Slow magneto-acoustic waves in simulations of a solar plage region carry enough energy to heat the chromosphere

Abstract

We study the properties of slow magneto-acoustic waves that are naturally excited due to turbulent convection and investigate their role in the energy balance of a plage region using three dimensional (3D) radiation-MHD simulations. We calculate the horizontally averaged (over the whole domain) frequency power spectra for both longitudinal and vertical (i.e. the component perpendicular to the surface) components of velocity. To compare our results with the observations we degrade the simulation data with Gaussian kernels having FWHM of 100 km and 200 km, and calculate horizontally averaged power spectra for the vertical component of velocity. The power spectra of the longitudinal component of velocity, averaged over field lines in the core of a kG magnetic flux concentration, reveal that the dominant period of oscillations shifts from around 6.5 minutes in the photosphere to around 4 minutes in the chromosphere. At the same time, the velocity power spectra, averaged horizontally over the whole domain, show that low frequency waves (approximately 6.5 minute period) may reach well into the chromosphere. Importantly, waves with frequencies above 5 mHz propagating along different field lines are found to be out of phase with each other even within a single magnetic concentration. The horizontally averaged power spectra of the vertical component of velocity at various effective resolutions show that the observed acoustic wave energy fluxes are underestimated, by a factor of three even if determined from observations carried out at a high spatial resolution of 200 km. Our results show that longitudinal waves carry (just) sufficient energy to heat the chromosphere in solar plage. We conjecture that current observations (with spatial resolution around 200 km) underestimate the energy flux by roughly a factor of three, or more if the observations have lower spatial resolution.