Diagnosing transient ionization in dynamic events

Abstract

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 Å and O iv 1401 Å, 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 Å 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 Å 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.

Nonequilibrium Processes in the Solar Corona, Transition Region, Flares, and Solar Wind (Invited Review)

“…Doyle et al (2012) and Doyle et al (2013) applied the GCR technique to predict the response of TR lines to short heating bursts occurring at loop footpoints. In Doyle et al (2013), the Si iv 1393.76Å and O iv 1401.16Å lines were studied.…”

“…Doyle et al (2012) and Doyle et al (2013) applied the GCR technique to predict the response of TR lines to short heating bursts occurring at loop footpoints. In Doyle et al (2013), the Si iv 1393.76Å and O iv 1401.16Å lines were studied. It was found that about 0.2 s after the occurrence of the burst, Si iv responds with an intensity increase by a factor of more than three, while the intensity of O iv is increased only weakly.…”

“…A plasma may only be out of ionization equilibrium for tens of seconds in some cases (or a few seconds in the case of Doyle et al, 2013), while the emission measure of the transient plasma may be very low (Section 5.4). Simultaneous coverage of consecutive ion stages of an element would allow transient ionization to be tracked through the atmosphere, while high sensitivity observations of strong lines are needed to capture the evolution at cadences of order seconds.…”

Differential emission measure analysis of active region cores and quiet Sun for the non-Maxwellianκ-distributions

“…Therefore, we believe that the presence of plasmas undergoing transient ionization (cf. Doyle et al 2013) can be neglected.…”

Signatures of the non-Maxwellianκ-distributions in optically thin line spectra

“…The ionization equilibrium is, however, an additional source of uncertainty because of the individual ionization and recombination rates, as well as possible departures from the ionization equilibrium (e.g. Bradshaw et al 2004;Bradshaw 2009;de Avillez & Breitschwerdt 2012;Reale et al 2012;Doyle et al 2012Doyle et al , 2013Olluri et al 2013) at low electron densities. The results presented here highlight the need for spectroscopic observations over a wide wavelength range, especially in the 170 Å − 370 Å range, which include many strongly temperature-sensitive line pairs, such as the Fe ix 244.909 Å/171.073 Å or 244.909 Å/188.497 Å, or analogous combinations from other ions investigated here.…”