Evidence of high-frequency/small-scale turbulence in the Cygnus region and anomalous Faraday rotation

Faraday effect --- a common and useful probe of cosmic magnetic fields --- is the result of magnetically-induced birefringence in plasmas causing rotation of the polarization plane of a linearly polarized electromagnetic wave. Classically, the rotation angle scales with the wavelength as $\Delta\phi=\text{RM}\, \lambda^2$, where $\text{RM}$ is the rotation measure. Although a typical $\text{RM}$ in the Milky Way is of the order of a few hundred to a few thousand, a famous Cygnus region shows anomalously small, even negative rotation measures. Moreover, Faraday rotation measurements seem to be inconsistent with the standard $\lambda^2$-law. We argue that fast micro-turbulence can cause this anomaly. We demonstrate that electromagnetic high-frequency and/or small-scale fluctuations can lead to effective plasma collisionality by scattering electrons over pitch-angle. We show that such quasi-collisionality radically alters Faraday rotation and other radiative transport properties, e.g., absorption, transmission and reflection. Thus, we explain the Cygnus puzzle by anomalous Faraday rotation in a thin ``blanket" of highly turbulent plasma at the front of an interstellar bubble/shock.