(tractable model used in specific kinds of spectral line analysis)
The Sobolev approximation is a means of approximating the solution
to the radiative transfer equation under specific challenging conditions,
i.e., within gas that has a very high velocity gradient.
In astrophysics, it is used in the analysis of spectral lines, i.e.,
to model the environment that produces observed lines.
The Sobolev approximation assumes local variation in the
velocity gradient are negligible as compared to variations over
longer lengths. The Sobolev length is a calculated distance
below which gradations are presumed ignorable.