The Photosphere - the "Surface" of the Sun

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 (65 KB JPEG). 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.

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.