  Anisotropic neutrino emission during the neutron star formation can be the
origin of the observed proper motions of pulsars. We derive a general
expression for the momentum asymmetry in terms of the neutrino energy flux
gradient, and show that a nonvanishing effect is induced at the lowest order by
a deformed neutrinosphere. In particular, this result is valid for a neutrino
flux transported through a spherical atmosphere with constant luminosity.
