Fr.: longueur de Sobolev
In the → Sobolev approximation, the length over which the conditions of the gas do not change and the approximation is valid. It is expressed by: ls = vth/(dv/dr), where vth is the thermal line width and (dv/dr) the velocity gradient. In other words, the length over which the profile function of a line is shifted through a distance equal to its own width by the macroscopic velocity gradients that exist in the moving medium.