f-mode strengthening from a localised bipolar subsurface magnetic field

Abstract

Recent numerical work in helioseismology has shown that a periodically varying subsurface magnetic field leads to a fanning of the f -mode, which emerges from a density jump at the surface.In an attempt to model a more realistic situation, we now modulate this periodic variation with an envelope, giving thus more emphasis on localised bipolar magnetic structures in the middle of the domain. Some notable findings are: (i) compared to the purely hydrodynamic case, the strength of the f -mode is significantly larger at high horizontal wavenumbers k, but the fanning is weaker for the localised subsurface magnetic field concentrations investigated here than the periodic ones studied earlier; (ii) when the strength of the magnetic field is enhanced at a fixed depth below the surface, the fanning of the f -mode in the kω diagram increases proportionally in such a way that the normalised f -mode strengths remain nearly the same in different such cases; (iii) the unstable Bloch modes reported previously in case of harmonically varying magnetic fields are now completely absent when more realistic localised magnetic field concentrations are imposed beneath the surface, thus suggesting that the Bloch modes are unlikely to be supported during most phases of the solar cycle;(iv) the f -mode strength appears to depend also on the depth of magnetic field concentrations such that it shows a relative decrement when the maximum of the magnetic field is moved to a deeper layer. We argue that detections of f -mode perturbations such as those being explored here could be effective tracers of solar magnetic fields below the photosphere before these are directly detectable as visible manifestations in terms of active regions or sunspots. ARTICLE HISTORY

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

“…Another motivation to study the QS comes from recent observational investigations (Singh et al 2016;Waidele et al 2022) that have reported a strengthening of the solar surface or the fundamental f -mode about one to two days before the formation of ARs using different kinds of local helioseismic techniques. Accompanied with numerical simulations that have given similar indications Singh et al (2014Singh et al ( , 2015Singh et al ( , 2020), this appears a very promising avenue for studying the origin of solar sub-surface magnetism and the mechanism of active region formation. The observational studies, however, suffer from a lack of proper calibration method against the QS.…”

Solar-Cycle Variation of quiet-Sun Magnetism and Surface Gravity Oscillation Mode

“…Another motivation to study the QS comes from recent observational investigations (Singh et al 2016;Waidele et al 2022) that have reported strengthening of solar surface or the fundamental f -mode about one to two days before the formation of ARs using different kinds of local helioseismic techniques. Accompanied with numerical simulations that have given similar indications Singh et al (2014Singh et al ( , 2015Singh et al ( , 2020, this seems as a very promising avenue of studying the origin of solar sub-surface magnetism and the mechanism of active region formation. The observational studies, however, suffer from a lack of proper calibration method against the QS.…”

Introduction

“…Returning to the Sun, several applications with the Pencil Code have previously determined the oscillation frequencies in stratified layers (Singh et al 2014(Singh et al , 2015. In their new work, Singh et al (2020) present results for a more complicated magnetic field geometry. In particular, they find that the unstable Bloch modes reported previously (Singh et al 2014) are now absent when more realistic localised flux concentrations are considered.…”