Line-of-sight statistical methods for turbulent medium: VCS for emission
  and absorption lines
  We present an overview of the Velocity Coordinate Spectrum (VCS), a new
technique for studying astrophysical turbulence that utilizes the line-of-sight
statistics of Doppler-broadened spectral lines. We consider the retrieval of
turbulence spectra from emission intensity observations of both high and low
spatial resolution and find that the VCS allows one to study turbulence even
when the emitting turbulent volume is not spatially resolved. This opens
interesting prospects for using the technique for extragalactic research. VCS
developed for spectral emission lines is applicable to absorption lines as well
if the optical depth is used instead of intensity. VCS for absorption lines in
point-source spectra benefit from effectively narrow beam and does not require
dense sky coverage by sampling directions. Even strongly saturated absorption
lines still carry the information about the small scale turbulence, albeit
limited to the wings of a line. Combining different absorption lines one can
develop tomography of the turbulence in the interstellar gas in all its
complexity.
