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