Helicity detection of the astrophysical magnetic fields from radio emission statistics

We discuss inverse problem of detection turbulence magnetic field helical properties using radio survey observations statistics. In this paper, we present principal solution which connects magnetic helicity and correlation between Faraday rotation measure and polarization degree of radio synchrotron emission. The effect of depolarization plays the main role in this problem and allows to detect magnetic helicity for certain frequency range of observable radio emission. We show that the proposed method is mainly sensitive to a large-scale magnetic field component.

💡 Research Summary

The paper tackles the long‑standing inverse problem of measuring magnetic helicity in astrophysical turbulence using radio observations. Magnetic helicity, a measure of the twist and linkage of magnetic field lines, is a key diagnostic for large‑scale dynamo action, plasma topology, and the evolution of cosmic magnetic fields, yet it is notoriously difficult to access directly. The authors propose a statistical method that exploits the correlation between the Faraday rotation measure (RM) and the degree of linear polarization (p) of synchrotron emission. Their central insight is that internal Faraday depolarization, which depends on the square of the wavelength (λ⁴), creates a frequency window where helicity‑induced asymmetries in the RM‑p relationship become observable.

The theoretical framework begins with a three‑dimensional turbulent magnetic field model, separating the magnetic energy spectrum E(k) from the helicity spectrum H(k). The RM is expressed as the line‑of‑sight integral of the product of electron density and the parallel magnetic field component, yielding a zero mean but a finite variance σ_RM² that encodes the strength of magnetic fluctuations. The observed polarization degree is reduced by an exponential depolarization factor D(ν)=exp