H2 excitation in turbulent interstellar clouds

We discuss the observational differences between lines of sight that intercept a group of turbulent dissipative structures and lines of sight that cross less-active regions. Using time-dependent calculations we show that the energy level distribution of the hydrogen molecule evolves in time in response to the local thermal phase. We find that relatively simple models can explain the observed properties of molecular hydrogen in diffuse interstellar clouds in terms of time evolution induced by collisional excitation in a low-density, high-temperature gas.