Licentiate Thesis: Exploring the diagnostic value of He I D3 in the solar chromosphere
Monday 14 November 2016
to 12:00 at
Tine Libbrecht (Stockholm University)
This thesis focusses on observations of the He I D 3 line at 5876 Å in the solar chromosphere. The He I D 3 line is part of the triplet system of neutral helium (as is the more commonly observed He I 10830 Å line). The triplet levels in helium are thought to be populated through a photoionization-recombination echanism. In active regions, helium is ionized by EUV photons from the transition region and corona which are backscattered towards the chromosphere. Electrons can recombine in both singlet and triplet states in neutral helium, giving rise to opacity in He I D 3 and He I 10830 Å.
Due to the line formation mechanism and sensitivity to magnetic fields, the He I D 3 line is a useful tool to measure magnetic fields and to study reconnection events and heating in the chromosphere. Observations of the He I D 3 line are challenging - especially on the solar disk - because the line is generally weak and entirely absent outside active regions. In the last decade, the limits of high-resolution imaging and spectro-polarimetry have been pushed by advances in adaptive optics and instrumentation. High-resolution imaging and spectro-polarimetry of He I D 3 have revealed (and will continue to reveal) spatial and spectral structure leading to new insights in chromospheric phenomena related to magnetic fields and heating, such as Ellerman Bombs (Libbrecht et al., 2016, henceforth Paper I).