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 to unlock the potential of RV surveys to discover Earth twins orbiting Sun-like stars. We show for the first time how a precise proxy of the unsigned magnetic flux (ΔαB²) can be obtained from Sun-as-a-star intensity spectra by harnessing the magnetic information contained in over 4000 absorption lines in the wavelength range from 380 to 690 nm. This novel activity proxy can thus be obtained from the same spectra from which RVs are routinely extracted. We derived ΔαB² from 500 randomly selected spectra from the HARPS-N public solar data set, which spans from 2015 to 2018. We compared our estimates with the unsigned magnetic flux values from the Solar Dynamics Observatory (SDO) finding excellent agreement (median absolute deviation: 4.9 per cent). The extracted indicator ΔαB² correlates with SDO's unsigned magnetic flux estimates on the solar rotational time-scale (Pearson correlation coefficient 0.67) and on the 3-yr time-scale of our data set (correlation coefficient 0.91). We find correlations of ΔαB² with the HARPS-N solar RV variations of 0.49 on the rotational time-scale and 0.78 on the 3-yr time-scale. The Pearson correlation of ΔαB² with the RVs is found to be greater than the correlation of the classical activity indicators with the RVs. For solar-type stars, ΔαB² therefore represents the best simultaneous activity proxy known to date.