Instability of vortex lines in the presence of axial normal fluid flow

We show that a normal mode analysis of the complete Hall two-fluid equations in rotating He II yields a critical velocity for axial normal fluid flow which is identical to the critical velocity derived in an earlier paper from the superfluid equation alone. We also present a simplified derivation of the critical velocity based on consideration of the forces acting on an isolated vortex line. The analysis is extended to³He-⁴He solutions and it is shown that rather small impurity concentrations can greatly increase the critical heat current in axial thermal counterflow.