Evolution and wave-like properties of the average solar supergranule

Abstract

Context. Solar supergranulation presents us with many mysteries. For example, previous studies in spectral space have found that supergranulation has wave-like properties. Aims. Here we study, in real space, the wave-like evolution of the average supergranule over a range of spatial scales (from 10 to 80 Mm). We complement this by characterizing the evolution of the associated network magnetic field. Methods. We used one year of data from the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory to measure horizontal near-surface flows near the solar equator by applying time-distance helioseismology (TD) on Dopplergrams and granulation tracking (LCT) on intensity images. The average supergranule outflow (or inflow) was constructed by averaging over 10 000 individual outflows (or inflows). The contemporaneous evolution of the magnetic field was studied with HMI line-of-sight observations. Results. We confirm and extend previous measurements of the supergranular wave dispersion relation to angular wavenumbers in the range 50 < kR⊙ < 270. We find a plateau for kR⊙ > 120. In real space, larger supergranules undergo oscillations with longer periods and lifetimes than smaller cells. We find excellent agreement between TD and LCT and obtain wave properties that are independent of the tracking rate. The observed network magnetic field follows the oscillations of the supergranular flows with a six-hour time lag. This behavior can be explained by computing the motions of corks carried by the supergranular flows. Conclusions. Signatures of supergranular waves in surface horizontal flows near the solar equator can be observed in real space. These oscillatory flows control the evolution of the network magnetic field, in particular they explain the recently discovered east-west anisotropy of the magnetic field around the average supergranule. Background flow measurements that we obtain from Doppler frequency shifts do not favor shallow models of supergranulation.