Primordial neutrinos decoupled in the early universe predominantly in helicity eigenstates. Their subsequent propagation through the residual cosmic and galactic magnetic fields partially flips their helicities. In view of the possibility of large neutrino magnetic moments arising from beyond-the-standard-model physics -- e.g., as the XENON1T experiment reported as a possible explanation of their low energy event excess -- we estimate the magnitudes of the helicity flips for relic Dirac neutrinos both in galactic and cosmic magnetic fields. Majorana neutrinos would not undergo such flips. The helicity flip probability is sensitive not only to the magnetic moment of neutrinos but also to the properties of galactic and cosmic fields, and thus can thus potentially probe astrophysical magnetic fields. We find that even a moment several orders of magnitude smaller than that possibly found by XENON1T could lead to significant helicity changes of Dirac neutrinos as they propagate to detectors on Earth. We also discuss the effects of gravitational inhomogeneities on the helicity of primordial neutrinos.
 G. Baym and J. C. Peng, Phys. Rev. Lett. 126, 191803 (2021).
 G. Baym and J. C. Peng, Phys. Rev. D 103, 123019 (2021).