The stability, during formation, of magnetohydrodynamic jets collimated by an azimuthal magnetic field

The stability, during formation, of jets is explored using a three-dimensional, nonrelativistic, ideal magnetohydrodynamic (MHD) code. An idealized calculation is performed to represent the generic problem of an azimuthal magnetic field pinching a plasma wind from an accretion disc. In particular, the stability of the jets to three-dimensional instabilities (the m = 1, helical, instability) is examined. It is found that the pinch effect of an azimuthal magnetic field readily generates a jet. At later times shear, arising from the flow of the jet as it reaches high speeds, appears to cause a reduction in the growth rates. At earlier times, during the jet formation, the jet is found to be unstable to the m = 1 instability, which disrupts the jet and also causes large opening angles, resulting in very poor time-integrated collimation. Thus it is shown that jets which are magnetically confined by an azimuthal field during the acceleration process are probably strongly unstable, at least in our idealized case.