Problems and Progress in Astrophysical Dynamos

Abstract

Abstract. Astrophysical objects with negligible resistivity are often threaded by large scale magnetic fields. The generation of these fields is somewhat mysterious, since a magnetic field in a perfectly conducting fluid cannot change the flux threading a fluid element, or the field topology. Classical dynamo theory evades this limit by assuming that magnetic reconnection is fast, even for vanishing resistivity, and that the large scale field can be generated by the action of kinetic helicity. Both these claims have been severely criticized, and the latter appears to conflict with strong theoretical arguments based on magnetic helicity conservation and a series of numerical simulations. Here we discuss recent efforts to explain fast magnetic reconnection through the topological effects of a weak stochastic magnetic field component. We also show how mean-field dynamo theory can be recast in a form which respects magnetic helicity conservation, and how this changes our understanding of astrophysical dynamos. Finally, we comment briefly on why an asymmetry between small scale magnetic and velocity fields is necessary for dynamo action, and how it can arise naturally.

Kiloparsec‐Scale Jets in FR I Radio Galaxies and the γ‐Ray Background

“…Hence, one can in principle suspect that amplification of the magnetic energy flux along the low-power jets between the subparsec and kiloparsec scales is required by the data, in agreement with the more detailed analysis of M87 . We note that amplification of a mean magnetic energy flux is indeed expected in any conductive plasma containing nonvanishing helicity of turbulent motions or a strong velocity shear, although the general applicability of the simple kinematic dynamo theory-usually considered in this context-to realistic astrophysical situations (like the relativistic outflows discussed here) is not clear (see, e.g., the recent review by Vishniac et al 2003). …”

Moving-mesh Simulations of Star-forming Cores in Magneto-gravo-turbulence

“…However, turbulence also acts to remove field lines from collapsing regions via reconnection diffusion and the magnetic field changes its topology in just an eddy turn over time (Lazarian & Vishniac 1999;Vishniac et al 2003;Lazarian et al 2004). Once rapid collapse begins, reconnection diffusion, which depends only on the properties of turbulence/turbulence amplitude, removes magnetic field from the contracting clouds in competition with the amplification the field experiences due to contraction and dynamo processes.…”

Isotherms clustering in cosmic microwave background

“…Technically, using the cosmic microwave pixel data map, we will calculate the cosmic microwave radiation temperature gradient measure using summation over pixel sets (by random walks method) instead of integration over subvolumes v r . The scaling of type (11) (if exists) will be then written as (12) where the metric scale r is replaced by number of the pixels, s, characterizing the size of the summation set (the random walk trajectory length). The χ s is a surrogate of the real 3D gradient measure χ r .…”