  Although several lines of evidence suggest that jets from young stars are
driven magnetically from accretion disks, existing observations of field
strengths in the bow shocks of these flows imply that magnetic fields play only
a minor role in the dynamics at these locations. To investigate this apparent
discrepancy we performed numerical simulations of expanding magnetized jets
with stochastically variable input velocities with the AstroBEAR MHD code.
Because the magnetic field B is proportional to the density n within
compression and rarefaction regions, the magnetic signal speed drops in
rarefactions and increases in the compressed areas of velocity-variable flows.
In contrast, B ~ n^0.5 for a steady-state conical flow with a toroidal field,
so the Alfven speed in that case is constant along the entire jet. The
simulations show that the combined effects of shocks, rarefactions, and
divergent flow cause magnetic fields to scale with density as an intermediate
power 1 > p > 0.5. Because p > 0.5, the Alfven speed in rarefactions decreases
on average as the jet propagates away from the star. This behavior is extremely
important to the flow dynamics because it means that a typical Alfven velocity
in the jet close to the star is significantly larger than it is in the
rarefactions ahead of bow shocks at larger distances, the one place where the
field is a measurable quantity. We find that the observed values of weak fields
at large distances are consistent with strong fields required to drive the
observed mass loss close to the star. For a typical stellar jet the crossover
point inside which velocity perturbations of 30 - 40 km/s no longer produce
shocks is ~ 300 AU from the source.
