Information on the S/N of the COS spectra used in this study. Left panel: distribution of the measured S/N per resolution element near Å 1240 for the COS spectra that are filament-related and those outside of filaments. Right panel: formal 3σ detection limits for H I Ly α absorption in these spectra, based on the equation given in Tumlinson et al. (2002). We note that these values reflect the detectability of Ly α absorption as a function of the local S/N under idealised conditions (no blending, no fixed-pattern noise, perfectly known continuum flux).