ADAS analysis of the differential emission measure structure of the inner solar corona

Application of the data adaptive smoothing approach to the SERTS-89 active region spectrum

¹ Dipartimento di Fisica e Astronomia, Università di Catania, via S. Sofia 78, 95123 Catania, Italy
² Dept. of Physics and Applied Physics, University of Strathclyde, 107 Rottenrow, Glasgow G4 0NG, UK
³ Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, UK
⁴ Laboratory for Astronomy and Solar Physics, Code 680, NASA-Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁵ Schlumberger GeoQuest Simulation Sofware Development, 11 Foxcombe Court, Abingdon, OX14 1DZ, UK

Corresponding author: A. C. Lanzafame,acl@sunct.ct.astro.it

Received: 14 September 2001
Accepted: 16 November 2001

Abstract

The differential emission measure (DEM) of a solar active region is derived from SERTS-89 rocket data between 170 and 450 Å (Thomas & Neupert [CITE]). The integral inversion to infer the DEM distribution from spectral line intensities is performed by the data adaptive smoothing approach (Thompson [CITE], [CITE]). Our analysis takes into account the density dependence of both ionisation fractions and excitation coefficients according to the collisional-radiative theory as implemented in ADAS, the Atomic Data and Analysis Structure (McWhirter & Summers [CITE]; Summers [CITE]; Summers [CITE]). Our strategy aims at checking, using observational data, the validity and limitations of the DEM method used for analysing solar EUV spectra. We investigate what information it is possible to extract, within defined limitations, and how the method can assist in a number of cases, e.g. abundance determination, spectral line identification, intensity predictions, and validation of atomic cross-sections. Using the above data and theory, it is shown that a spurious multiple peak in the DEM distribution between and 6.7, where T_e is the electron temperature, may derive from an inaccurate treatment of the population densities of the excited levels and ionisation fractions or from using an integral inversion technique with arbitrary smoothing. Therefore, complex DEM structures, like those proposed for solar and stellar coronae by several authors, must be considered with caution. We address also the issue of systematic differences between iso-electronic sequences and show that these cannot be unambiguously detected in the coronal lines observed by SERTS. Our results indicate that a substantial improvement is required in the atomic modelling of the complex element Fe. The elemental abundance ratio Si/Ne is found to be close to its photospheric value. The same result may be true for the Fe/Ne abundance, but this latter result is uncertain because of the problems found with Fe.