Excitation temperature T₃₄ between the J = 3 and J = 4 levels as a function of the excitation temperature T₃₅ between the J = 3 and J = 5 levels. Green squares and blue stars correspond to values inferred from absorption measurements toward extragalactic targets and nearby OB stars observed with FUSE (Wakker 2006; Shull et al. 2021). Red points and arrows show measurements and upper limits, respectively, obtained along MRS background lines of sight after excluding flagged data. The dashed black line shows a power-law relation derived between the two temperatures (Eq. (2)), and the shaded region is an envelope obtained by varying each fit parameter independently within its marginalized ±3σ uncertainty, which encloses 78% of the data points.