  A massless Dirac particle is considered, moving along the x-axis while
Pauli-coupled by its anomalous magnetic moment to a piecewise constant magnetic
field along the same axis, with stochastically varying sign. The motion is
approximated as a quantum walk with unitary noise, for which the evolution can
be found exactly. Initially ballistic, the motion approaches a classical
diffusion on a time-scale determined by the speed of light, the size of the
magnetic moment, the strength of the field and the time interval between
changes in its direction. It is suggested that a process of this type could
occur in the Sun's corona, significantly affecting the solar fluxes of one or
more neutrino types.
