Panel a: black: daily disc-integrated intensity excess at the continuum and core of the Fe I 6173 Å line and 1700 Å (top to bottom), calculated with the empirical intensity contrast model derived in this study (Eq. (2) and Table 1). Red: fit of the daily disc-integrated intensity excess at the Ca II K line, calculated with the relationship between Ca II K intensity excess and B_los reported by Harvey & White (1999). The R² between the two time series is annotated. Panel b: 81-day moving average. Panel c: zoomed inset of 2014.