Umbral oscillations in the photosphere. A comprehensive statistical study

Abstract: It is well-known that the global acoustic oscillations of the Sun's atmosphere can excite resonance modes within large-scale magnetic concentrations. These structures are conduits of energy between the different layers of the solar atmosphere, and understanding their dynamics can explain the processes behind coronal heating and solar wind acceleration. In this work, we studied the Doppler velocity spectrum of more than a thousand large-scale magnetic structures (i.e., sunspots) in the solar photosphere that crossed near the disk centre of the Sun. We exploited the excellent stability and seeing-free conditions of the Helioseismic and Magnetic Imager (HMI) instrument onboard the Solar Dynamics Observatory (SDO) to cover nearly seven years of observations, providing the most comprehensive statistical analysis of its kind. Here, we show that the power spectra of the umbra of sunspots in the photosphere is remarkably different from the one of quiet-Sun regions, with both exhibiting a primary peak at 3.3 mHz, but the sunspot umbrae also displaying a closely packed series of secondary peaks in the $4-6$~mHz band. Understanding the origin of such peaks is a challenging task. Here, we explore several possible explanations for the observed oscillations, all pointing toward a potential resonant interaction within these structures and an unknown driver. Our observational result provides further insight into the magnetic connectivity between the different layers of the dynamic atmosphere of the Sun.