What Causes P-mode Asymmetry Reversal?

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 $\tau_{\rm cont} = 1$ have the opposite asymmetry as is observed for the intensity and velocity spectra. At a fixed geometrical depth, corresponding to $=1$, however, the temperature and velocity spectra have the same asymmetry. This indicates that the asymmetry reversal is produced by radiative transfer effects and not by correlated noise.Comment: 16 pages, 10 figures, submitted to Astrophysical Journa

Local helioseismology and correlation tracking analysis of surface structures in realistic simulations of solar convection

We apply time-distance helioseismology, local correlation tracking and Fourier spatial-temporal filtering methods to realistic supergranule scale simulations of solar convection and compare the results with high-resolution observations from the SOHO Michelson Doppler Imager (MDI). Our objective is to investigate the surface and sub-surface convective structures and test helioseismic measurements. The size and grid of the computational domain are sufficient to resolve various convective scales from granulation to supergranulation. The spatial velocity spectrum is approximately a power law for scales larger than granules, with a continuous decrease in velocity amplitude with increasing size. Aside from granulation no special scales exist, although a small enhancement in power at supergranulation scales can be seen. We calculate the time-distance diagram for f- and p-modes and show that it is consistent with the SOHO/MDI observations. From the simulation data we calculate travel time maps for surface gravity waves (f-mode). We also apply correlation tracking to the simulated vertical velocity in the photosphere to calculate the corresponding horizontal flows. We compare both of these to the actual large-scale (filtered) simulation velocities. All three methods reveal similar large scale convective patterns and provide an initial test of time-distance methods.Comment: 15 pages, 9 figures (.ps format); accepted to ApJ (tentatively scheduled to appear in March 10, 2007 n2 issue); included files ms.bbl, aabib.bst, aabib.sty, aastex.cl

ASYMMETRY REVERSAL IN SOLAR ACOUSTIC MODES

The power spectra of solar acoustic modes are asymmetric, with velocity having more power on the low frequency side of the peak and intensity having more power on the high frequency side. This effect exists in both observations and simulations, and it is believed to be caused by the correlated background noise. We study the temperature near the solar surface by means of a 3D hydrodynamic simulation of convection with a detailed treatment of radiation. The temperature spectrum at optical depth τ cont = 1 has opposite asymmetry to the velocity spectrum, whereas the temperature measured at a fixed geometrical depth, corresponding to < τ cont> = 1, has the same asymmetry as velocity. We believe that the asymmetry reversal in temperature at τ cont = 1 (and therefore in intensity) occurs partly because of the radiative transfer effects. High temperature sensitivity of the opacity suppresses temperature fluctuations on opposite sides of the mode peaks differently, thus causing the asymmetry reversal