Axion-photon conversion in stochastic magnetic fields

We investigate axion-photon conversion in stochastic magnetic fields, focusing on the evolution of the photon intensity and polarizations induced by conversion into axions. Assuming Gaussian magnetic fields characterized by the power spectra of their helical/nonhelical components, we express the expectation values and variances of the photon intensity and linear/circular polarizations after conversion in terms of these spectra. We find nontrivial dependencies of these statistical quantities on the characteristic magnetic-field correlation length, the propagation distance, and the axion mass. Moreover, we find that nontrivial polarizations emerge even if the photons are initially unpolarized, that the variances of these observables become suppressed in specific frequency regions, and that a peak structure arises in the expectation value of the circular polarization in the presence of statistically helical magnetic fields. We also point out consistency relations among these statistical quantities that hold independently of the specific forms of the magnetic-field power spectra.