  Magneto-hydrodynamic (MHD) turbulence is ubiquitous and a fundamental
ingredient underlying many astrophysical phenomena. The multiphase nature of
interstellar medium and diversity of driving mechanisms give rise to spatial
variation of turbulence properties, particularly their plasma properties. There
has been no observational diagnosis of the plasma modes so far beyond the solar
system. Here we report the identification of different plasma modes in various
Galactic environments, including active star forming zones and supernova
remnants, based on our synchrotron polarization analysis. The observed high
degree of consistency between the $\gamma-$ray excess in Cygnus cocoon and the
location of magnetosonic modes provides strong observational evidence for the
long-advocated theory that magnetosonic modes dominate the cosmic ray (CR)
scattering and acceleration. Our results open up a new avenue for the study of
interstellar turbulence and demonstrate the indispensability to account for
their plasma property in all the relevant processes including CR transport and
star formation.
