Ambipolar diffusion in young stellar object jets

We address the issue of ambipolar diffusion in Herbig-Haro (HH) jets. The c urrent consensus holds that these jets are launched and collimated via MI-I D forces. Observations have, however, shown that the jets can be mildly to weakly ionized. Beginning with a simple model for cylindrical equilibrium b etween neutral, plasma, and magnetic pressures, we calculate the characteri stic 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 MI-ID equilibri a established at the base of an HH jet may not be maintained as the jet pro pagates far from its source. For typical jet parameters one finds that the length scale at which ambipolar diffusion should become significant corresp onds to the typical size of large (parsec) scale jets. We discuss the signi ficance of these results for the issue of magnetic fields in parsec-scale j ets.