Supergranulation
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In
solar physics Solar physics is the branch of astrophysics that specializes in the study of the Sun. It intersects with many disciplines of pure physics and astrophysics. Because the Sun is uniquely situated for close-range observing (other stars cannot be re ...
and
observation Observation in the natural sciences is an act or instance of noticing or perceiving and the acquisition of information from a primary source. In living beings, observation employs the senses. In science, observation can also involve the percep ...
, supergranulation is a pattern of
convection cell In 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 convection currents, which are the key characteristics o ...
s in the Sun's
photosphere The photosphere is a star's outer shell from which light is radiated. It extends into a star's surface until the plasma becomes opaque, equivalent to an optical depth of approximately , or equivalently, a depth from which 50% of light will esc ...
. The individual convection cells are typically referred to as supergranules. The pattern was discovered in the 1950s by A.B. Hart using
Doppler The Doppler effect (also Doppler shift) is the change in the frequency of a wave in relation to an observer who is moving relative to the source of the wave. The ''Doppler effect'' is named after the physicist Christian Doppler, who described ...
velocity measurements showing horizontal flows on the
photosphere The photosphere is a star's outer shell from which light is radiated. It extends into a star's surface until the plasma becomes opaque, equivalent to an optical depth of approximately , or equivalently, a depth from which 50% of light will esc ...
(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 an ''affluent'', is a stream or river that flows into a larger stream ('' 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 into which the ...
. 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). {{The Sun Solar phenomena