supergranulation
supergranulation
(soo-per-gran-yŭ-lay -shŏn) A network of large-scale (30 000 km diameter) convective cells in the solar photosphere. The individual cells have an upward velocity of about 0.1 km s–1 and exhibit a horizontal flow of material, outward from the center, with a velocity of about 0.4 km s–1; their average lifetime is about one day (compare granulation). The space between adjacent cells is the preferred birthplace of sunspots, whose intense localized magnetic fields are dispersed by differential rotation and a cyclonic rotation of the cells, to form a weak polar field in both hemispheres.Supergranulation is not readily visible but may be detected in velocity-canceled spectroheliograms. (These are obtained by the photographic cancellation of two spectroheliograms taken simultaneously in the wings, i.e. on either side of the core, of a strong Fraunhofer line.) It is seen as alternate bright–dark elements away from the center of the Sun's disk, corresponding to a predominantly horizontal flow of material in the line of sight that has been rendered visible by its doubled Doppler shift. See also chromospheric network; spicules.