Diurnal relative variability in the GOES-16 and GOES-18 EUVS-C MgII_EXIS index components (Eq. (8)) for August 9, 2022. Diurnal variability in the blue and red wing irradiances arises from Doppler shifts caused by the spacecraft orbital motion that is not corrected in the operational data reduction algorithm (Sect. 4.2.1). Differing satellite longitudes (GOES-16 at 75.2° W and GOES-18 at 137.0° W) cause the profiles of the two instruments to peak at different UTC positions.