Gruzinov and Diamond (1994); Kleeorin and Rogachevskii (1999); Kleeorin, Moss, Rogachevskii and Sokoloff (2000); Blackman and Field (2000); Brandenburg and Subramanian (2005); etc. In the absence of the magnetic helicity flux, i.e., catastrophic quenching (Vainshtein and Cattaneo, 1992) In the presence of the flux of magnetic helicity:

  26. The shear-current nonlinear dynamo (algebraic and dynamic nonlinearities) Mean magnetic field:

  27. References • I. Rogachevskii and N. Kleeorin, Phys. Rev. E68, 036301 (2003). • I. Rogachevskii and N. Kleeorin, Phys. Rev. E 70, 046310 (2004). • I. Rogachevskii, N. Kleeorin, A. D. Chernin and E. Liverts, Astron. Nachr. 327, 591-594 (2006). • I. Rogachevskii, N. Kleeorin and E. Liverts, Geophys. Astroph. Fluid Dyn. 100, 537-557 (2006). • I. Rogachevskii andN. Kleeorin, Phys. Rev. E 75, 046305 (2007). • N. Kleeorin and I. Rogachevskii, Planet. Space Sci.55, 2315-2318 (2007). • N. Kleeorin and I. Rogachevskii, Phys. Rev. E 77, 036307 (2008). • T. A. Yousef, T. Heinemann, A.A. Schekochihin, N. Kleeorin, I. Rogachevskii, A.B. Iskakov, S.C. Cowley and J.C. McWilliams,Phys. Rev. Lett. 100, 184501 (2008).

  28. Conclusions • Generation of large-scale magnetic field is caused by a new ''shear-current" effect which acts even in a nonrotating and nonhelical homogeneous turbulence. • The shear-current dynamo and generation of large-scale vorticitycan occur only when Re>1 and Rm > 1 • During the growth of the mean magnetic field, the nonlinear ''shear-current" effect is not quenched and it only changes its sign at some value of the mean magnetic field which can determine the level of the saturated mean magnetic field.

  29. Conclusions • We have taken into account the transport of magnetic helicity as dynamical nonlinearity. The magnetic helicity flux strongly affects the magnetic field dynamics during the nonlinear shear-current dynamo. The level of the saturated mean magnetic field is of the order of the equipartition field. • The shear current dynamo can occur in laboratory dynamo experiments. • The estimated saturated large-scale magnetic field for merging protogalactic clouds and colliding giant galaxy clusters is about several microgauss, and for merging protostellar clouds is of the order of several tenth of microgauss.

  30. THE END

  31. Shear-current dynamo with generated mean vorticity Growing perturbations of vorticity:

  32. Generation of the mean magnetic field (kinematic dynamo) The growth rate of B: Critical dynamo number: Solution for the antisymmetric mode: The magnetic scale at maximum :

  33. Anisotropic Turbulent Magnetic Diffusion

  34. Anisotropic Turbulent Viscosity

  35. Method of Derivation Equations for the correlation functions for: • The velocity fluctuations • The magnetic fluctuations • The cross-helicity tensor The spectral t-approximation (the third-order closure procedure)

  36. The shear-current nonlinear dynamo (algebraic and dynamic nonlinearities) Mean magnetic field: Magnetic part of alpha effect: The nonlinear function: Dynamical nonlinearity: magnetic helicity evolution

  37. Astrophysical clouds We apply the universal mechanism of generation of large-scale magnetic fields due to shear-current effect to several astrophysical objects: merging protostellar clouds merging protogalactic clouds colliding giant galaxy clusters Interactions of protostellar clouds, or colliding protogalactic clouds or giant galaxy clusters produce large-scale shear motions which are superimposed on small-scale turbulence.

  38. Chernin (1993). Non-central collision

  39. Different cloud sizes, Chernin (1993)

  40. Parameters

  41. Parameters

  42. Parameters

  43. Necessary condition for the shear-current dynamo The parameter : The growth rate of B: The Kolmogorov Scaling (large Re and Rm): Rogachevskii and Kleeorin (2003): there isshear-current dynamo Small Re and Rm (random flow): In a good agreement with: there is no dynamo for: Rädler and Stepanov (2006) (SOCA) Rüdiger and Kitchatinov (2006)

  44. Necessary condition for the shear-current dynamo Large Re and arbitrary Rm

  45. Generation of the mean vorticity in turbulence with mean velocity shear Elperin, Kleeorin and Rogachevskii, PRE, 68, 016311 (2003) Mean velocity shear: The growth rate of the mean vorticity

  46. Necessary condition for the shear-current dynamo Re > 1 and arbitrary Rm

  47. Necessary condition for the shear-current dynamo Kraichnan - Kazantsev model: - correlated in time random velocity field There is no shear-current dynamo The shear-current dynamo (as well as effect of shear)requiresfinite correlation timeof turbulent velocity field

  48. Magnetic Field and Vorticity Induction equation for magnetic field: Equation for vorticity:

  49. Generation of the mean vorticity and magnetic field in sheared turbulence Mean velocity shear: The mean magnetic field The mean vorticity The growth rate ofB The growth rate of

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