Interpretation of solar-cycle variability in high-degree p-mode frequencies

Abstract

RECENTLY Libbrecht and Woodard¹ and Elsworth et al.² have demonstrated that the frequencies of solar acoustic p-mode oscillations vary significantly over the solar cycle. We have previously suggested that cyclic variations in the magnetic activity of the Sun could modulate the p-mode frequencies in a similar way. In particular, we investigated^3–5simple models of the 'magnetic canopy', which permeates the solar atmosphere and overlies all of the Sun's surface, to determine its influence on p-mode frequencies. Here we make a comparison of our model predictions with the observations of Libbrecht and Woodard. We find that, despite the simplicity of our model, we are able to obtain good agreement with the observed frequency shifts for modes of frequency less than 4 mHz, through a mechanism in which an increasing magnetic field induces 'stiffening' of the Sun's chromosphere. Above this frequency there is clearly something missing from our model. We speculate that the behaviour above 4 mHz is related to a cutoff frequency in the solar atmosphere, above which waves are trapped only partially.