Strengthening of the f mode due to subsurface magnetic fields in simulations of convection

Localised strengthening of the f mode precedes the emergence of active regions on the Sun by one to three days. While such an effect has also been demonstrated in highly simplified numerical models, those studies used a prescribed forcing function whose properties are different from those of a convectively driven flow. Our aim is to demonstrate magnetic effects on the f mode in convectively driven flows. We carried out non-linear simulations of convection with imposed magnetic fields at a range of depths. The power spectrum of the vertical velocity (with the depth and the horizontal wave vector kept fixed) took the form of a broad continuum with superposed peaks corresponding to various modes. We measured the mode amplitudes near the top of the domain. Overall, if the continuum is modelled by a power law, its power law index is consistent with ω^-2 (where ω is the angular frequency). We find that the f mode is strengthened when a super-equipartition magnetic field is imposed near the top of the domain. However, neither a magnetic field of equal strength near the bottom of the domain, nor an equipartition magnetic field near the top of the domain are seen to exert any significant effect. The magnetic effects we find are qualitatively consistent with those previously found in simplified numerical models. Future applications to observations should account for the fact that the observed effects are transient. Furthermore, we suggest that the continuum should be fit by a power law, rather than by unmotivated linear or polynomial fits.