The research question of my PhD is in a way a simple one: what can observations of the He I D₃ line teach us about the solar chromosphere? This optical spectral 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 He I D₃ 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 during my PhD has provided new insights in Ellerman bombs and flares. Our He I D₃ observations have suggested that the temperature of Ellerman Bombs is higher than 2×10⁴ K based on the discovery of helium emission signatures in these events. This result is unexpected, since previous modeling in the literature estimates the temperatures of Ellerman Bombs below 10⁴ K. Subsequently, 3D non-LTE radiative transfer calculations have revealed the detailed physical mechanisms to generate He I D₃ emission in these events. The calculations also confirmed that temperatures between 2×10⁴ - 10⁶ K are required to populate the helium triplet levels.

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.