Diagnosing transient ionization in dynamic events

Aims. The present study aims to provide a diagnostic line ratio that will enable the observer to determine whether a plasma is in a state of transient ionization. Methods. We use the Atomic Data and Analysis Structure (ADAS) to calculate line contribution functions for two lines, Si iv 1394 A and O iv 1401 A, formed in the solar transition region. The generalized collisional-radiative theory is used. It includes all radiative and electron collisional processes, except for photon-induced processes. State-resolved direct ionization and recombination to and from the next ionization stage are also taken into account. Results. For dynamic bursts with a decay time of a few seconds, the Si iv 1394 A line can be enhanced by a factor of 2–4 in the first fraction of a second with the peak in the line contribution function occurring initially at a higher electron temperature due to transient ionization compared to ionization equilibrium conditions. On the other hand, the O iv 1401 A does not show such any enhancement. Thus the ratio of these two lines, which can be observed with the Interface Region Imaging Spectrograph, can be used as a diagnostic of transient ionization. Conclusions. We show that simultaneous high-cadence observations of two lines formed in the solar transition region may be used as a direct diagnostic of whether the observed plasma is in transient ionization. The ratio of these two lines can change by a factor of four in a few seconds owing to transient ionization alone.