Supersonic ambipolar diffusion: Estimating the magnetic field strength in protostellar outflows

The magnetic field plays a crucial role in star formation. It is involved in rotational braking, collapse braking, outflow formation and jet collimation. Direct observations of the field are difficult. However, the field can be indirectly estimated through the field-cushioned C-shocks which produce strong infrared molecular emission lines. In particular, a high field in the outflows will generate the 'shock absorber' signature: very broad H2 lines. Such lines are indeed observed. Here we summarise recent progress in C-shock formation and stability. We demonstrate numerically that the Shock Absorbers are evolutionary and stable. The widths of H2 lines then limit the magnetic field strength. A field of 6 mG is suggested for HH 212.