Comments on the observability of coronal variations

The appropriateness of the presently available X-ray and XUV multilayer emission lines for studies of solar outer atmosphere and corona is discussed. The spectral regions within which solar emission lines are formed are compared with the available multilayer passbands, with particular attention given to lines which are of interest for studying the various temperature regimes and variability time constants known to be present in the solar atmosphere. It is emphasized that, for many of the typical regions found in the outer solar atmosphere, the observable time scales for variability are determined by the ionization and recombination time scales of the plasma rather than by the inherent nature of the physical process causing the variability.