He I(2³S) transmission spectrum. Left panel: in gray F_in/F_out obtained with the transmission spectroscopy analysis. A sinusoidal shape is evident. The red line is the sinusoidal fit obtained using the relation described in the text. Right panel: gray transmission spectrum divided by the sinusoidal red fit. Black dots represent the binned spectra with equally spaced bins of 0.3 Å, while vertical dashed green lines show the positions of the theoretical He I(2³ S) lines in vacuum.