COLLIMATION OF MAGNETICALLY DRIVEN JETS FROM ACCRETION DISCS

We argue that the toroidal fields developing in a magnetically acceler ated jet are sufficiently unstable that they cannot contribute much to collimation. We show how an initially collimated jet is decollimated by decay of the toroidal field and by the build-up of internal pressur e due to kink instability. We propose that most of the collimation of observed jets is due to the poloidal field anchored in the disc. We sh ow how the collimation by this mechanism depends on the distribution o f poloidal field strength in the disc. We find that the maximum achiev able collimation increases with the ratio of outer to inner disc radiu s, and can be of the order of 1 degrees. This dependence is found to b e consistent with the available data, in particular the absence of col limated outflows from cataclysmic variables. Because of the decay of t he toroidal field a new characteristic distance plays a role: the coll imation distance, z(c), of the order of the disc radius or less. Beyon d z(c) the jet is entirely ballistic and only weakly magnetic. No exte rnal medium is needed beyond this distance to explain observed narrow opening angles of jets, and no interaction with an external medium is necessary to explain the parallel field orientation observed in fast j ets.