Numerical simulations of radiation and heating from non-thermal electrons in solar flares

QB Astronomy;QC Physics

Pollock, Jennifer A. ; Fletcher, Lyndsay

University:University of Glasgow

Department:School of Physics and Astronomy

关键词: solar physics, hard x-rays, non-thermal electrons, numerical simulations, magnetic loops, solar flares;   

Others  :  http://theses.gla.ac.uk/331/1/2008pollockphd.pdf

来源: University of Glasgow

【 摘 要 】

This thesis investigates heating and thermal and non-thermal X-ray emission from magnetic loops in active regions of the solar atmosphere using numerical simulations.The simulations also allow investigation of Type III radio emission.In our model we vary a number of physical parameters such as magnetic field configuration and density models, investigating the effect they have on emission and loop heating as a result of the propagation of a beam of fast electrons moving through the ambient coronal plasma.Chapter 1 presents an overview of the Sun and the magnetic processes at work in the solar atmosphere.It also contains a summary of observations and current work corresponding to the phenomena discussed in later chapters, as well as current theories of particle acceleration and transport in an active region loop.Chapter 2 describes the theory behind, and implementation of, the numerical simulations used, and initial tests of the accuracy of the simulations by comparing results with analytical results for simplified models.The simulations are built on a core which models the evolution of the electron distribution function through stochastic processes.We derive the Fokker-Plank equation from which we obtain the expressions describing the progress along a magnetic field of an electron undergoing Coulomb collisions with particles of a background plasma.We describe the field and density models used, and consider the effects of gradient and curvature drifts on particles.In Chapter 3 we present results showing the non-thermal X-ray emission from magnetic loops with various density models and field configurations.We show results from a straightforward field model with no curvature as described in MacKinnon & Brown (1990), and then results from a more complex (and more realistic) X-point field model as described in Priest & Forbes (2000).These results illustrate the significant effects the field model and density of the background coronal plasma have on the loop emission, both in intensity and position (i.e. at which part of the loop the emission originates from).We also investigate the correlation between loop footpoint size and X-ray intensity, theoretically verifying work done by Schmahl et al. (2006) in which they present observations showing that X-ray intensity increases with footpoint size.In Chapter 4 we present results showing the evolution of the loop temperature profile over time.As the fast electrons collide with the particles of the ambient background plasma they lose energy, which is transferred to the plasma, increasing it's temperature.We include in these calculations the effects of radiative and conductive cooling of the loop, but we do not consider chromospheric evaporation (whereby heated plasma from the photosphere rises into the loop at the footpoints as a result of bombardment by the beam of fast electrons) or other bulk plasma effects.This would require a combination of stochastic and hydrodynamic simulations, which we do not cover in this work.Again, we show the effects of changing density and field models on the temperature profile.In Chapter 5we investigate the thermal X-ray emission from the particles of the background plasma in the heated loop.We then combine the thermal and non-thermal emission to produce X-ray spectra from photon energies 6 - 100 keV, similar to those observed by satellites such as the Reuven-Ramaty High Energy Solar Spectroscopic Imager (RHESSI), thus verifying that our simulations successfully model some of the processes present in active regions.We also consider the limitations of our simulations and models and discuss what parameters and changes would produce results close to observational data. Chapter 6 is separate from the preceding chapters and is a brief study of the production of Reverse-Drift Type III radio bursts in a loop, specifically the position in the loop at which the condition for their development originates, given various plasma densities and particle injection profiles.In a beam of injected electrons, the faster (higher energy) electrons propagate along the loop more quickly than the slower particles, causing an instability to develop in the beam distribution.This instability leads to the growth of Langmuir waves, which in turn result in emission at radio wavelengths.We show the development of the condition leading to this emission from a loop as a function of time and position, with various field and density models and particle injection profiles.Chapter 7 summarises the main body of work in this thesis and discusses possible further development of these methods in investigating the physical processes and parameters at work in active region magnetic loops.