Extreme ultraviolet spectroscopy of impulsive phase solar flare footpoints

QB Astronomy;QC Physics

Graham, David Robert ; Fletcher, Lyndsay

University:University of Glasgow

Department:School of Physics and Astronomy

关键词: Sun, solar, flare, x-ray, extreme-ultraviolet, observation, chromosphere, transition region;   

Others  :  http://theses.gla.ac.uk/5017/1/2013grahamphd.pdf

来源: University of Glasgow

【 摘 要 】

This thesis is primarily concerned with the atmospheric structure of footpoints during the impulsive phase of a solar flare. Through spectroscopic diagnostics in Extreme-Ultraviolet wavelengths we have made significant progress in understanding the depth of flare heating within the atmosphere, and the energy transport processes within thefootpoint.Chapter 1. introduces the Sun and its outer atmosphere, forming the necessary background to understand the mechanisms behind a solar flare and their observationalcharacteristics. The standard flare model is presented which explains the energy source behind a flare, through to the creation of the EUV and X-ray emission. In Chapter 2 the basics of atomic emission line spectroscopy are introduced, covering the processes driving electron excitation and de-excitation, the formation of Gaussian line profiles, and the formation of density sensitive line ratios. The concept of a differ-ential emission measure is also derived from first principles, followed by a description of all of the instruments used throughout this thesis.Chapter 3 presents measurements of electron density enhancements in solar flare footpoints using diagnostics from Hinode/EIS. Using RHESSI imaging and spectroscopy,the density enhancements are found at the location of hard X-ray footpoints and are interpreted as the heating of layers of increasing depth in the chromosphere to coronaltemperatures.Chapter 4 shows the first footpoint emission measure distributions (EMD) obtained from Hinode/EIS data. A regularised inversion method was used to obtain the EMDfrom emission line intensities. The gradient of the EMDs were found to be compatible with a model where the flare energy input is deposited in an upper layer of the flarechromosphere. This top layer then cools by a conductive flux to the denser plasma below which then radiates to balance the conductive input. The EUV footpoints arefound to be not heated directly by the injected flare energy.In Chapter 5 electron densities of over 1013 cm−3 were found using a diagnostic at transition region temperatures. It was shown to be difficult to heat plasma at thesedepths with a thick-target flare model and several suggestions are made to explain this; including optical depth effects, non-ionisation equilibrium, and model inaccuracies.Finally, Chapter 6 gathered together both the density diagnostic and EMD results to attempt to forward fit model atmospheres to observations using a Genetic Algorithm.The results are preliminary, but progress has been made to obtain information about the T (z) and n(z) profiles of the atmosphere via observation.