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Title: Transverse waves in the solar corona
Author: White, R. S.
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2014
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Transverse loop oscillations (TLOs) are standing oscillations of coronal loops that are interpreted as kink waves in Magneto-hydrodynamic theory. These waves are particularly important as they can be used to infer local plasma properties such as the magnetic field strength through the technique of coronal seismology. In this thesis TLOs are studied both observationally, primarily with the Atmospheric Imaging Assembly (AIA) instrument on the Solar Dynamics Observatory (SDO) satellite, and numerically using the Lare2D code. In the first observational study, eleven transversely oscillating loops in three active regions are analysed in detail. The time series analysis revealed periods between 1.7 and 10 minutes and damping times between 2.9 and 13 minutes and are compared with previously observed events. Coronal seismology of the kink mode is applied to determine the range of the internal Alfvén speed and the magnetic field strength for each loop. The magnetic field strength is found to have a range of 4 − 18 G. The second observational study presents the first observation of a transverse oscillation in a hot coronal loop following a linked coronal-flare mass-ejection event. The TLO is observed exclusively in the 131 Å and 94 Å bandpasses, indicating a loop plasma temperature in the range of 9 - 11 MK. Furthermore, the loop is not observed to cool into the other AIA channels, but just disappears from all bandpasses at the end of the oscillation. The time series analysis reveals a period of 302 ± 14 s (291 ± 9 s) and a damping time of 306 ± 43 s (487 ± 125 s) for the first (second) loop strand and a spatial phase shift along the loop of approximately 180° suggests that a higher order harmonic is observed. We show that the excitation mechanism of this unusual oscillation is directly connected with the reconnection processes that form the post flare loop. This differs from the blast wave excitation mechanism often proposed as the cause of cooler transverse loop oscillations. The third observation presents another unusual TLO event that is also shown to have a direct link to the local magnetic field topology and the flare reconnection processes. A transversely oscillating coronal loop seen in the 171 Å bandpass is seen to oscillate in antiphase with respect to adjacent larger loops seen in the 193 Å and 211 Å bandpasses. The local magnetic topology of the active region is modelled using potential field source surface (PFSS) extrapolation which reveals that the loops are anchored in different topological regions with footpoint locations identified either side of the EUV flare peak emission source. Lastly, numerical simulations using the Lare2D code are performed to further investigate the anti-phase TLO observations. Loops positioned in different topological regions above and below a magnetic X-point are impulsively excited and we observe inwardly directed anti-phase oscillations of the loops. In addition we show that both the loop oscillations and the oscillatory behaviour of the X-point are independent of the resistivity.
Supervisor: Not available Sponsor: Science and Technology Facilities Council (Great Britain)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QB Astronomy