Magnetic Field Transport in Accretion Disks

Abstract

The leading models for launching astrophysical jets rely on strong poloidal magnetic fields threading the central parts of their host accretion disks. Numerical simulations of magneto-rotationally turbulent disks suggest that such fields are actually advected from the environment by the accreting matter rather than generated by internal dynamos. This is puzzling from a theoretical point of view, since the reconnection of the radial field across the midplane should cause an outward drift on timescales much shorter than the accretion time. We suggest that a combination of effects are responsible for reducing the radial field near the midplane, causing efficient inward advection of the poloidal field. Magnetic buoyancy in subsonic turbulence pushes the field lines away from the midplane, decreasing the large-scale radial field in the main body of the disk. In magneto-rotationally driven turbulence, magnetic buoyancy dominates over the effects of turbulent pumping, which works against it, and turbulent diamagnetism, which works with it, in determining the vertical drift of the magnetic field. Balancing buoyancy with diffusion implies that the bending angle of the large-scale poloidal field can be very large near the surface, as required for outflows, but vanishes near the midplane, which impedes turbulent reconnection and outward diffusion. This effect becomes less efficient as the poloidal flux increases. This suggests that accretion disks are less likely to form jets if they have a modest ratio of outer to inner radii or if the ambient field is very weak. The former effect is probably responsible for the scarcity of jets in cataclysmic variable systems.

Model-free Maps of Interstellar Neutral Hydrogen Measured with IBEX between 2009 and 2018

“…This is achieved thanks to the different energies of the two species: ISN H reaches IBEX-Lo with a relative energy <40 eV at any time or viewing direction (based on the WTPM predictions). Thus, ISN H can only create a detectable H − signal in energy bins 1 (11-21 eV) and 2 (20-41 eV), whereas ISN He, with relative energy inside energy bin 4 (78-155 eV, Swaczyna et al 2018), causes a H − and O − signal via sputtering in all energy bins from 4 down to 1 (Möbius et al 2012). Exploiting these distinctions, we found three useful methods for deriving maps of ISN H: 1.…”

“…This cutoff energy increased from 65±5 eV to 75±5 eV (blue dashed-dotted line in lower panel of Figure 4) when the PAC changed in 2012. Because the ISN energy decreases with higher latitude (Swaczyna et al 2018), the H3_inflight approaches relying on H − counts in energy bin 3 cannot be applied to derive ISN H in energy bins 1 and 2 for ecliptic latitudes beyond ±54°for 2009-2012 or beyond ±48°a fter 2012. The corresponding latitudes or spin angles in the ISN H maps were therefore blanked out.…”

“…This leaves us ->before the PAC change (top) and g3 He H ->after the PAC change (bottom) vs. incoming He energy. The conversion factors from all years at the same instrument settings are plotted against He energy derived with the model parameters for the primary and secondary ISN He (Kubiak et al 2016;Swaczyna et al 2018). The black symbols are the raw data points, the red solid lines denote the linear fits, and the red dashed lines indicate the 1σ uncertainties of the fits.…”

Stringent limits on the magnetic field strength in the disc of TW Hya

“…The poloidal component of the disc magnetic field is the result of magnetic flux that is dragged into the disc during protostellar collapse, and its strength is expected to be greater than the molecular cloud magnetic field (e.g. Ferreira & Pelletier 1995;Jafari & Vishniac 2018). However, recent models show that the toroidal magnetic field is likely the dominant component.…”

Neutrinos and gravitational waves from magnetized neutrino-dominated accretion discs with magnetic coupling

“…The magnetic field in the inner parts of the disc is complicated and currently not well understood (see a detailed study by Jafari & Vishniac 2018), and many authors have recently researched this topic (see e.g., Lovelace, Rothstein & Bisnovatyi-Kogan 2009;Bisnovatyi-Kogan & Lovelace 2007. To simplify the calculations, we assumed that the magnetic fields in the disc vary as BD ∝ ξ −n , as described in Blandford (1976).…”