## EUV spectroscopy of cool stars## III. Interpretation of EUVE spectra in terms of quasi-static loops
^{1} Sterrekundig Instituut Utrecht, Postbus 80 000,
3508 TA Utrecht, The Netherlands^{2} Stanford-Lockheed Institute for Space Research, Dept.
H1-12, Bldg. 252, 3251 Hanover Street, Palo Alto, CA 94304, USA^{3} SRON Laboratory for Space Research, Sorbonnelaan 2,
3584 CA Utrecht, The Netherlands
We discuss the limitations of coronal spectroscopy to derive physical parameters of stellar magnetic loops. We distinguish between the intrinsic non-uniqueness of emitted spectra for models of quasi-static coronal loops, and the supplemental ambiguity introduced by both instrumental effects and spectral line formation. We demonstrate that the spectrum emitted by loops with constant cross-sections is the same for a large range of values of the conductive flux at the base when the apex temperature is fixed. Because it is impossible to estimate the conductive flux at the base from observations, it is also impossible to determine the volume heating rate and the loop length uniquely. For geometrically expanding (tapered) loops, the emitted spectrum depends on the expansion and on the conductive flux at the base, and there is a trade off between them without significant changes in the spectrum. We show that loop length and heating rate can only be derived if the density is known, but that even then a large intrinsic uncertainty remains for these loop parameters. We conclude that there is no unambiguous relationship between loop parameters and emitted spectra: modeling the spectra as the sum of spectra from discrete loops cannot result in a unique determination of coronal structure. Based on spectra observed with the Extreme Ultra Violet
Explorer (
## Contents- 1. Introduction
- 2. Spectral models and fitting techniques
- 3.
*EUVE*observations - 4. Quasi-static loop models and their DEMs
- 5. Application of loop models to
*EUVE*DEMs - 6. Discussion and conclusions
- Acknowledgements
- References
© European Southern Observatory (ESO) 1997 Online publication: April 8, 1998 |