Equivalent width (EW) in the O₂-A band as a function of phase angle α. The symbols indicate the different sceneries (Pacific and Atlantic). Full lines and gray areas indicate the linear regression and its errors. Both regression curves are distinct by ≈ 2σ from each other, and offset over the full range of phase angles covered. A very low probability found in a two-sample KS-test indicates that both samples are not drawn from the same underlying population. Red lines with different line styles refer to models of Stam (2008) and star symbols refer to the two models of Emde et al. (2017).