  Since decoupling in the early universe in helicity states, primordial
neutrinos propagating in astrophysical magnetic fields precess and undergo
helicity changes. In view of the XENON1T experiment possibly finding a large
magnetic moment of solar neutrinos, we estimate the helicity flipping for relic
neutrinos in both cosmic and galactic magnetic fields. The flipping probability
is sensitive both to the neutrino magnetic moment and the structure of the
magnetic fields, thus potentially a probe of the fields. As we find, even a
magnetic moment well below that suggested by XENON1T could significantly affect
relic neutrino helicities and their detection rate via inverse tritium beta
decay.
