J.G. Doyle, A. Giunta, M.S. Madjarska, H. Summers, M. O’Mullane and A. Singh

Diagnosing Transient Ionization in Dynamic Events

Fig. 1. Line contribution function for Si IV 1394 Å and O IV 1401 Å in cm−3 s−1 at ionization equilibrium and for transient ionization. The electron density was fixed at 1010 cm−3, with an initial starting temperature of 30 000K.

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 an 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.

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Last Revised: 2013 July 29th