The photosphere is the visible "surface" of the Sun (left). Sunspots are often visible "on" the photosphere. A close-up view (right) shows the granulation pattern on the photosphere.
Click on image for full size
Images courtesy of SOHO/NASA/ESA and The Royal Swedish Academy of Sciences and Oddbjorn Engvold, Jun Elin Wiik, and Luc Rouppe van der Voort - University of Oslo.
The Photosphere - the "Surface" of the Sun
Most of the energy we receive from the Sun is the visible (white) light emitted from the photosphere. The photosphere is one of the coolest
regions of the Sun (6000 K), so only a small fraction (0.1%) of the gas is
ionized (in the plasma state). The photosphere is the densest
part of the solar atmosphere, but is still tenuous compared to
Earth's atmosphere (0.01% of the mass density of air at sea level).
The photosphere looks somewhat boring
at first glance: a disk with some dark spots. However, these
sunspots
are the site of strong magnetic fields. The solar magnetic field is believed to drive the complex
activity seen on the Sun.
Magnetographs measure the solar magnetic field at the photosphere.
Because of the tremendous heat coming from the solar core, the solar interior below
the photosphere (the convection zone) bubbles like a pot of boiling water.
The bubbles of hot material welling up from below are seen at the photosphere
as slightly brighter regions. Darker regions occur where cooler plasma
is sinking to the interior. This constantly churning pattern of convection
is called the solar granulation pattern.
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