ON THE EVIDENCE FOR MESOGRANULES IN SOLAR POWER SPECTRA

The power spectrum of the horizontal component of the solar convective velocity field has recently been estimated from observations of the Doppler shifts of surface flows at and near disk center (Chou et al.). From their analysis, the authors assert that "there is no evidence of apparent energy excess at the scale of mesogranulation." We show in this paper that their conclusion is incorrect and that the shape of the observational spectrum does indeed confirm the presence of both supergranules and mesogranules in the solar convective flow. To establish this claim we have extended existing kinematic models of convection at the solar surface (Simon and Weiss; Simon et al.) and have introduced power spectra diagnostics. We find that models with supergranule cells alone do not produce spectra that match the observations, but if mesogranules are included then there is excellent agreement between the model and observational spectra when the model parameters are chosen to be consistent with proper motion and Doppler measurements. Since the magnitudes of our model spectra are consistent with earlier Doppler measurements of the rms horizontal velocity field, but are significantly less than those of the observed spectrum, we must call into question the analysis of Chou et al.