Supergranulation is a particular pattern of
convection cell
In the field of fluid dynamics, a convection cell is the phenomenon that occurs when density differences exist within a body of liquid or gas. These density differences result in rising and/or falling currents, which are the key characteristics ...
s on the
Sun
The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
's surface called supergranules. It was discovered in the 1950s by A.B.Ha
using
Doppler effect, Doppler velocity measurements showing horizontal flows on the
photosphere
The photosphere is a star's outer shell from which light is radiated.
The term itself is derived from Ancient Greek roots, φῶς, φωτός/''phos, photos'' meaning "light" and σφαῖρα/''sphaira'' meaning "sphere", in reference to it ...
(flow speed about 300 to 500 m/s, a tenth of that in the smaller
granules). Later work (1960s) by Leighton, Noyes and Simon established a typical size of about 30000 km for supergranules with a lifetime of about 24 hours.
Origin
Supergranulation has long been interpreted as a specific convection
scale, but its origin is not precisely known. Although the presence of granules in the 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
A tributary, or affluent, is a stream or river that flows into a larger stream or main stem (or parent) river or a lake. A tributary does not flow directly into a sea or ocean. Tributaries and the main stem river drain the surrounding drainage ...
. It should however be stressed that this picture is highly speculative and might turn out to be false in the light of future discoveries. Recent studies
show some evidence that mesogranulation was a ghost feature caused by averaging procedures.
See also
*
Dopplergram
References
External links
a SOHO/MDI Dopplergram showing supergranular speed pattern* Michel Rieutord and François Rincon, "The Sun’s Supergranulation", Living Rev. Solar Phys. 7, (2010), 2
online article(cited on June 15, 2010).
Solar phenomena
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