The structure of the cold diffuse interstellar medium

Abstract

The cold diffuse interstellar medium (ISM) is turbulent withsupersonic, most likely transAlfvénic motions. All its tracers,including velocity, are structured down to the smallest scalesaccessible to observations. This structure, though, is not random andcan be quantified using the statistics of its Fourier phaseincrements. The quantity of structure of interstellar clouds is largerthan that of fractal Brownian motion (fBm) fields and of mildlycompressible turbulence. We propose that this structure is a responseof the medium to a general property of turbulence, the space-timeintermittency of its dissipation. The bursts of turbulentdissipation, because they temporarily (over a few hundred years) andlocally heat the diffuse medium up to ~10³ K, irreversiblymodify the subsequent thermal and chemical evolution of the gas.Preliminary models of such bursts predict chemical abundances andexcitation of a subset of molecules that are in agreement with theobservations. Timescales of the velocity fluctuations are shortenough to locally decouple the neutrals from the ionized species inthe diffuse medium and generate helical magnetic fields. Small scalefluctuations of the polarization properties of the diffuse ISM aretherefore expected.