Exploring the diagnostic value of Helium I D3 in the chromosphere

We present high-resolution observations of the chromospheric He I D3 line at 5876 Å, with SST/TRIPPEL and SST/CRISP. The SST observations have the highest spatial and spectral resolution for He I D3 up till date, and our goal is to explore its diagnostic value in the upper chromosphere.
The lower level of D3 gets populated via the photoionization-recombination mechanism. Therefore, the line intensity is sensitive to EUV-radiation from the corona and from the transition region. We investigate the He I D3 line behaviour for different targets on the sun: flares, active regions, plages, off-limb, ... and we interprete the spectra in context of space-borne co-observations with IRIS and SDO. Moreover, the He I D3 line is magnetically sensitive and allows for high-resolution spectro-polarimetric observations with SST/CRISP. We will present preliminary results of inversions of the He I D3 line with the inversion code HAZEL, to obtain the full magnetic field vector in the upper chromosphere.