Hydromagnetic Dynamo in Astrophysical Jets

Abstract

A b s t r a c t . The origin of a regular magnetic field in astrophysical jets is discussed. It is shown that jet plasma flow can generate a magnetic field provided the streamlines are helical. The dynamo of this type, known as the screw dynamo, generates magnetic fields with the dominant azimuthal wave number m = 1 whose field lines also have a helical shape. The field concentrates into a relatively thin cylindrical shell and its configuration is favorable for the collimation and confinement of the jet plasma.K e y words: The screw dynamo -Astrophysical jets Long, thin, well-collimated jets are a widespread companion of energetic activity in radio galaxies and young stars (see, e.g., the reviews of Begelman et al., Lada, 1985), and even in the Galactic center region (Mirabel et ai, 1992). Polarization observations of the synchrotron emission of extragalactic jets indicate the presence of magnetic fields of a few /iG strength at the kiloparsec scale. A high polarization of the synchrotron emission indicates a rather high degree of ordering of the magnetic field, although purely random two-dimensional magnetic field configurations can also produce polarized emission (Laing, 1981). It seems only reasonable to conclude that magnetic fields in the jets contain both regular and random components, as do the fields in all other astronomical objects. In many cases, polarization observations are compatible with a helical shape for the field lines of the ordered field.In contrast to the rather detailed knowledge of the magnetic field morphology in extragalactic jets, very little is known about the plasma velocity patterns. This is due to the fact that extragalactic jets emit mainly a continuous, synchrotron emission, so that the classical method of the observational study of velocity fields based on the Doppler shifts of spectral lines does not work. As a result, our knowledge of the jet velocity fields is only rudimentary. However, there is an indirect evidence of a helical shape for the streamlines in the jets of quasars and BL Lac objects as discussed by Camenzind and Krockenberger (1992). The helical shape of both the streamlines and the magnetic lines is also a property of many self-similar MHD jet models.The observational situation with stellar jets is quite the opposite. They contain rather dense thermal plasma, so that their velocity fields can be studied, at least in principle, by conventional methods of astronomy. Up to now, observational attempts have focused on the longitudinal velocity in stellar jets but, undoubtedly, information on the transverse velocity components and the velocity profiles across the jet will follow. However, up to now there is hardly any information about magnetic fields in stellar bipolar outflows, even though magnetic field is believed to play an important role in the formation, collimation and confinement of both extragalactic and stellar jets (see Begelman et al., 1984;Camenzind, 1990). As long as we might believe that the mechanisms of acceleration and collimation of the jet

History and results of the Riga dynamo experiments

“…One promising candidate is the "double helix nebula" which was detected recently in the outflow from the galactic centre [8]. The basic idea that cosmic jets could work as a Ponomarenko-like dynamo was already discussed in an early paper by Shukurov and Sokoloff [9].…”