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MHD waves and oscillations in solar coronal waveguides
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Goddard, Christopher R. (2018) MHD waves and oscillations in solar coronal waveguides. PhD thesis, University of Warwick.
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Official URL: http://webcat.warwick.ac.uk/record=b3227892~S1
Abstract
Perturbations of coronal structures by impulsive events such as solar flares generate waves which are interpreted with MHD theory. These waves allow plasma processes to be studied, and seismology of the local plasma parameters to be performed. The focus of this thesis is the detailed observational study of these waves.
A statistically significant number of kink oscillations of coronal loops were analysed. The measured periods scale linearly with the estimated loop length, as expected from the standard interpretation of the waves as the global fundamental standing mode. A typical kink speed of Ck=(1300±50) kms⁻¹ is obtained. A linear scaling of the damping time with period is observed, and non-exponential damping profiles were noted. The study was then extended to determine if there is any scaling between the quality factor of the oscillations and the oscillation amplitude. Selected events from the kink oscillation catalogue were analysed in detail, and it was found that the damping profiles of several oscillations were better fit by a
Gaussian envelope than an exponential one. These damping profiles were then used to perform seismological inversions, including the transverse density structure of the loops. The obtained transverse density profile was compared to the observed intensity profile for one loop, using forward modelling and Bayesian inference, where good agreement was found.
The intensity cross-sections of 233 coronal loops were analysed. Assuming an isothermal and cylindrical cross-section the transverse density structure of the coronal loop plasma was inferred. Several models for the transverse density profile were quantitatively compared. Very strong evidence was found for the existence of an inhomogeneous layer where the density varies smoothly between the rarefied background plasma and the dense centre of the loop. In a significant number of cases the width of this layer was high enough to conclude that the loop does not have a core at all and has a continuously varying transverse density profile.
Finally, a flaring event was analysed which excites a series of propagating EUV intensity perturbations, and simultaneously produces a series of features in radio spectrometer data. This is the first observation which links quasi-periodic fast waves observed in the EUV band to quasi-periodic features in radio spectra. 2D numerical simulations of impulsively generated wave trains in coronal density enhancements are presented. This aims to establish how these waves are affected by initial perturbations which enter the non-linear regime, thereby establishing the feasibility of some of the mechanisms by which the observations presented could be explained.
| Item Type: | Thesis (PhD) | ||||
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| Subjects: | Q Science > QB Astronomy | ||||
| Library of Congress Subject Headings (LCSH): | Magnetohydrodynamic waves, Solar activity, Helioseismology, Sun -- Loop prominences, Sun -- Corona | ||||
| Official Date: | April 2018 | ||||
| Dates: |
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| Institution: | University of Warwick | ||||
| Theses Department: | Department of Physics | ||||
| Thesis Type: | PhD | ||||
| Publication Status: | Unpublished | ||||
| Supervisor(s)/Advisor: | Nakariakov, V. M. (Valery M.) | ||||
| Format of File: | |||||
| Extent: | xviii, 143 pages : illustrations, charts | ||||
| Language: | eng |
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