Turbulence in the Diffuse Interstellar Medium

Abstract

Abstract. The diffuse interstellar medium (ISM) hosts the first steps of interstellar chemistry and the seeds of dense structures. Since its turbulent pressure by far exceeds its thermal pressure, turbulence must play a prominent role in its evolution. Fed at galactic scales, turbulent energy cascades down to the dissipation scales, but as in both laboratory and atmospheric turbulence, it does so in an intermittent way : only a tiny fraction of the small-scales is fed by the turbulent cascade, so that dissipation occurs in bursts. In diffuse molecular clouds, where they can be observed, the signatures of intermittency are: (1) the non-Gaussian statistics of velocity increments, and (2) the existence of coherent structures of intense velocity-shear that appear to channel the large-scale turbulent energy down to milliparsec scales. Attempts at modelling the warm chemistry triggered in the diffuse ISM by bursts of turbulent dissipation are promising : in this framework, the so far unexplained molecular richness observed in this medium is naturally understood, in particular its CH + , HCO + and CO abundances. Turbulent dissipation is also likely at the origin of the H2 rotational line emission of the diffuse ISM and of a significant fraction of its [C ii] emission.

A parameter study of strato-rotational low-frequency modulations: impacts on momentum transfer and energy distribution

“…The modulations observed in the momentum transport may have an impact on accretion disc theory taking into account that constant or intermittent regimes might lead to different conditions of how matter collapses in the centre of the discs. Note that signatures of an intermittent transport was already measured in astrophysical contexts, such as in the measurements of molecular clouds in the north Celestial pole loop performed by Falgarone et al [48], where the turbulent energy dissipation was reported to occur in bursts [50]. These systems guard some similarities with the SRI geometry, such a cylindrical gap morphology, and energy spectra of dust clouds surrounding young stars with decay rates of k−11/3 (the same decay rate of Eϕ obtained in the numerical simulations presented in §3).…”

“…This decay has already been associated with different phenomena, such as the influence of potential energy available in turbulent diffusion of quasi-geostrophic cascades [46], or with Kelvin waves propagating along a vortex line [47]. Curiously, in an astrophysics context, m−11/3 power law was measured over a broad length-scale domain in energy spectra of diffuse molecular clouds surrounding young stars [48], as well as proposed in an MHD context [49]. These astrophysical observations were performed in the North Celestial Pole loop, which has a cylindrical gap morphology, with a cylindrical radial velocity profile fitting its radial expansion [50].…”