The supergranulation spectrum

Abstract

Full-disk Dopplergram observations obtained at the 60-foot tower of the Mount Wilson Observatory with the Cacciani sodium magneto-optical filter were analyzed to determine the spectrum of the solar supergranulation. Individual Dopplergrams were averaged together using a weighted average over 20-minute intervals to remove the p-mode oscillations. The Doppler signals due to the motion of the observer, the solar rotation, differential rotation, and limb shift were then removed from the data to produce Dopplergrams dominated by the supergranular flows. These data were mapped to heliographic coordinates and projected onto the spherical harmonics. The resulting spectrum exhibits a peak at spherical harmonic degree ℓ } 100, which corresponds to typical cell diameters of about 40 Mm. Synthetic data were constructed and passed through the same analysis procedures to determine the actual spectrum required to reproduce the results. A good fit was obtained with a kinetic energy spectrum which peaks at ℓ } 100 and decreases exponentially out to ℓ } 500 with an e-folding range of Δℓ } 90. A power law fit to the spectrum over this range in ℓ yields an exponent of about −2.75. No corrections for seeing were included in the analysis. Although the image was sampled at 8 arcsec resolution, the effects of seeing may alter the actual slope of the spectrum and make it somewhat flatter. The spectrum does not exhibit any evidence for a distinct mesogranulation component out to ℓ } 500 (corresponding to cell diameters of about 8 Mm).