Constructing mean electron distributions from hard X-ray spectra of solar flares

Scullion, Eamon (2006) Constructing mean electron distributions from hard X-ray spectra of solar flares. MSc(R) thesis, University of Glasgow.

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Abstract

The Ramaty High Energy Solar Spectroscopic Imager (RHESSI) mission provides information on high resolution X-ray spectra emitted by collisional bremsstrahlung of thermal and non-thermal electrons with ions in solar flares. One of the aims of the mission is to infer information about the acceleration and transport mechanisms of hard X-ray emitting electrons and 7-ray emitting ions in solar flares. I investigate events observed during distinct attenuator states of the satellite, which have unique characteristics in their photon and mean electron distributions. Such characteristics include evidence of low energy cutoff features and other such physically real bump and dip features. In the framework of a colli-sionally thin bremsstrahlung model and an adjustable thermal function I forward fit these to the background-removed count flux data, using least squares minimization via OSPEX (Object Spectroscopy Executive). Along with Chi-Square Tests, I present random and non- systematic residuals to show goodness of fit. Conversion from count rate to count flux and then to photon flux spectra is a traditional approach to modelling hard X-ray spectra, but is significantly unreliable due to its dependence on parametric electron distribution approximations. Model independent hard X-ray spectra can be non-trivially calculated using the Detector Response Matrix (DRM) and are computed using a sequence of algorithms solving a linear system of equations, which present an inversion problem characterized by numerical instability due to the non-diagonal nature of the DRM. The extent of such instability is unique for different DRM configurations, hence different under each attenuator state. We then address the solution of this inversion problem- by using a regularization algorithm with the aim of inferring accurate and useful electron distribution spectra. Adjustment of this procedure to convert directly between counts and electrons, which is a one step rather than two step process, presents more robust and detailed information about features of electron dynamics during flares. Significant features such as low and high energy cutoffs tell us much about electron acceleration properties and energy losses in the flare evolution. Comparing regularized solutions in each event through different approaches will allow for confirmation of the existence or non-existence of such features. This innovative regularization technique is capable of portraying a truer interpretation of both hard X-ray and mean electron spectra.

Item Type: Thesis (MSc(R))
Qualification Level: Masters
Additional Information: Adviser: John C Brown
Keywords: Astronomy
Date of Award: 2006
Depositing User: Enlighten Team
Unique ID: glathesis:2006-71479
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 10 May 2019 14:34
Last Modified: 10 May 2019 14:34
URI: http://theses.gla.ac.uk/id/eprint/71479

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