Non-stellar sources of turbulence in the interstellar medium

Turbulence is believed to be important to star formation both within GMCs (affecting the IMF and the SFE), and on larger scales in the ISM (affecting GMC formation rates and properties). The traditional view of the ISM attributes most of the turbulent driving to stellar sources - in particular, supernovae and HII regions. However, evidence suggests that sources other than star formation must contribute significantly to large-scale turbulent driving in the diffuse ISM, thus also affecting the turbulence that GMCs inherit. I review recent investigations of alternative sources proposed for driving ISM turbulence, including thermal instabilities, the magnetorotational instability, large-scale self-gravitating modes, and instabilities in spiral shocks. I summarize results based on numerical simulations regarding the levels of turbulence that can be driven, and how these amplitudes depend on galactic conditions. This recent work shows that, under certain circumstances, very large-amplitude (trans-sonic with respect to the warm gas) motions and magnetic fields can be driven even without stellar energy inputs. Since turbulence can either trigger or limit star formation, understanding these potentially large non-stellar driving sources is crucial for a developing a theory of star formation regulation in the Milky Way and other disk galaxies.