  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. 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. 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. 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.
