Astrophysical Turbulence

Coordinators: Eve C. Ostriker, Ellen G. Zweibel

It is difficult to exaggerate the importance of turbulence in astrophysics, or the challenges which it poses. Turbulence is responsible for dynamical pressure support, energy transport, angular momentum transport, chemical mixing, and magnetic field generation and evolution in a host of astrophysical settings. Turbulent astrophysical flows differ from terrestrial forms of turbulence which have been studied traditionally by virtue of their inherent compressibility, strong radiative cooling, self-gravity, and the importance in many environments of magnetic fields.

Recent years have seen important advances in several distinct areas of astrophysical turbulence theory -- including modeling of turbulence in stars, accretion disks, and the interstellar medium, as well basic studies of MHD turbulence which provide the framework for all these applications. In one star, the Sun, helioseismologic data are allowing increasingly sophisticated comparison of observations with the theory of turbulent stellar interiors. Attempts to model the solar differential rotation has shown clearly that turbulent angular momentum transport is an essential ingredient, and attempts to model it are improving. Solar dynamo calculations are only slightly behind. Models of turbulent accretion disks are becoming increasingly realistic, with the dynamical role of magnetic fields a crucial element, and global, time-dependent modeling now feasible. It seems likely that the alpha\'\' viscosity parameter will be soon be supplanted by ab initio calculations of the accretion rate. Although the presence of interstellar turbulence has long been recognized, recent theoretical studies have significantly increased our understanding of its effects, particularly in the cold ISM where it plays a dominant role. Self-consistent dynamical studies will soon be able to identify how strong turbulence evolves and shapes the internal structure of magnetized interstellar clouds. Basic studies in MHD turbulence have made substantial recent progress in such longstanding problems as the nature of the turbulent cascade, dynamo generation of fields, and the process of magnetic reconnection.

In all of these studies, a crucial new ingredient has been computational advances that now make possible direct hydrodynamic/MHD simulations of three-dimensional, time-dependent turbulence with inertial dynamic ranges of more than two orders of magnitude. These advances in numerical experimentation are inspiring new analytical work, new comparisons between models and observations, and advances in observations and data analysis themselves. The ITP program on Astrophysical Turbulence will provide a forum for intensive interaction among analytical theorists, computational physicists, and observers from all of the subspecialties, with prospects for major research progress through interdisciplinary discussions and collaborations.