31 research outputs found
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
have the opposite asymmetry as is observed for the
intensity and velocity spectra. At a fixed geometrical depth, corresponding to
, 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 Ï„ cont = 1 has opposite asymmetry to the velocity spectrum, whereas the temperature measured at a fixed geometrical depth, corresponding to < Ï„ cont> = 1, has the same asymmetry as velocity. We believe that the asymmetry reversal in temperature at Ï„ 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
Heat transport in the convective zone and deviations from the mixing length theory models
Sound Speed Variations Near the Photosphere due to Entropy Perturbations in 3D Numerical Experiments
Results on how the temperature distribution near the solar photosphere is altered by perturbing the entropy of rising fluid in the convection zone several megameters below the surface are presented. Effects on the emergent intensity and implications for helioseismic observations are described. 1 Introduction Considering the very upper layers of the solar convection zone, it would be interesting to check how temperature inhomogeneites, caused e.g. by magnetically driven entropy fluctuations deeper in convection zone, are affected and diffused by turbulent motions and what is the role of such changes in producing the observable photospheric intensity excess. Several efforts were made in order to answer this question (Kuhn 1994, 1996). Here we discuss this problem with some new results. 2 Tools To simulate solar convection, the 3D hydrodynamic code of Nordlund - Stein was utilised. The code includes radiative transfer (LTE) on the upper boundary. Horizontal boundaries are periodic, while ..