Abstract

Magnetic forces seem likely to play the dominant role in both the acceleration and initial collimation of relativistic jets in AGNs. I describe recent developments in the theory of hydromagnetic jets and winds.

Introduction

Hydromagnetic propulsion as a mechanism for accelerating jets has become attractive largely through a process of elimination. Other mechanisms, such as acceleration by gas or radiation pressure, have been examined and found inadequate. The observational case for relativistic flow velocities, on both pc and kpc scales, continues to mount (see, e.g., and for recent reviews). Models involving acceleration by radiation pressure would have to be stretched to extremes in order to account for the Lorentz factors ∼ 2 – 10 which are needed to explain most instances of onesidedness and superluminal motion. Losses due to catastrophic cooling place even more severe constraints on acceleration by gas pressure. Recent observations of “intra-day” radio variability, may require Lorentz factors as high as ∼ 100 if catastrophic Compton losses are to be avoided.

Magmetic propulsion has other attractive features besides the ability to produce the high speeds indicated by observations. Chief among these is the tendency of magnetically driven flows to “self-collimate” due to the development of a predominantly toroidal magnetic field. Thus, it may not be necessary to invoke a “funnel” or external confining medium to explain the collimation of jets.