Ambipolar diffusion in young stellar object jets

We address the issue of ambipolar diffusion in Herbig-Haro (HH) jets. The current consensus holds that these jets are launched and collimated via MHD forces. Observations have, however, shown that the jets can be mildly to weakly ionized. Beginning with a simple model for cylindrical equilibrium between neutral, plasma, and magnetic pressures, we calculate the characteristic timescale for ambipolar diffusion. Our results show that a significant fraction of HH jets will have ambipolar diffusion timescales equivalent to, or less than, the dynamical timescales. This implies that MHD equilibria established at the base of an HH jet may not be maintained as the jet propagates far from its source. For typical jet parameters one finds that the length scale at which ambipolar diffusion should become significant corresponds to the typical size of large (parsec) scale jets. We discuss the significance of these results for the issue of magnetic fields in parsec-scale jets.