This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of Physics. Please check back later for the full article.
The sun, the nearest star to us and the biggest object in our solar system, serves as a reference for fundamental astronomical parameters such as stellar mass, luminosity, and elemental abundances. It also serves as a plasma physics laboratory. A great deal of our understanding of the sun comes from its electromagnetic radiation, which is close to that of a blackbody in the near UV, visible, and infrared, peaking in the visible spectral range at around 5000 Å. The bulk of this radiation escapes from the solar surface, from a layer that is a mere 100 km thick (corresponding roughly to the local pressure scale-height). This surface from where the photons escape into the heliosphere and beyond, as well as the roughly 400–500 km thick lowest atmospheric layer above it (where the temperature keeps dropping outward), is termed the solar photosphere.
Observations of the solar photosphere led to some important discoveries in modern-day astronomy and astrophysics. At low spatial resolution the photosphere, with an effective temperature of 5780 K, is nearly featureless, but observations made in the early 1600s have shown there to be blemishes or spots extending more than 10,000 km at the surface. Continued observations of these sunspots later revealed that they appear and disappear with a period of about 11 years, and that they actually are due to the magnetic field (representing the first account of an extraterrestrial magnetic field). This established the presence of magnetic cycles on the sun responsible for observed periodic solar activity with geological implications (e.g., geomagnetic storms, aurorae). Such magnetic activity is now known to exist in other stars as well.
Superimposed on the solar blackbody spectrum are numerous spectral lines from different atomic species that arise due to the absorption of photons at certain wavelengths by those atoms, in the cooler photospheric plasma overlying the solar surface. These spectral lines provide diagnostics of the properties and dynamics of the underlying plasma (e.g., the granulation due to convection and the solar p-mode oscillations) and of the solar magnetic field. Since the early 20th century, researchers have used these spectral lines and the accompanying polarimetric signals to decode the physics of the solar photosphere and its magnetic structures, including sunspots. Modern observations with high spatial (0.1 arcsec, corresponding to 70 km on the solar surface) and spectral (10 mÅ) resolutions reveal a tapestry of the magnetized plasma with structures down to tens of kilometers at the photosphere (three orders of magnitude smaller than sunspots). Such observations combined with advanced numerical models provide further clues to the very important role of the magnetic field in solar and stellar structures, and the variability in their brightness. Being the lowest directly observable layer of the sun, the photosphere is also a window into the solar interior by means of helioseismology, which makes use of the p-mode oscillations. Furthermore, being the bottom layer of the solar atmosphere including the extended corona (invisible to the naked eye except during a total solar eclipse), the photosphere provides key insights into another long-standing mystery, that after the temperature-minimum (~500 km above the surface), the plasma is heated to temperatures up to1,000 times higher than at the visible surface. The physics of the solar photosphere is thus central to our understanding of many solar and stellar phenomena.