Line formation of He I D3 and He I 10 830 Å in a small-scale reconnection event

Context. Ellerman bombs (EBs) and UV bursts are small-scale reconnection events that occur in the region of the upper photosphere to the chromosphere. It has recently been discovered that these events can have emission signatures in the He I D₃ and He I 10 830 Å lines, suggesting that their temperatures are higher than previously expected.
Aims: We aim to explain the line formation of He I D₃ and He I 10 830 Å in small-scale reconnection events.
Methods: We used a simulated EB in a Bifrost-generated radiative magnetohydrodynamics snapshot. The resulting He I D₃ and He I 10 830 Å line intensities were synthesized in 3D using the non-local thermal equilibrium (non-LTE) Multi3D code. The presence of coronal extreme UV (EUV) radiation was included self-consistently. We compared the synthetic helium spectra with observed raster scans of EBs in He I 10 830 Å and He I D₃ obtained at the Swedish Solar Telescope with the TRI-Port Polarimetric Echelle-Littrow Spectrograph.
Results: Emission in He I D₃ and He I 10 830 Å is formed in a thin shell around the EB at a height of ∼0.8 Mm, while the He I D₃ absorption is formed above the EB at ∼4 Mm. The height at which the emission is formed corresponds to the lower boundary of the EB, where the temperature increases rapidly from 6 × 10³ K to 10⁶ K. The synthetic line profiles at a heliocentric angle of μ = 0.27 are qualitatively similar to the observed profiles at the same μ-angle in dynamics, broadening, and line shape: emission in the wing and absorption in the line core. The opacity in He I D₃ and He I 10 830 Å is generated through photoionization-recombination driven by EUV radiation that is locally generated in the EB at temperatures in the range of 2 × 10⁴ − 2 × 10⁶ K and electron densities between 10¹¹ and 10¹³ cm⁻³. The synthetic emission signals are a result of coupling to local conditions in a thin shell around the EB, with temperatures between 7 × 10³ and 10⁴ K and electron densities ranging from ∼10¹² to 10¹³ cm⁻³. This shows that both strong non-LTE and thermal processes play a role in the formation of He I D₃ and He I 10 830 Å in the synthetic EB/UV burst that we studied.
Conclusions: In conclusion, the synthetic He I D₃ and He I 10 830 Å emission signatures are an indicator of temperatures of at least 2 × 10⁴ K; in this case, as high as ∼10⁶ K.