Granule (solar physics)

Solar photosphere, with North America superimposed for scale. Each black or white bar on the graphic represents 1000 km.

Granules on the photosphere of the Sun are caused by convection currents (thermal columns, Bénard cells) 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 where the plasma is hotter. The outer edge of the granules is darker due to the cooler descending plasma. In addition to the visible appearance, which would be explained by convective motion, Doppler shift measurements of the light from individual granules provides evidence for the convective nature of the granules.

A typical granule has a diameter on the order of 1,500 kilometers^[1] and lasts 8 to 20 minutes before dissipating. At any one time, the Sun's surface is covered by about 4 million granules. Below the photosphere is a layer of "supergranules" up to 30,000 kilometers in diameter with lifespans of up to 24 hours.

References

The Sun Internal structure Core Radiation zone Tachocline Convection zone Atmosphere Photosphere Supergranulation Granule Faculae Sunspot Chromosphere Plage Spicule Moreton wave Corona Transition region Coronal hole Coronal loop Coronal mass ejection Prominence Helmet streamer Variation Solar cycle List of solar cycles Solar maximum Solar minimum Wolf number Solar wind Flare Helioseismology Heliosphere Current sheet Termination shock Heliosheath Heliopause Bow shock Related topics Solar activity Solar astronomy Solar dynamo Eclipse Solar energy Solar physics Solar System Solar telescope Solar time Radiation Spectral class: G2V