Poynting Jets from Accretion Disks

Abstract

We give further consideration to the problem of the evolution of a coronal, force-free magnetic field that threads a differentially rotating, conducting Keplerian disk, extending the recent work of Li and coworkers. This situation is described by the force-free Grad-Shafranov (GS) equation for the flux function É(r, z) that labels the poloidal field lines (in cylindrical coordinates). The GS equation involves a function H(É) describing the distribution of the poloidal current, which is determined by the differential rotation or '' twist '' of the disk that increases linearly with time. We numerically solve the GS equation in a sequence of volumes of increasing size corresponding to the expansion of the outer perfectly conducting boundaries at (R m , Z m ). The outer boundaries model the influence of an external nonmagnetized plasma. The sequence of GS solutions provides a model for the dynamical evolution of the magnetic field in response to (1) the increasing twist of the disk and (2) the pressure of external plasma. We find solutions with magnetically collimated Poynting jets in which there is a continuous outflow of energy, angular momentum, and toroidal magnetic flux from the disk into the external space. This behavior contradicts the commonly accepted '' theorem '' of solar plasma physics that the motion of the footpoints of a magnetic loop structure leads to a stationary magnetic field configuration with zero power, angular momentum, and flux outflows. In addition, we discuss magnetohydrodynamic simulations that show quasi-stationary collimated Poynting jets similar to our GS solutions. In contrast with the GS solutions, the simulations show a steady uncollimated hydromagnetic (nonforce-free) outflow from the outer part of the disk. The Poynting jets are of interest for the understanding of the jets from active galactic nuclei, microquasars, and possibly gamma-ray burst sources.

Polarization and Structure of Relativistic Parsec-Scale AGN Jets

“…The development of instabilities would lead to the dissipation of poloidal current and a decrease in B φ /B z (Colgate & Li 1998). Calculations of relativistic force-free expanding helices produced by the twisting of the magnetic field lines by the accretion disc also indicate that B φ ∼ B z in the inner collimated part of the expanding magnetic helix (Lovelace et al 2002). Thus, if the modest observed degrees of polarization are indeed due to the presence of comparable toroidal and poloidal fields in the jet frame, this would imply that all relativistic jets are dominated by the toroidal magnetic field component in the observer's frame.…”

“…Choudhuri & Konigl 1986). Secondly, it is probable that jets are electromagnetically dominated on parsec scales (Lynden-Bell 1996Ustyugova et al 2000;Li et al 2001;Lovelace et al 2002;; see also Section 6), so that their structure is determined by relativistic force-free electrodynamics. In this work, we assume that the jets are strongly magnetized and are described by relativistic force-free electrodynamics.…”

Propeller-driven Outflows and Disk Oscillations

“…Furthermore, there is a strong outflow of angular momentum carried by the twisted open magnetic field lines from the star , the Poynting flux, and the closed field AN S f lines connecting the star and the disk (bottom panel of Fig. 4; see also Lovelace et al 2002). The time-averaged total angular momentum flux from the star is …”

EPISODIC HIGH-VELOCITY OUTFLOWS FROM V899 Mon: A CONSTRAINT ON THE OUTFLOW MECHANISMS*

“…Magnetic pressure driven conical winds can also arise from the inner disc-magnetosphere boundary irrespective of the truncation radius (Romanova et al 2009;Lii et al 2012). Along the polar direction, one can have radiatively driven stellar winds, as well as magnetic pressure driven winds, like in the case of a Poynting jet in the propeller regime (Lovelace et al 2002;Ustyugova et al 2006) when the magnetosphere is rotating fast enough. All the above mentioned outflow mechanisms have their own characteristic velocity, outflow angle, as well as stability timescale.…”