Granulation of
the Photosphere

Granulation near sunspot group
The photosphere under close observation exhibits a mottled appearance that is called granulation. This is a consequence of heat convection below the photosphere.

Observation of Granulation

The image adjacent right shows such granulation as the mottled gray regions surrounding a darker sunspot group (click on the image to display a larger version that shows the granulation more clearly). The image below left is a highly enlarged region near a sunspot showing granulation above and to the left of the dark sunspot.

Granulation region

Cause of Granulation

Granulation is due to the convection operating below the photosphere that we already mentioned in the section on the solar interior. This convection produces columns of rising gas just below the photosphere that are about 700 to 1000 km in diameter.

In these columns hot gas rises with a velocity of several kilometers per second (this can be confirmed by Doppler shift measurements). The tops of these columns are the brighter gray-white cells seen in the granulation images. The hot gas then cools at the top of the column and sinks down in the darker regions surrounding each granule.

Thus, granulation just represents the tops of convection currents that are transferring heat from below the solar surface to the surface. In that sense, granules are a little like the tops of cumulus clouds in the Earth's atmosphere, which are also associated with convection currents.


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