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On the formation of Lyman $β$ and the O I 1027 and 1028 Å spectral lines

We study the capabilities of Lyman $β$ and the O I 1027 and 1028 Å spectral lines for understanding the properties of the chromosphere and transition region. The oxygen transitions are located in the wing of Lyman $β$ that is a candidate spectral line for the solar missions Solar Orbiter/SPICE and Solar-C (EUVST). We examine general spectroscopic properties of the three transitions in the quiet Sun by synthesizing them assuming non-local thermal equilibrium taking into account partial redistribution effects. We estimate the heights where the spectral lines are sensitive to the physical parameters computing the response functions to temperature and velocity using a 1D semi-empirical atmospheric model. We also synthesize the intensity spectrum using the 3D enhanced network simulation computed with the BIFROST code. The results indicate that Lyman $β$ is sensitive to the temperature from the middle chromosphere to the transition region while it is mainly sensitive to the line-of-sight velocity at the latter atmospheric layers, around 2000 km above the optical surface. The O I lines form lower in the middle chromosphere, being sensitive to the LOS velocities at lower heights than those covered by Lyman $β$. The spatial distribution of intensity signals computed with the BIFROST atmosphere, as well as the inferred velocities from the line core Doppler shift confirm the previous results. Therefore, these results indicate that the spectral window at 1025 Å contains several spectral lines that complement each other to seamlessly trace the thermal structure and gas dynamics from the middle chromosphere to the lower transition region.