Magnetically powered jets

This is an update on a previous, more extended conference review text [61], with emphasis on outstanding problem areas. The role of dissipation of magnetic energy in accelerating the flow is discussed, and its importance for explaining high Lorentz factors. The transition between disk and outflow is one of the least understood parts of the magnetic theory; its role in setting the mass flux in the wind, in possible modulations of the mass flux, and the problems in treating it realistically are discussed. Current views on most of these problems are still strongly influenced by the restriction to 2 dimensions (axisymmetry) in previous analytical and numerical work; 3‐D effects likely to be important are suggested. An interesting problem area is the nature and origin of the strong, preferably highly ordered magnetic fields known to work best for jet production. The presence or absence of such fields may well be the ‘second parameter’ governing not only the presence of jets but also the X‐ray spectra and timing behavior of X‐ray binaries.