Granule (solar physics)
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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, Doppler shift measurements of the light from individual granules provides evidence for the convective nature of the granules.
The diameter of a typical granule is on the order of 1000km and lasts 8 to 20 minutes before dissipating. Amid the typical granules are supergranules up to 30,000 kilometers in diameter with lifespans of up to 24 hours.
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