Advection of Magnetic Fields in Accretion Disks: Not So Difficult After All

Abstract

We show that a large-scale, weak magnetic field threading a turbulent accretion disk tends to be advected inward, contrary to previous suggestions that it will be stopped by outward diffusion. The efficient inward transport is a consequence of the diffuse, magnetically dominated surface layers of the disk, where the turbulence is suppressed and the conductivity is very high. This structure arises naturally in three-dimensional simulations of magnetorotationally unstable disks, and we demonstrate here that it can easily support inward advection and compression of a weak field. The advected field is anchored in the surface layer but penetrates the main body of the disk, where it can generate strong turbulence and produce values of (i.e., the turbulent stress) that are large enough to match observational constraints; typical values of the vertical magnetic field merely need to reach a few percent of equipartition for this to occur. Overall, these results have important implications for models of jet formation that require strong, large-scale magnetic fields to exist over a region of the inner accretion disk.

Turbulent resistivity evaluation in magnetorotational instability generated turbulence

“…In fact, because the accretion velocity in jet-driving disks is larger than in standard accretion disks, this requirement overestimates the necessary turbulent resistivity, which turns out to be comparable to the turbulent viscosity in self-consistent accretion-ejection models (see e.g. Casse & Ferreira 2000; see also Lovelace 2008 andLovelace et al 2009, for a simplified version of the same argument). This large turbulent Prandtl number argument is also often invoked to justify that an outer standard disk cannot transform into an inner jet-launching one in the accretion process, but the validity of this conclusion relies heavily on the assumed vertical structure of the models considered (in particular the magnetic structure of the corona), a point that has not been appropriately taken into account in the literature up to now.…”

Launching of jets by cold, magnetized disks in Kerr metric

“…The possibility of dragging of these fields by accreting plasma, originally suggested by Bisnovatyi-Kogan & Ruzmaikin (1974), is still debated (Lubow et al 1994;Spruit & Uzdensky 2005;Bisnovatyi-Kogan & Lovelace 2007;Rothstein & Lovelace 2008;Beckwith et al 2009). The radial distribution of the magnetic field line inclination is another question that is related to this lack of a profound model for the magnetic field evolution in accretion disks.…”

Jet launching and field advection in quasi-Keplerian discs

“…This is kind of problematic as our current view of turbulence would suggest instead P m ∼ o(1) Lesur & Longaretti (2009). However, they did not take into account the disc vertical equilibrium and, as pointed out by Rothstein & Lovelace (2008), advection could still be occurring. In fact, what is requested in order to switch from an outer SAD to an inner JED is only to reach a large enough magnetization µ.…”