title: Tracking oscillatory signals through the flaring solar atmosphere
Creators: name: Millar, David C. L.
Publisher: name: University of Glasgow
Year: 2023

description: Fluctuations in the light we receive from the Sun happen constantly. Much of this variation can be thought of as random, however within solar timeseries there are many true periodic signals to be found, if one knows where to look. Identifying the source of these oscillations can give us a wealth of information about the underlying physical environment of the solar atmosphere. One area which has been particularly exploited is the search for oscillations due to solar flares which can be short-lived, bursty, and variable. Periodic signals detected during solar flares can help us understand the physics of flares, as well as how they interact with the lower layers of the atmosphere. The lower layers of the atmosphere have been known to present periodic fluctuations on timescales of a few minutes in quiet Sun conditions, and there have been some recent reports of energetic events such as flares affecting the quiescent oscillations, even exciting them. This thesis aims to investigate the effect of flares on the lower atmosphere by identifying and categorising oscillatory signals in a variety of solar data. In Chapter 1 an overview of necessary background physics is given. The layers of the Sun’s atmosphere and the standard model of solar flares are described. An introduction to different periodic signals which are present in solar data is given, including quiet Sun periodicities and those observed during transient phenomena. The methods used throughout the thesis are presented in Chapter 2, starting with descriptions of the instrumentation which obtained the analysed datasets. A discussion is presented on best practices when searching for oscillations in data, and the spectral fitting method is outlined. Techniques used to obtain physical solar parameters from observations are also introduced. In Chapter 3, a study of chromospheric intensity oscillations in a flaring active region is presented. Maps were produced showing the oscillatory signals which were present before and after the occurrence of the flare. When comparing the results after the flare to those before, oscillations were found to have changed their locations and typical periods in the vicinity of a sunspot located near the flare ribbons. These changes were interpreted as the result of a changing magnetic environment connected to the flare. The interpretations of these results were investigated using magnetohydrodynamic simulations in Chapter 4. Three different sets of simulations were carried out, to identify the most likely effect to have been responsible for the results of Chapter 3. The sets of simulations were concerned with the magnetic field inclination, the chromospheric temperature profile, and the length of the chromospheric cavity. The inclination angle was found to be the most likely of these three effects to change the periodic signals. Chapter 5 presents analysis of a second dataset, from an active region which featured much less powerful flare activity than the event from Chapter 3. A similar analysis was performed, and extended to also feature Doppler velocity data. Velocity oscillations were identified in pixels which exhibited some of the strongest flare heating. The possible cause of the induced velocity oscillations was explored using measurements of the magnetic field from spectropolarimetric observations, and inversions using the STiC code. In Chapter 6 concluding remarks are made, including a summary of the main results of the thesis and descriptions of possible future avenues for investigation.