Evolution and wave-like properties of the average solar supergranule

Abstract

Solar supergranulation presents us with many mysteries. For example, previous studies in spectral space found that supergranulation has wave-like properties. 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. We use one year of data from the Helioseismic and Magnetic Imager (HMI) to measure horizontal near-surface flows near the solar equator by applying time-distance helioseismology on Dopplergrams and granulation tracking on intensity images. The average supergranule outflow (or inflow) is constructed by averaging over 10000 individual outflows (or inflows). The contemporaneous evolution of the magnetic field is studied with HMI line-of-sight observations. 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. 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.

arXiv comment13 pages, 14 figures (plus appendix), accepted for publication in A&A

References in corpus (15)

• Solar supergranulation as a wavelike phenomenon
• The Solar Internetwork. I. Contribution to the Network Magnetic Flux
• The Emergence of Solar Supergranulation as a Natural Consequence of Rotationally-Constrained Interior Convection
• On the power spectrum of solar surface flows
• Spatially resolved vertical vorticity in solar supergranulation using helioseismology and local correlation tracking
• Point spread function of SDO/HMI and the effects of stray light correction on the apparent properties of solar surface phenomena
• Supergranulation as the largest buoyantly driven convective scale of the Sun
• Supergranulation and multiscale flows in the solar photosphere: Global observations vs. a theory of anisotropic turbulent convection
• Variation of Supergranule Parameters With Solar Cycles: Results From Century-Long Kodaikanal Digitized Ca II K Data
• Relation between trees of fragmenting granules and supergranulation evolution
• Additional Evidence Supporting a Model of Shallow, High-Speed Supergranulation
• The continuum intensity as a function of magnetic field II. Local magnetic flux and convective flows
• Anisotropy of the solar network magnetic field around the average supergranule
• Transport of internetwork magnetic flux elements in the solar photosphere
• Relationships between fluid vorticity, kinetic helicity and magnetic field at the small-scale (quiet-network) on the Sun

Cited by in corpus (10)

• Analyzing supergranular power spectra using helioseismic normal-mode coupling
• Imaging the Sun's near-surface flows using mode-coupling analysis
• Plasma flows and sound-speed perturbations in the average supergranule
• Characterising solar surface convection using Doppler measurements
• Recipe for inferring sub-surface solar magnetism via local mode-coupling using Slepian basis functions
• Characterizing the spatial pattern of solar supergranulation using the bispectrum
• Texture of average solar photospheric flows and the donut-like pattern
• Testing solar surface flux transport models in the first days after active region emergence
• Supergranulation and Poleward Migration of the Magnetic Field at High Latitudes of the Sun
• Swaying oscillations in Rayleigh-Bénard convection cast new light on solar convection