A theoretical lower limit to the magnetic field strength required for critical ionization velocity (vc) interaction is found to correspond to the requirementv<vA(1+&bgr;e)1/2, which can also be expressed as &ohgr;pe/&ohgr;ce<(c/v)[(1+&bgr;e) ×me/mi]1/2. This limit is found to agree, within the experimental uncertainties, with the results from all thevcexperiments in the impact configuration. The different experiments have very different physical characteristics, and cover large ranges in plasma densities, stream velocities, and magnetic field strengths. A theoretical upper limit to the magnetic field strength is also found, corresponding to the requirement &ohgr;pe/&ohgr;ce≫(me/mn)1/2.