On the Asymmetry of Solar Acoustic Line Profiles

Abstract

We study a simplified model of solar acoustic oscillations and show how asymmetries in spectral lines depend both on the acoustic source depth, as previously recognized, and on the acoustic source type. We provide a unified description of modal line asymmetries and high-frequency pseudomode locations, suggesting an inversion on power spectra minima to determine source properties and a correction to Lorentzian line shapes based upon the relative locations of spectral peaks and valleys. We also consider nonadiabatic effects due to Newtonian cooling and demonstrate that these do not lead to notable differences between velocity and intensity power spectral line shapes. We argue more generally that it is unlikely that any nonadiabatic effect can be responsible for the observed differences. Finally, we discuss the importance of both multiplicative and additive background power to the spectra and show how additive noise can reduce the apparent line asymmetry of a mode. We note that information on solar convective motions can be potentially extracted from three components of the acoustic power spectra: the additive background yielding information on the spectrum of nonoscillatory motions at the height of observation, the multiplicative background reflecting the source spectrum, and the power minima providing the source depth and physical nature. For stochastically excited linear waves only the first of these contributes significantly to spectral differences between observed variables.