The value of density measurements in stellar coronae
  The grating instruments on board Chandra and XMM-Newton now allow measurements of electron densities. These rely on the ratios of fluxes in emission lines, where one line depends on both collisional and radiative decay rates. The electron density is required to constrain the physical extent of the emitting region, and large samples of measurements are of interest in the context of trends in coronal activity. Here we discuss the important He {\sc i}-like ions and the differences in densities that result when different current data bases are used. 
Jan-Uwe Ness, Carole Jordan (Rudolf Peierls Centre for Theoretical   Physics, University of Oxford)