  We calculate the Cherenkov process nu -> nu+photon in the presence of a
homogeneous magnetic field. The neutrinos are taken to be massless with only
standard-model couplings. The magnetic field fulfills the dual purpose of
inducing an effective neutrino-photon vertex and of modifying the photon
dispersion relation such that the Cherenkov condition is fulfilled. Our effect
is closely related to photon splitting that occurs in magnetic fields and that
may be astrophysically important in the strong magnetic fields of pulsars. It
is also closely related to magnetic-field enhanced radiative decays nu ->
nu'+photon that have been extensively discussed in the recent literature. In
the appropriate limits we agree with these results, but we disagree with
earlier explicit calculations of the Cherenkov process. For a field strength
B_crit = m_e^2/e = 4.41E13 Gauss and for E=2m_e the Cherenkov rate is about
6E-11/sec and thus too small to be of practical importance for pulsar physics.
