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The properties of non-statistical equilibrium ionization of silicon andoxygen ions are analyzed in this work. We focus on four solar targets (quietsun, coronal hole, plage, quiescent active region, AR, and flaring AR) asobserved with the Interface Region Imaging Spectrograph (IRIS). IRIS is bestsuited for this work due to the high cadence (up to 0.5s), high spatialresolution (up to 0.32"), and high signal to noise ratios for O IV and Si IV.We find that the observed intensity ratio between lines of three times ionizedsilicon and oxygen ions depends on their total intensity and that thiscorrelation varies depending on the region observed (quiet sun, coronal holes,plage or active regions) and on the specific observational objects present(spicules, dynamic loops, jets, micro-flares or umbra). In order to interpretthe observations, we compare them with synthetic profiles taken from 2Dself-consistent radiative MHD simulations of the solar atmosphere, where thestatistical equilibrium or non-equilibrium treatment silicon and oxygen isapplied. These synthetic observations show vaguely similar correlations as inthe observations, i.e. between the intensity ratios and their intensities, butonly in the non-equilibrium case do we find that (some of) the observations canbe reproduced. We conclude that these lines are formed out of statisticalequilibrium. We use our time-dependent non-equilibrium ionization simulationsto describe the physical mechanisms behind these observed properties.

Time Dependent Non-Equilibrium Ionization of Transition Region Lines Observed with IRIS