Granulation of the Photosphere
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 adjacent right figure , which is a digitally enhanced black-and-white image,
shows such
granulation. (Also visible are two dark sunspots near the center.) Although
the granules look tiny on this scale, they can be a thousand kilometers in diameter
(the size of a large state), and each individual
one lasts for only a few minutes.
The image below left is a highly enlarged region
near a sunspot showing granulation below and to the left of the dark sunspot.
Cause of Granulation
Granulation is caused by the convection operating below the photosphere.
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, as confirmed by Doppler shift measurements.
The tops of these columns are the brighter gray-white cells seen in the
granulation images; they are brighter because they are hotter than the surrounding region.
The hot gas then cools at the top of the column and sinks
down in the darker regions surrounding each granule. Thus, granules are very much like
rising bubbles in boiling water.
Supergranulation
In addition to the granule structure, there is a larger scale structure that organizes the
solar surface into supergranules perhaps 30,000 km in diameter on average and containing
hundreds of granules. Granules are the size of a large state but supergranules are
typically several times the size of the Earth. This supergranulation appears to be
associated with very slow rising currents and the supergranules have lifetimes of about a day.
Although not completely understood, it is
thought that the supergranules may be traces of gas flow currents below the surface
on scales larger than those responsible for
granulation.