The Spectrum of the Solar Supergranulation: Multiple Nonwave Components

Abstract

It has recently been suggested that the solar supergranulation undergoes oscillations, with a spectrum of superposed traveling waves of unknown origin showing excess prograde power to yield superrotation. We show here that the observed supergranular spectrum does not necessarily imply a wave origin but is instead consistent with two components of nonoscillatory bulk motions having differing rotation rates and somewhat asymmetrically distributed in space. The two components are identified with solar mesogranulation and supergranulation, and the spatial asymmetry is shown to be caused by a weak north-south alignment of the supergranular flows. The source of both the supergranular alignment and its enhanced rotation is likely underlying giant cell motions. Because no single rotation rate characterizes all components of a solar image, the spectral properties, including the Fourier dispersion relation, are extremely sensitive to the rate at which the solar disk is tracked when making up the time series. A spuriously wavelike spectrum is obtained when the image tracking rate falls between the actual mesogranular and supergranular rotation rates.