Jet Collimation by Small‐Scale Magnetic Fields

Abstract

A popular model for jet collimation is associated with the presence of a large-scale and predominantly toroidal magnetic field originating from the central engine (a star, a black hole, or an accretion disk). Besides the problem of how such a large-scale magnetic field is generated, in this model the jet suffers from the fatal long-wave mode kink magnetohydrodynamic instability. In this paper we explore an alternative model: jet collimation by small-scale magnetic fields. These magnetic fields are assumed to be local, chaotic, and tangled, but are dominated by toroidal components. Just as in the case of a large-scale toroidal magnetic field, we show that the "hoop stress" of the tangled toroidal magnetic fields exerts an inward force which confines and collimates the jet. The magnetic "hoop stress" is balanced either by the gas pressure of the jet or by centrifugal force if the jet is spinning. Since the length scale of the magnetic field is small (< the cross-sectional radius of the jet ≪ the length of the jet), in this model the jet does not suffer from the long-wave mode kink instability. Many other problems associated with the large-scale magnetic field are also eliminated or alleviated for small-scale magnetic fields. Though it remains an open question how to generate and maintain the required small-scale magnetic fields in a jet, the scenario of jet collimation by small-scale magnetic fields is favored by the current study on disk dynamo which indicates that small-scale magnetic fields are much easier to generate than large-scale magnetic fields.