Astron. Astrophys. 358, 332-342 (2000)

An empirical test of different ionization balance calculations in an isothermal solar plasma

R. Allen ¹, E. Landi ^2,3, M. Landini ² and G.E. Bromage <sup>1

1</sup> Centre for Astrophysics, University of Central Lancashire, Preston, UK
² Department of Astronomy and Space Science, University of Florence, Italy
³ Max-Planck-Institute für Aeronomie, Katlenburg-Lindau, Germany

Received 16 November 1999 / Accepted 24 March 2000

Abstract

By examining solar observations using the Normal Incidence Spectrometer (NIS) within the Coronal Diagnostic Spectrometer (CDS) on board SOHO, an isothermal region in the lower solar corona was chosen for analysis by three different temperature diagnostic techniques. These techniques are the line-ratio method, the Differential Emission Measure and the Emission Measure analysis. All three methods should in theory yield the same temperature. Using these powerful diagnostic methods, the reliability of all widely used ionisation balance calculations, namely those of Shull & Van Steenberg (1982), Arnaud & Rothenflug (1985) (with revisions by Arnaud & Raymond 1992) and Mazzotta et al. (1998) have been empirically tested.

It has been found that the temperature obtained does not depend on the ionization balance calculation used. It is also concluded that the three ionization balance calculations give the same results, within their uncertainties.

A new variant Emission Measure analysis is presented. This provides a diagnostic tool that is stable against any distorting effects arising when a few lines have large deviations from the mean.

Key words: atomic data Sun: corona Sun: UV radiation

Send offprint requests to: r.allen@uclan.ac.uk

This article contains no SIMBAD objects.

Contents

• 1. Introduction
• 2. The observations
  • 2.1. Choosing an isothermal region
• 3. Wavelength calibration and line identification
  • 3.1. Presence of transition region lines
• 4. Theory of the intensity integral in an isothermal plasma
• 5. The differential emission measure (DEM )
• 6. Line ratio diagnostics applied to isothermal plasmas
  • 6.1. The line ratio diagnostic of temperature
  • 6.2. The line ratio diagnostic of density
• 7. An emission measure analysis for isothermal plasmas
  • 7.1. The emission measure analysis as a temperature diagnostic
• 8. Discussion
• 9. Conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 2000

Online publication: June 26, 2000
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