“Mesogranulation” — A Convective or An Oscillatory Phenomenon?

Abstract

We address the “mesogranulation” phenomenon by analyzing a two dimensional spectral time series. The main goal of this observation is to measure simultaneously the vertical and horizontal velocity of low frequency, convective flows in the solar photosphere with a single observation, in order to test the agreement of investigations based on either spectroscopic and local correlation techniques. The power spectra of the horizontal velocities determined by local correlation tracking are in good agreement with the power spectra of the vertical velocity. The structures of “mesogranular” scale, most prominent in the divergence of the horizontal flow field, emerge as the algorithm removes all flows at smaller scales and not as an evidence for a distinct regime of mesogranulation. A detailed analysis reveals a rather low correlation between the divergence of the horizontal velocity and the vertical velocity. We propose a corruption of the local correlation technique by the presence of horizontally propagating internal gravity waves as one of the possible sources for this disagreement.