Spectral diagnostics with the SDO EVE flare lines^⋆

¹ DAMTP, Centre for Mathematical Sciences, Wilberforce road, Cambridge CB3 0WA, UK
e-mail: g.del-zanna@damtp.cam.ac.uk
² Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado 20305, USA

Received: 21 December 2012
Accepted: 5 April 2013

Abstract

The diagnostic use of the soft X-ray and EUV lines observed with the Solar Dynamics Observatory (SDO) Extreme ultraviolet Variability Experiment (EVE) is discussed. We focus on all the flare lines observed in the 80–640 Å range (mainly due to Fe xviii − Fe xxiv), showing their use to measure temperatures, emission measures, densities, and chemical abundances. We discuss their identification at the EVE resolution, by using the latest atomic data, and by assessing possible sources of blending, taking into account higher-resolution solar spectra. We present observations of four flares, and study in more detail the gradual phase peak of the 7 March 2012 X5.6 flare. Good agreement between observations and theory is found in most cases, and the best lines for diagnostics are recommended. We found reasonable densities (10^11.2 cm^-3 from Fe xxi lines), but isothermal temperatures (12 MK) lower than those estimated with GOES. We show that EVE can be used to measure relative elemental abundance, and find photospheric argon/iron and calcium/iron abundances. We also show that lines normally formed in the quiet Sun in the low transition region such as O III are the best to study the impulsive phase.