The Diagnostic Potential of Transition Region Lines under-going Transient Ionization in Dynamic Events

J.G. Doyle (1), A. Giunta (2,3), A. Singh (1,4), M.S. Madjarska (1), H. Summers (2,3), B.J. Kellett (3) & M. O’Mullane (2,3) ((1) Armagh Observatory, College Hill, Armagh BT61 9DG, N. Ireland; (2)Department of Physics, University of Strathclyde, 107 Rottenrow, Glasgow, G4 0NG, Scotland; (3) Space Science and Technology Department, STFC Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, OX11 0QX, UK; (4) Dept.of Physics and Electronics, Deen Dayal Upadhyaya College, University of Delhi, India)

We discuss the diagnostic potential of high cadence ultraviolet spectral data when transient ionization is considered. For this we use high cadence UV spectra taken during the impulsive phase of a solar flares (observed with instruments on-board the Solar Maximum Mission) which showed excellent correspondence with hard X-ray pulses. The ionization fraction of the transition region ion O V and in particular the contribution function for the O V 1371Å line are computed within the Atomic Data and Analysis Structure, which is a collection of fundamental and derived atomic data and codes which manipulate them. Due to transient ionization, the O V 1371Å line is enhanced in the first fraction of a second with the peak in the line contribution function occurring initially at a higher electron temperature than in ionization equilibrium. The rise time and enhancement factor depend mostly on the electron density. The fractional increase in the O V 1371Å emissivity due to transient ionization can reach a factor of 2–4 and can explain the fast response in the line flux of transition regions ions during the impulsive phase of flares solely as a result of transient ionization. This technique can be used to diagnostic the electron temperature and density of solar flares observed with the forth-coming Interface Region Imaging Spectrograph.

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