Abstract
The solar chromosphere forms the interface between the cool photosphere and the hot corona. It is made up of fast-moving charged plasma and strong magnetic fields, which both push and pull on each other, resulting in highly dynamic and complex structures. While the Balmer Hα 6563 Å line is the most studied spectral line in the chromosphere, the Balmer Hβ 4861 Å line provides a different view of the chromosphere due to its lower opacity. The Swedish 1-m Solar Telescope (SST) on La Palma is equipped with the CRISP and CHROMIS tunable filtergraph instruments, capable of observing spectrally resolved images in, respectively, the Hα and Hβ lines. This allows for high-resolution, co-spatial and co-temporal observations in Hα and Hβ. Furthermore, multiple images taken on a grid pattern within a short time span can be combined to form a “mosaic” image which covers a larger field of view than that of a single telescope pointing. These mosaics can capture the context in which both small- and large-scale structures are formed with very high resolution. Depending on the technique with which the individual observations are assembled, the resulting mosaic images are useful for applications such as magnetic field extrapolation, as well as comparison with co-temporal observations of the solar corona from space telescopes, e.g. NASA’s Solar Dynamics Observatory (SDO). We used two sets of mosaic observations from the SST to compare the different mosaic assembly techniques, and found that the focus should be on capturing mosaics quickly line-by-line, and including the full polarity of active regions. We also used a cospatial and co-temporal SST time series of Hα and Hβ to investigate the properties of the spectral lines, and the processes that make them sensitive to chromospheric structures. We did pixel-to-pixel comparisons on the cross-correlated datasets and found that both the Hα and Hβ lines are sensitive to the same chromospheric structures, and that the chromosphere is less optically thick for the Hβ line than for the Hα line. Both lines showed indications of being affected by opacity broadening in accordance with the curve of growth, however, this applied only for the darkest fibrils within our observed field of view.
Supervisor: Professor Luc Rouppe van der Voort, Institute of Theoretical Astrophysics, UiO
Intern. assessor: Professor David F. Mota, Institute of Theoretical Astrophysics, UiO
Extern. assessor: Senior Researcher Jaime de la Cruz Rodriguez, Stockholm University