Abstract
Supergranules in the quiet Sun are outlined by a web-like structure of enhanced magnetic field strength, the so-called magnetic network. We aim to map the magnetic network field around the average supergranule near disk center. We use observations of the line-of-sight component of the magnetic field from the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO). The average supergranule is constructed by coaligning and averaging over 3000 individual supergranules. We determine the positions of the supergranules with an image segmentation algorithm that we apply on maps of the horizontal flow divergence measured using time-distance helioseismology. In the center of the average supergranule the magnetic (intranetwork) field is weaker by about 2.2 Gauss than the background value (3.5 Gauss), whereas it is enhanced in the surrounding ring of horizontal inflows (by about 0.6 Gauss on average). We find that this network field is significantly stronger west (prograde) of the average supergranule than in the east (by about 0.3 Gauss). With time-distance helioseismology, we find a similar anisotropy. The observed anisotropy of the magnetic field adds to the mysterious dynamical properties of solar supergranulation.
Highlights
Solar supergranules are surrounded by the network magnetic field that can be observed, for instance, in Ca K emission lines in the solar chromosphere (e.g., Simon & Leighton 1964)
We find that this network field is significantly stronger west of the average supergranule than in the east
In this Letter, we present photospheric maps of the magnetic field of the average supergranule using data from the Helioseismic and Magnetic Imager (HMI; Schou et al 2012) onboard the Solar Dynamics Observatory (SDO) at full resolution
Summary
Solar supergranules are surrounded by the network magnetic field that can be observed, for instance, in Ca K emission lines in the solar chromosphere (e.g., Simon & Leighton 1964). The network field is built up through the advection of magnetic field by supergranular flows (e.g., Rieutord & Rincon 2010). Not much is known about the dynamical interaction of supergranulation and the network field, and the dynamics of supergranulation itself is not understood (e.g., Gizon et al 2003; Rieutord & Rincon 2010). The average supergranule is constructed as an ensemble average of individual supergranules that are identified in maps of the horizontal flow divergence from time-distance helioseismology (Duvall & Gizon 2000)
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