Unsigned magnetic flux proxy from solar optical intensity spectra

The photospheric unsigned magnetic flux has been shown to be highly correlated with radial velocity (RV) variations caused by solar surface activity. This activity indicator is therefore a prime candidate unlock the potential of RV surveys discover Earth twins orbiting Sun-like stars. We show for first time how precise proxy ($\Delta\alpha B^2$) can obtained from Sun-as-a-star intensity spectra harnessing information contained in over 4000 absorption lines wavelength range 380 690 nm. novel thus same which RVs are routinely extracted. derived $\Delta\alpha B^2$ 500 randomly selected HARPS-N public data set, spans 2015 2018. compared our estimates values Solar Dynamics Observatory (SDO) finding excellent agreement (median absolute deviation: 4.9 per cent). extracted correlates SDO's on rotational timescale (Pearson correlation coefficient 0.67) and three-year set (correlation 0.91). find correlations 0.49 0.78 timescale. Pearson found greater than classical indicators RVs. For solar-type stars, represents best simultaneous known date.