PhD Thesis Defenses

PhD Thesis Defence: The diagnostic potential of the He I D3 spectral line in the solar atmosphere

The research question of my PhD is in a way a simple one: what can observations of the He I D3 spectral line teach us about the solar chromosphere? This optical line at 5876 Å is generally formed in the upper chromosphere, and is sensitive to the local magnetic field. The He I D3 line is also indirectly sensitive to heating of the transition region and corona, since it is resulting from a transition that occurs between levels in the triplet system of neutral helium. These levels are generally populated via an ionization-recombination mechanism under the influence of EUV radiation originating in the transition region and corona. The He I D3 line was used as a flare diagnostic in the seventies and in the subsequent decades also to measure magnetic fields in prominences. However, due to the poor spatial resolution and low signal-to-noise of that data, almost exclusively off-limb targets have been studied. The on-disk absorption of \hed is very weak and localized. Recent instrumental developments allow for the acquisition of high spatial resolution on-disk spectroscopic and spectro-polarimetric data of He I D3 with different instruments at the SST, opening the possibility of studying all types of targets in the chromosphere in a new light. During my PhD, I have focused on the study of reconnection targets via high-resolution observations of He I D3 with TRIPPEL and CRISP at the SST, in co-observation with space-borne instruments. Subsequently, a theoretical study has aimed at in-depth understanding of He I D3 line formation in small-scale reconnection events. The data which I have obtained and analyzed in during my PhD has provided new insights in Ellerman bombs and flares. Our HeI D3 observations have set constraints on the temperature of Ellerman Bombs via the discovery of helium emission signatures in these events. Subsequently, 3D non-LTE radiative transfer calculations have increased our understanding of the line-formation of these emission signatures, demonstrating that the link to temperature is not straightforward. In the context of flares, we measured strong downflows in the chromosphere via He I D3, revealing detailed dynamics in the deep atmosphere during a flare. Spectro-polarimetry was used to measure the magnetic field during a flare and to propose its magnetic topology. In conclusion, the He I D3 line is an excellent probe for reconnection targets in the solar atmosphere. Detailed dynamics as well as the magnetic field configuration can be derived using the line. Our findings encourage the use of the He I D3 spectral line as a diagnostic for the chromosphere and open up a range of applications that is yet to be exploited.