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Title: Nonlinear and multi-modal oscillatory processes in active regions of the solar atmosphere
Author: Kolotkov, Dmitrii
ISNI:       0000 0004 6423 1342
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2017
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For longer than a century, analysis of a quasi-periodic variability of the Sun on various time scales has been attracting great interest among the research community. These signals are continuously detected throughout the whole electromagnetic spectrum, and often have non-stationary oscillation periods and irregular profile shapes. The physical mechanisms responsible for such variations are usually hidden, and their revealing always require an advanced combination of powerful spectral techniques and strong theoretical foundations. This thesis considers oscillatory phenomena occurring in the solar atmosphere from these two perspectives. For the spectral analysis of solar periodicities the self-adaptive Hilbert–Huang transform (HHT) method is employed. With the statistics of coloured noises incorporated, it allowed for the detection of a long-period oscillation of a small-scale photospheric magnetic structure, whose period grows from 80 to 230 min and positively correlates with the amplitude. A multi-modal nature of the solar flare quasiperiodic pulsations was also revealed with HHT. The detected intrinsic modes have mean periods of 15, 45, and 100 s, and can be interpreted as the kink and sausage magnetohydrodynamic oscillations of a flaring loop. Analysis of much longer solar periodicities associated with the magnetic activity cycles 22, 23, and 24 was also successfully performed with HHT, revealing a broad range of intrinsic modes with periods from about a month to several years (including the 11 yr cycle). From the theoretical point of view, the one-dimensional equilibrium and dynamical models of current sheets in the corona have been developed. The equilibrium model provides an inhomogeneous distribution of macroscopic plasma parameters across the current sheet, as found for realistic physical conditions. The dynamical model describes nonlinear oscillations of the current sheet formed by the coalescence of two magnetic flux ropes. The oscillation period is found to be about the ion plasma period or longer, and is prescribed by the current sheet thickness, the plasma parameter β, and the oscillation amplitude. Analytical modelling of finite amplitude transverse oscillations in quiescent prominences situated in a magnetic field dip, is also performed in the thesis. The model is based on the line-current concept and accounts for the interaction of the prominence current with the electrically conductive photosphere. In the linear regime, the horizontal and vertical motions are considered independently, and the mechanical stability of the system is analysed. The oscillation periods are determined by the prominence current, its mass and height above the photosphere, and the properties of the magnetic dip. In the case of finite amplitudes, the horizontal and vertical modes were found to be nonlinearly coupled with each other, and a metastable equilibrium state of the prominence was revealed. The periods of nonlinear oscillations are shown to depend upon the oscillation amplitude.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QB Astronomy