Vanreppelen, Luka

Master thesis supervised by Magdalenic, Jasmina; Berghmans, David (KULeuven)

Abstract: In this thesis, observations from the Extreme Ultraviolet Imager (EUI) aboard Solar Or- biter were reprojected into an equidistant azimuthal (ARC) coordinate system centered on the solar poles. The reprojection is essential because it minimizes foreshortening effects and enables accurate analysis of coronal structures near the poles. Longitude- time maps were constructed for several heliographic latitudes, and the angular ve- locities of coronal features were determined by manually tracing persistent structures within these maps. Statistical analysis showed that the distributions of the measured angular velocities are approximately Gaussian, with small non-Gaussian deviations appearing mainly in the outer quantiles. It was found that the angular velocities systematically decrease with increasing heliographic latitude, and this result generally agrees with other empirical differential rotation laws obtained using the coronal bright point tracers. Moreover, the results obtained in this study also improve and refine the description of the existing differential rotation trends in the Sun’s high latitudes, which are prone to the projection effect. The results demonstrate that polar reprojection of Solar Orbiter’s EUI observations pro- vides a robust framework for investigating differential rotation and slow coronal dynam- ics in the polar regions of the Sun. This study employs novel, out-of-the-ecliptic-plane observations by Solar Orbiter for the first time, in a study of the Sun’s differential rota- tion, and lays the methodology for future studies.

Keyword(s): Solar Orbiter ; EUI ; solar poles ; differential rotation