WHAT CAUSES p-MODE ASYMMETRY REVERSAL?

摘要

The solar acoustic p-mode line profiles are asymmetric. Velocity spectra have more power on the low-frequency sides, whereas intensity profiles show the opposite sense of asymmetry. Numerical simulations of the upper convection zone have resonant p-modes with the same asymmetries and asymmetry reversal as the observed modes. The temperature and velocity power spectra at optical depth τ_(cont) = 1 have the opposite asymmetry, as is observed for the intensity and velocity spectra. At a fixed geometrical depth, corresponding to <τ_(cont)> = 1, however, the temperature and velocity spectra have the same asymmetry. This indicates that the asymmetry reversal in the simulation is produced by radiative transfer effects and not by correlated noise. The cause of this reversal is the nonlinear amplitude of the displacements in the simulation and the nonlinear dependence of the H~- opacity on temperature. Where the temperature is hotter the opacity is larger and photons escape from higher, cooler layers. This reduces the fluctuations in the radiation temperature compared to the gas temperature. The mode asymmetry reversal in the simulation is a small frequency-dependent differential effect within this overall reduction. Because individual solar modes have smaller amplitudes than the simulation modes, this effect will be smaller on the Sun.