Using images from the Helioseismic and Magnetic Imager aboard the \textit{Solar Dynamics Observatory} (SDO/HMI), we extract the radial-velocity (RV) signal arising from the suppression of convective blue-shift and from bright faculae and dark sunspots transiting the rotating solar disc. We remove these rotationally modulated magnetic-activity contributions from simultaneous radial velocities observed by the HARPS-N solar feed to produce a radial-velocity time series arising from the magnetically quiet solar surface (the 'inactive-region radial velocities'). We find that the level of variability in the inactive-region radial velocities remains constant over the almost 7 year baseline and shows no correlation with well-known activity indicators. With an RMS of roughly 1 m/s, the inactive-region radial-velocity time series dominates the total RV variability budget during the decline of solar cycle 24. Finally, we compare the variability amplitude and timescale of the inactive-region radial velocities with simulations of supergranulation. We find consistency between the inactive-region radial-velocity and simulated time series, indicating that supergranulation is a significant contribution to the overall solar radial velocity variability, and may be the main source of variability towards solar minimum. This work highlights supergranulation as a key barrier to detecting Earth twins.
@article{arxiv.2311.16076,
title = {The magnetically quiet solar surface dominates HARPS-N solar RVs during low activity},
author = {Ben S. Lakeland and Tim Naylor and Raphaëlle Haywood and Nadège Meunier and Federica Rescigno and Shweta Dalal and Annelies Mortier and Samantha J. Thompson and Andrew Collier Cameron and Xavier Dumusque and Mercedes López-Morales and Francesco Pepe and Ken Rice and Alessandro Sozzetti and Stéphane Udry and Eric Ford and Adriano Ghedina and Marcello Lodi},
journal= {arXiv preprint arXiv:2311.16076},
year = {2023}
}