An investigation of Fe XV emission lines in solar flare spectra

¹ Department of Physics and Astronomy, Queen's University Belfast, Belfast BT7 1NN, Northern Ireland, UK e-mail: F.Keenan@qub.ac.uk
² Center for Theoretical Studies of Physical Systems, Clark Atlanta University, Atlanta, GA 30304, USA
³ Computational Physics, Inc., 8401 Braddock Road, Springfield, VA 22151, USA and Naval Research Laboratory, Washington, DC 20375-5320, USA

Received: 10 August 2005
Accepted: 16 December 2005

Abstract

Previously, large discrepancies have been found between theory and observation for  emission line ratios in solar flare spectra covering the 224-327 Å  wavelength range, obtained by the Naval Research Laboratory's S082A instrument on board Skylab. These discrepancies have been attributed to either errors in the adopted atomic data or the presence of additional atomic processes not included in the modelling, such as fluorescence. However our analysis of these plus other S082A flare observations (the latter containing  transitions between 321-482 Å), performed using the most recent  atomic physics calculations in conjunction with a chianti synthetic flare spectrum, indicate that blending of the lines is primarily responsible for the discrepancies. As a result, most  lines cannot be employed as electron density diagnostics for solar flares, at least at the spectral resolution of S082A and similar instruments (i.e. ~0.1 Å). An exception is the intensity ratio I(3s3p ³P₂–3p^{2 3}P₁)/I(3s3p ³P₂–3p^{2 1}D(321.8 Å)/I(327.0 Å), which appears to provide good estimates of the electron density at this spectral resolution.