Supergranulation
Encyclopedia
Supergranulation is a particular pattern on the Sun
's surface. It was discovered in the 1950s by A.B.Hart using Doppler
velocity measurements showing horizontal flows on the photosphere
(flow speed about 300 to 500 m/s).
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.
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
. It should however be stressed that this picture is highly speculative and might turn out to be false in the light of future discoveries.
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
's surface. It was discovered in the 1950s by A.B.Hart using Doppler
Doppler effect
The Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842 in Prague, is the change in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from...
velocity measurements showing horizontal flows on the photosphere
Photosphere
The photosphere of an astronomical object is the region from which externally received light originates. The term itself is derived from Ancient Greek roots, φῶς, φωτός/phos, photos meaning "light" and σφαῖρα/sphaira meaning "sphere", in reference to the fact that it is a spheric surface perceived...
(flow speed about 300 to 500 m/s).
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.
Supergranulation has long been interpreted as a specific convection scale, but its origin is not precisely known. Although the presence of granules
Granule (solar physics)
Granules on the photosphere of the Sun are caused by convection currents of plasma within the Sun's convective zone. The grainy appearance of the solar photosphere is produced by the tops of these convective cells and is called granulation.The rising part of the granules is located in the center...
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
Tributary
A tributary or affluent is a stream or river that flows into a main stem river or a lake. A tributary does not flow directly into a sea or ocean...
. 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
- Michel Rieutord and François Rincon, "The Sun’s Supergranulation", Living Rev. Solar Phys. 7, (2010), 2. online article (cited on June 15, 2010).
