Top panel: Comparison of the GOES-16 MgII_EXIS/GOES-18 MgII_EXIS ratio for an eclipse day (black) and non-eclipse day (red). Indices were calculated using the fixed-mask operational algorithm. Displacements or the black curve relative to the red result from abrupt changes in instrument temperature that occur during eclipse entry. Lower Panel: Ratio comparisons for eclipse and non-eclipse days with indices calculated using the operational algorithm after shifting the spectra from each instrument to its noontime pixel scale as described in Section 4.2.1.