[Previous] | [Session 5] | [Next]
D. H. Hathaway (NASA/MSFC), J. G. Beck, R. S. Bogart (Stanford University), K. T. Bachmann, G. Khatri, J. M. Betitto (Birmingham-Southern College), S. Han, J. Raymond (Tennessee Technological University)
Spectra of the cellular photospheric flows are determined from observations acquired by the MDI instrument on the SOHO spacecraft. Spherical harmonic spectra are obtained from the full-disk observations. Fourier spectra are obtained from the high-resolution observations. The p-mode oscillation signal and instrumental artifacts are reduced by temporal filtering of the Doppler data. The resulting spectra give power (kinetic energy) per wavenumber for effective spherical harmonic degrees from 1 to over 3000. Significant power is found at all wavenumbers, including the small wavenumbers representative of giant cells. The time evolution of the spectral coefficients indicates that these small wavenumber components rotate at the solar rotation rate and thus represent a component of the photospheric cellular flows. The spectra show distinct peaks representing granules and supergranules but no distinct features at wavenumbers representative of mesogranules or giant cells. The observed cellular patterns and spectra are well represented by a model that only includes only two distinct modes --- granules and supergranules.
The radial component of the supergranular flow is determined by examining the center-to-limb variation of the Doppler velocity signal. Doppler velocity images are constructed from sections of the spectrum representing cells of different sizes. The center-to-limb variation of the mean squared signal in each of these images is fit over the central portion of the disk out to where foreshortening begins to affect the signal. The results of this analysis suggest that the radial flows for typical supergranules have speeds about 9% that of their associated horizontal flows or about 30 m/s. The ratio of the radial to horizontal flow speed increases from 9% to about 13% as the size of the cells decreases from >30 Mm to <5 Mm. Data simulations are used to confirm these conclusions.