Solar-cycle variation of quiet-Sun magnetism and surface gravity oscillation mode

Funding Information: We are indebted to Dr. Harsha Raichur, the developer of the original f-mode analysis pipeline, wherefrom this work stems. We also acknowledge the fruitful discussions with Prof. Nishant Singh on the theoretical interpretation of the results and practical insights to the analysis. All SDO data used are publicly available from the Joint Science Operations Center (JSOC) at Stanford University supported by NASA Contract NAS5- 02139 (HMI), see http://jsoc.stanford.edu/ . The data analysis has been carried out on supercomputers in the facilities hosted by the CSC – IT Center for Science in Espoo, Finland, which are financed by the Finnish ministry of education. The data were also processed at the German Data Center for SDO (GDC-SDO), funded by the German Aerospace Center (DLR), and hosted by the Max Planck Institute for Solar System Research (Göttingen, Germany). This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (Project UniSDyn, grant agreement no 818665). | openaire: EC/H2020/818665/EU//UniSDynContext. The origins of quiet-Sun magnetism (QS) is still under debate and investigating the solar cycle variation observationally in greater detail can provide clues on how to resolve the ensuing controversies. Aims. We investigate the solar cycle variation of the most magnetically quiet regions and their surface gravity oscillation (f-) mode-integrated energy, Ef. Methods. We used 12 years of Helioseismic and Magnetic Imager (HMI) data and applied a stringent selection criteria based on spatial and temporal quietness to avoid any influence from active regions (ARs). We developed an automated high-throughput pipeline to go through all available magnetogram data and to compute the value of Ef for the selected quiet regions. Results. We observed a clear solar cycle dependence of the magnetic field strength in the most quiet regions containing several supergranular cells. For patch sizes smaller than a supergranular cell, no significant cycle dependence was detected. The Ef at the supergranular scale is not constant over time. During the late ascending phase of Cycle 24 (SC24, 2011-2012), it is roughly constant, but starts diminishing in 2013, as the maximum of SC24 is approached. This trend continues until mid-2017, when hints of strengthening at higher southern latitudes are seen. Slow strengthening continues, stronger at higher latitudes than at the equatorial regions, but Ef never returns to the values seen in 2011-2012. In addition, the strengthening trend continues past the solar minimum, to the years when SC25 is already clearly ascending. Hence, the Ef behavior is not in phase with the solar cycle. Conclusions. The dependence of Ef on the solar cycle at supergranular scales is indicative of the fluctuating magnetic field being replenished by tangling from the large-scale magnetic field and not solely due to the action of a fluctuation dynamo process in the surface regions. The absence of variations on smaller scales might be an effect of the limited spatial resolution and magnetic sensitivity of HMI. The anticorrelation of Ef with the solar cycle in gross terms is expected, but the phase shift of several years indicates a connection to the large-scale poloidal magnetic field component rather than the toroidal one. Calibrating AR signals with the QS Ef does not reveal significant enhancement of the f-mode prior to AR emergence.Peer reviewe