Opacity in the upper atmospheres of active stars II. AD Leonis
  We present FUV and UV spectroscopic observations of AD Leonis, with the aim of investigating opacity effects in the transition regions of late-type stars. The C III lines in FUSE spectra show significant opacity during both the quiescent and flaring states of AD Leonis, with up to 30% of the expected flux being lost during the latter. Other FUSE emission lines tested for opacity include those of O VI, while C IV, Si IV and N V transitions observed with STIS are also investigated. These lines only reveal modest amounts of opacity with losses during flaring of up to 20%. Optical depths have been calculated for homogeneous and inhomogeneous geometries, giving path lengths of ~20-60 km and \~10-30 km, respectively, under quiescent conditions. However path lengths derived during flaring are ~2-3 times larger. These values are in excellent agreement with both estimates of the small-scale structure observed in the solar transition region, and path lengths derived previously for several other active late-type stars. 
D.J. Christian, M. Mathioudakis, D.S. Bloomfield, J. Dupuis, F.P.   Keenan, D.L. Pollacco, R.F. Malina