Abstract
We show that it is possible to measure Alfvénic Mach numbers, defined as
the shock velocity in the flow divided by the Alfvén velocity, for
low-velocity (V\$\_shock\$ \$łesssim\$ 100 km\$\,\$s\$^-1\$) radiative shocks. The
method combines observations of bright forbidden lines with a measure of the
size of the cooling zone, the latter typically obtained from spatial separation
between the Balmer emission lines and the forbidden lines. Because magnetic
fields become compressed as gas in the postshock region cools, even relatively
weak preshock magnetic fields can be detected with this method. We derive
analytical formulae that explain how the spatial separations relate to
emission-line ratios, and compute a large grid of radiatively-cooled shock
models to develop diagnostic diagrams that can be used to derive Alfvénic
Mach numbers in flows. Applying the method to existing data for a bright knot
in the HH 111 jet, we obtain a relatively low Alfvénic Mach number of \$\sim\$
2, indicative of a magnetized jet that has super-magnetosonic velocity
perturbations within it.
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