Extreme Ultraviolet Spectroscopy
of the Solar Corona

Giulio Del Zanna

A thesis submitted in
partial fulfilment
of the requirements
for the degree of
Doctor of Philosophy.


Centre for Astrophysics
University of Central Lancashire


July 1999

A thesis dedicated to the memory of Brunella Monsignori Fossi.





Abstract

New Extreme Ultra-Violet (EUV) observations of the solar corona, obtained by the Coronal Diagnostic Spectrometer (CDS) instrument on board the NASA/ESA Solar and Heliospheric Observatory (SOHO) are presented. The CDS instrument for the first time has provided the opportunity of observing a large number of emission lines from a wide range of ions of different elements. The spectral and spatial resolution of the CDS instrument has allowed the simultaneous application of a wide range of spectroscopic diagnostic techniques to determine the temperature distribution, densities and elemental abundances in the solar plasma. A differential emission measure (DEM) diagnostic technique has been used to infer the temperature structure and the element abundances. The importance of including DEM effects in the element abundance analysis is demonstrated. A complete in-flight cross-calibration between all the CDS detectors (NIS and GIS, 150-785 Å), is presented here for the first time. The level of accuracy and completeness of the CHIANTI atomic database, used throughout this thesis, has allowed the identification of the many hundreds of spectral lines observed in the CDS spectra, resulting in several new line identifications. An assessment of the GIS solar spectra is presented, showing that, in spite of complexities, GIS spectra are useful for diagnostic analyses. Many discrepancies (in particular with the Li-like ions) between theory and CDS observations are highlighted. It is shown that some of these may be explained by inaccurate ionization equilibrium calculations. Coronal hole densities, temperatures, DEMs, and relative element abundances from both off-limb (plume and inter-plume regions) and on-disc observations are derived and compared with quiet sun values. These included (August 1996) a large equatorial hole, the Elephant's Trunk, probably the best example of a coronal hole observed by SOHO. Coronal hole densities in both coronal (Ne @ 1 x108 cm-3) and transition region (Ne @ 1 x1010 cm-3) plasma were found to be about a factor of 2 lower than in the quiet sun, with much lower emission measures at temperatures above 106 K. Moreover, in the transition region, the cell-centres in both coronal holes and quiet sun regions show consistently higher densities (factor of 2) compared to those in the network. Relative element abundances show approximately photospheric values for the coronal holes, with an indication of element abundance variation (neon in particular) with the supergranular structure. On-disc EUV observations of coronal hole plumes have been performed. This allowed a spectroscopic characterisation of plumes to be obtained for the first time, leading to the first identification of a low-latitude plume near sun-centre. Plumes are shown to be quasi-isothermal structures, with temperatures T @ 7-8 x105 K. Abundance analyses (using transition region lines) reveal a small FIP effect in these plumes, together with a decreased Ne/O abundance ratio (compared to photospheric).




Acknowledgments

I wish to thank all the many people and institutions that with their support helped me during the course of my PhD. These include, with no particular order:

my first and second supervisors, Barbara and Gordon Bromage for their support and guidance throughout this period.

The University of Central Lancashire for a three-year full-time research studentship, which supported my work and living expenses during that time. In particular, the Centre for Astrophysics for additional financial support for some of my trips.

All the local organising committees and agencies (NATO in particular) that funded my many trips to summer schools, meetings, and congresses.

Deianira. Without her help during the last year, I would have not been able to finish on time.

Barbara Hassall and Ken Phillips for having accepted the task of examining me, and for reading this rather long thesis.

Massimo Landini for his precious collaboration and support from the very beginning.

Enrico Landi, a good collaborator and companion during the many conferences I went to.

Helen Mason, Ken Dere and Peter Young for stimulating discussions and collaborations.

Andy Adamson and Pete Newman, for their continuous support as system managers, during a difficult period including office and building moves and disruptions. [Much of the data reduction, processing and analysis detailed in this thesis has been done with the Interactive Data Language (IDL) and using STARLINK facilities].

The whole of the CDS operation staff at RAL (UK) and GSFC (USA) for their assistance and hospitality during visits there. In particular: Richard Harrison and Dave Pike for their assistance in the planning and execution of the observations; Bob Bentley, without whom most of the Elephant's Trunk coronal hole observations would have not been taken place; Andrzej Fludra, for the stimulating collaboration in working with CDS and in producing results. William Thompson, Eddie Breeveld for their technical support.

Stein Vidar Haugan, for his much appreciated support regarding CDS data analysis and IDL programming.

Sarah Gibson and Douglas Biesecker for the collaboration for the Whole Sun Month Campaign, and support for the attendance of the related workshops.

Barbara Thompson and the EIT consortium for providing the EIT images.

Marta, for her continuous moral support and great long-distance long-lasting friendship.

Vincenzo, for his excellent hospitality and friendship.

Susanna, for an excellent friendship and for cheering me up when I needed.

All the many colleagues and friends, with whom I shared an enjoyable social life in these years. These include (it is virtually impossible to mention all of them): Christophe, Fiona, Eleonora, Andy, Alessandro, Agostino, Vinicio, Katerina, Marianna, Dalila, Nathalie, Turandot, Julia, John, Francesca, Bill, Gabriel, Cormac, Carl, Guillaume, Till, Emmanuel, Carina, Chris, Mark, Pete, Laurence, Lucia, Barbara, Alice, Mikko, Pedro, Maria (2).

I would also like to thank Janey. With her, I learned much, including english.

Finally, I thank my father and all my family, for their help and support when I needed it.

This thesis is dedicated to the memory of Brunella Monsignori Fossi.
She suddenly died the morning of monday 22nd January 1996, in front of her terminal. She spent long part of her active life studying the coronae of cool stars and the sun, and in the last years in preparation for SOHO. I had the opportunity, working closely with her, in Arcetri, to appreciate her qualities outside research.
She would have very much enjoyed studying the solar corona with SOHO and CDS, as I did.

In the following, you can find links to the (g-zipped) postscript files of each Chapter. Note that: