(a) Mosaic observed in M43. For each position, the [¹²C II] line profileis shown in the top box. Below the spectra, we show the windows for the base line subtraction (−65,−45) (−10,30) (60,80) km s⁻¹. The bottom box shows a zoom to the [¹³C II] satellites. (b) M43 mosaicof the seven positions observed by upGREAT. For each position, we show in the top panel a comparison between [¹²C II] (in black) and [¹³C II] (in red), the latter averaged over the hyperfine satellites and scaled-up by the assumed value of α⁺ = 67. The red line corresponds to 1.5 σ scaled-up by α⁺. Bottom panels: for all observation above 1.5 σ, the [¹²C II]/[¹³C II] intensity ratio per velocity bin (in gray) and the optical depth from the zeroth-order analysis (blue).