The spectrum of magnetized turbulence in the interstellar medium

Abstract

The interstellar medium (ISM) of our Galaxy is magnetized, compressible and turbulent, influencing many key ISM properties, such as star formation, cosmic-ray transport, and metal and phase mixing. Yet, basic statistics describing compressible, magnetized turbulence remain uncertain. Utilizing grid resolutions up to 10,080³ cells, we simulated highly compressible, magnetized ISM-style turbulence with a magnetic field maintained by a small-scale dynamo. We measured two coexisting kinetic energy cascades, \({{\mathcal{E}}}_{{\rm{kin}}}(k)\propto {k}^{-n}\), in the turbulence, separating the plasma into scales that are non-locally interacting, supersonic and weakly magnetized (n = 2.01 ± 0.03 ≈ 2) and locally interacting, subsonic and highly magnetized (n = 1.465 ± 0.002 ≈ 3/2), where k is the wavenumber. We show that the 3/2 spectrum can be explained with scale-dependent kinetic energy fluxes and velocity–magnetic field alignment. On the highly magnetized modes, the magnetic energy spectrum forms a local cascade (n = 1.798 ± 0.001 ≈ 9/5), deviating from any known ab initio theory. With a new generation of radio telescopes coming online, these results provide a means to directly test if the ISM in our Galaxy is maintained by the compressible turbulent motions from within it.