Asymmetries of Solar p-mode Line Profiles

Abstract

Recent observations indicate that solar p-mode line profiles are not exactly Lorentzian, but rather exhibit varying amounts of asymmetry about their respective peaks. We analyze p-mode line asymmetry using both a simplified one-dimensional model and a more realistic solar model. We find that the amount of asymmetry exhibited by a given mode depends on the location of the sources exciting the mode, the mode frequency, and weakly on the mode spherical harmonic degree, but not on the particular mechanism or location of the damping. We calculate the dependence of line asymmetry on source location for solar p-modes, and provide physical explanations of our results in terms of the simplified model. A comparison of our results to the observations of line asymmetry in velocity spectra reported by Duvall et al. (1993) for modes of frequency $\sim$ 2.3 mHz suggests that the sources for these modes are located more than 325 km beneath the photosphere. This source depth is greater than that found by Kumar (1994) for acoustic waves of frequency $\sim$ 6 mHz. The difference may indicate that waves of different frequencies are excited at different depths in the convection zone. We find that line asymmetry causes the frequency obtained from a Lorentzian fit to a peak in the power spectrum to differ from the corresponding eigenfrequency by an amount proportional to a dimensionless asymmetry parameter and to the mode linewidth.