... motions

Here and in the following we use the term turbulence in the spectroscopic sense (which is wider than in hydrodynamics) referring to any stochastic motion leading to line broadening. The terms micro-, meso-, and macroturbulence refer to the way in which the turbulent motions affect the line broadening which depends on the ratio of the mean free path of a photon to the correlation length of the velocity field.

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