Identification of magnetosonic modes in Galactic turbulence with synchrotron polarization.

The equipartition of magnetic and thermal energy in the interstellar medium (ISM) indicates the magneto-hydrodynamic nature of the interstellar turbulence, which can be decomposed into three wave modes: Alfv\'en, fast and slow magnetosonic modes\cite{Herlofson50,LG01,CL03}. Even for studies performed in the case of subsonic turbulence, the magnetosonic modes have been revealed to be more effective than the Alfv\'enic modes for processes such as cosmic ray (CR) transport and acceleration \cite{YL04,Lynn14}. The multiphase nature of ISM and diversity of driving mechanisms give rise to spatial variation of turbulence properties. Nevertheless, the employed model of magneto-hydrodynamic turbulence is often oversimplified being assumed to be only Alfv\'enic or even hydrodynamic due to a lack of observational evidence. Here we report the employment of our novel method, the signature from polarization analysis (SPA), on unveiling the plasma modes in interstellar turbulence. Its application leads to the first discovery of magnetosonic modes in the Cygnus X region. It is found that the magnetosonic modes overlap to a high degree with Fermi cocoon, consistent with theoretical expectations. Moreover, through comparison with the spectrum at other wavelengths, the plasma modes of turbulence are unveiled in active star formation zones. The SPA casts light on the plasma modes composition of the Galactic turbulence, and marks the onset of a new era in the study of interstellar turbulence and accordingly our understandings of relevant processes including cosmic ray transport and star formation.