Supergranulation

(astronomy) A system of convective cells, with typical diameters of 12,000 miles (20,000 kilometers), that cover the sun's surface.

A system of convective cells, with typical diameters of 20,000 km (12,000 mi), that cover the Sun's surface. Solar convection cells are invisible in ordinary photographs.

High-resolution photographs of the Sun's visible surface reveal the granulation, a closely packed cellular grid having bright (hot) centers surrounded by dark (cool) lanes. Granules have lifetimes of 10–30 minutes and average diameters of 1000 km (620 mi). In the 1930s, L. Biermann suggested that, in the outer 30% of the Sun, heat from the Sun's interior is transported to the surface by convection, just like hot rising bubbles in a pot of boiling soup heated from below. The granules are the boiling bubbles (plumes) of this convective process. Theorists in the 1950s proposed that the bubble sizes are approximately equal to the local scale height H (the distance in which density or pressure changes by a factor e ≈ 2.7). Since H varies from about 1000 km (620 mi) at the Sun's surface to 100,000 km (62,000 mi) at the base of the convection zone, a very large range of plume sizes was hypothesized. See also Convection (heat).

In 1959, R. Leighton modified the spectroheliograph to image the Sun using the Doppler and Zeeman effects. Images of line-of-sight (approaching and receding) gas motions (Dopplergrams) and of magnetic fields (magnetograms) could be obtained. The Dopplergrams showed a new cell structure with area 400 times that of the granulation, having a mainly horizontal flow pattern. Lifetimes of most supergranules are 1–2 days, but observations in 1998 with the Michelson Doppler Imager (MDI) on the Solar and Heliospheric Observatory (SOHO) spacecraft showed that some live longer than 4 days. The cells form an irregular polygonal structure, with most of the down-flow occurring at the polygon vertices. The supergranules fit neatly within an essentially identical network (grid) structure seen in magnetograms. The kinetic energy of the supergranular motions at the Sun's surface exceeds the magnetic field's ability to resist such motions, and thus the magnetic field is dragged forcibly to the supergranule boundaries until the two patterns coincide. This magnetic field, in turn, causes local heating of the upper solar atmosphere (chromosphere), producing a similar chromospheric network pattern seen in high-temperature spectral lines. See also Doppler effect; Fraunhofer lines; Solar magnetic field; Spectrograph; Spectroheliograph; Sun; Zeeman effect.

Supergranulation is a particular pattern on the Sun surface. It was discovered in the 1950s by A.B.Hart on Doppler velocity measurements showing horizontal flows on the photosphere (flow speed about 300 to 500 m/s).

Later works (1960's) by Leighton, Noyes and Simon established a typical size of about 30000 km for supergranules with a lifetime of about 24 hours. Supergranulation has long been interpreted as a specific convection scale, however its origin is not precisely known. As long as the presence of granules in solar photosphere is a well documented phenomenon, there is still much debate on the true nature or even the existence of higher-order granulation patterns. Some authors suggest the existence of three distinct scales of organization: granulation (with typical diameters of 150-2500 km), mesogranulation (5000-10000 km) and supergranulation (over 20000 km). Granules are typically considered as being signs of convective cells forming a hierarchic structure: supergranules would be thus fragmented in their uppermost layers into smaller mesogranules, which in turn would split into even smaller granules at their surface. The solar material would flow downward in dark "lanes" separating granules with the divisions between supergranules being the biggest concentrations of cold gas, analogous to rivers connecting smaller tributaries. It should however be stressed that this picture is highly speculative and might turn out to be false in the light of future discoveries.

External links

• a SOHO/MDI Dopplergram showing supergranular speed pattern
• NASA: The Sun Does The Wave
• Information at Nature.com

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