Comments on the observability of coronal variations

We discuss the observable variability of spectral lines in the soft X-ray and XUV region. Rapid variability of coronal emission, both in flaring and non-flaring structures has been reported and is particularly prominent when high spatial resolution is available. Examination of the ionization and recombination time-scales for the formation and removal of ions with prominent solar emission lines shows that, even though ionization equilibrium generally prevails, the observable variability time-scales are often limited by these atomic processes, independent of the physical process which is causing the change in the solar atmosphere. Rapid heating can lead to an initial freezing-in of abundances of some ions; observations of at least one low- and one high-excitation line from such an ion would permit studies of the time evolution of the emission measure and temperature. In a very limited number of cases, rapid cooling leads to freezing-in of the abundance of an ion and observations of a low-excitation line of this ion will not yield accurate information about the thermal evolution. Thus, future observations of Mgx 609 Å should be augmented by simultaneous observation at another wavelength, such as 63 Å. In addition, with the ability to produce images in isolated spectral lines it becomes possible to select those for which rapid variability is observable, such asOvii, rather than lines which were selected on the basis of previous hardware constraints, such asOvii.