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tokens: e.g., or Mm, the 1027 line reaches higher layers. We also included, as reference, an isothermal contour at 7000 K that seems to follow relatively closely the height where the optical depth is unity for these lines. In the case of Ca ii , the near-UV transition forms at heights similar to those covered by the O i lines, while the infrared spectral line scans lower layers in the low to mid chromosphere. In the region , magnetic loops are stratified and the TR shifts to higher layers. Following this trend, the spectral lines also form higher in the atmosphere, with the exception of Ca ii IR, which is sensitive to the low chromosphere. 3.3.3. Inferring the LOS Velocity One of the main targets of future spectroscopic observations (e.g., done by Solar-C (EUVST) and SO/SPICE) is to examine the dynamics of the solar phenomena from the chromosphere to the outer atmosphere. We aim to estimate the capabilities of the spectral lines of interest to tackle this task. We do this by computing the Doppler shift of the line core of Lyman β and the two O i 1027 and 1028 Å lines. We believe a single Gaussian fitting is not accurate enough for Lyman β because the line shows multiple lobes, similar to the Mg ii UV spectral lines. Therefore, using the analysis of the IRIS observations as a reference, we perform the following steps. First, we detect the extrema of each spectral profile. Among them, we select the extremum that is nearest to the line core rest wavelength, and we perform a parabola fitting around the extremum to obtain the corresponding line core wavelength. In the case of the O i transitions, we compute the line core Doppler shift using a single Gaussian fitting of the intensity profile. We tried more complex procedures than we do for Lyman β , but we did not achieve noticeable improvements. The inferred LOS velocities for the entire snapshot FOV are plotted in the upper row of Figure 9 . We include in the middle row the LOS velocity at the height where the optical depth is unity for the line core wavelength (similar to what we did for the temperature in Figure 6 ). Additionally, we show in the bottom row the joint probability density functions (JPDFs) of the velocity obtained by the fitting and that from the B ifrost atmosphere (top and middle rows). In some pixels, the derived velocities were larger than 20 km s −1 . We believe those large values correspond to a bad fit of our method. The pixels where the velocity is larger than 20 km s −1 were excluded from the JPDF plots and from the computation of the Pearson correlation rate (although they are shown in the top row panels). Examining the JPDF results, we can say that both methods provide a similar spatial distribution of the LOS velocity for the three spectral lines. We can trace the velocities along the loop-like structures as well as inside the magnetic field elements. The differences among these LOS velocities can be seen in the JPDF plots and the Pearson correlation between both cases. For the