Speaker
A. Hankla
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P4.4019.pdf
Dependence of the Nonhelical Dynamo on Shear: Numerical Exploration
of the Magnetic Shear-Current and Stochastic-α Effects
A. Hankla1 , C. Fendt1
1 Max Planck Institute for Astronomy, Heidelberg, Germany
Under certain conditions, a well-ionized plasma can generate its own magnetic field in what is
known as a “dynamo”. The need for a theory to describe dynamo action is evident from e.g.
the Earth (whose magnetic field would have decayed long ago if not for a dynamo); however,
the standard “α effect” mechanism fails in systems that lack reflectional symmetry, for instance
near the midplane of accretion disks around black holes. Two competing models attempt to
fill this shortcoming: the “stochastic α” effect, relying on random fluctuations of this same α
coefficient, and the “magnetic shear-current” effect, suggesting that the off-diagonal resistive
term ηyx can be negative, hence providing a sort-of reverse diffusion of the field [1]. The goal
of this work is to disentangle the contributions of these models by investigating the dependence
of stress and magnetic energy production on a simple system’s degree of shear.
Using unstratified shearing boxes, we investigate a range of shearing parameters q, defined
such that the orbital velocity Ω ∼ r−q , from near rigid body (q = 0) to Keplerian (q = 1.5) rota-
tion. Although many parameters behave according to predictions, an unexpected break occurs
in others (e.g the ratio of Maxwell stress to magnetic energy) around q = 1.2. This effect is
only present in “tall” boxes whose length in the z-direction is at least twice as long as the ra-
dial length, supporting Ref. [2]’s hypothesis that the dynamo can only act when longer vertical
modes are allowed. We investigate the transport coefficients to shed light on this break.
Another as-yet unexplained feature of these dynamos is the periodic reversal of the toroidal
magnetic field, similar to the sun’s 11-year cycle, which could explain the origin of knots in
black hole jets [3]. We characterize the unstratified dynamo’s reversal as a function of shear
(complementary to Ref. [4]’s analysis for stratified shearing boxes) to motivate future theoreti-
cal work on the origin of the reversals within the context of the magnetic shear-current effect.
References
[1] J. Squire and A. Bhattacharjee, J. Plasma Phys. 82 (2016)
[2] J. Shi, J. Stone, and C. Huang, MNRAS 456 (2016)
[3] D. Stepanovs, C. Fendt, and S. Sheikhnezami, ApJ 796, 29 (2014)
[4] O. Gressel and M. Pessah, ApJ 810, 59 (2015)
