Abstract
Solar magnetohydrodynamic (MHD) waves have been studied extensively in the past decades. They have been known to be the result of many interesting and observable phenomena in the solar atmosphere, such as umbral flashes and running penumbral waves, seen in sunspots’ chromosphere. The MHD waves are thought to be one of the prime means of transporting energy through the solar atmosphere. For this reason, scientists have theorized that waves could be an explanation for one of the most important unsolved question about the Sun, the heating of the outer solar atmosphere. Theoretically, since the gas density decreases outward from the surface, the temperature should also have an equivalent trend. However, in the solar atmosphere, the temperature (after a decrease in the photosphere) increases outward, first shallowly, then exponentially. Many causes of this unique problem have been debated, one of which has been the solar MHD waves.
This thesis investigates oscillations and waves in a sunspot’s atmosphere using high spatial and temporal-resolution images obtained with the Swedish 1-m Solar Telescope (SST). Several atmospheric layers are sampled by spectropolarimetric observations at various wavelength positions of Hα 656.3 nm and Ca II 854.2 nm spectral lines. These correspond to several heights between the low photosphere and the high chromosphere of the Sun. This thesis aims to show how waves of different frequency propagate through the sunspot’s atmosphere. The data set obtained by SST in this report is particularly interesting to the study of waves in sunspots due to its high spatial and temporal resolution. Spatial resolution has been shown to be directly correlated with wave power (Wedemeyer-Böhm and Wöger 2007). A relatively low resolution data, employed in many of previous studies, have difficulty resolving high frequency waves of diverse phases. As a result, high frequency waves become washed out in Doppler and intensity oscillations in low spatial-resolution observations and the corresponding power becomes suppressed. Low temporal resolution also limits the detection of high frequency oscillations because they are limited by the Nyquist frequency. Our data set with a 20-second cadence and high spatial (and spectral) resolution, is of great importance to detect higher frequency waves. The SST observations were also recorded for ~2.5 hours, which makes them additionally unique since the longer a time-series is, the higher frequency resolution is obtained. Although many wave studies have already been conducted in sunspots, the data set used in this thesis are unique in their clarity and produced high quality images to be analyzed.
In Chapter 1, we have reviewed some background physics related to the Sun as well as the structures and phenomena we studied in the thesis. The employed data and methods are briefly described in Chapters 2 and 3. We have detailed our analysis and results in Chapter 4 and finally, we have summarized our findings in Chapter 5, where some of our results are also compared with those reported in the literature.
Veiledere: Professor Luc Rouppe van der Voort, Institutt for teoretisk astrofysikk, UiO
Medveileder: Researcher Dr. Shahin Jafarzadeh, Institutt for teroretisk astrofysikk, UiO
Intern sensor: Professor Øystein Elgarøy, Institutt for teoretisk astrofysikk, UiO
Ekstern sensor: Researcher Dr. Jaime de la Cruz Rodriguez, Department of Astronomy, Stockholm University, Sweden