The Photosphere

The photosphere is the visible “surface” of the Sun, but is not a true or solid surface because the Sun is completely gaseous. Moving outward from the core of the Sun, the density, temperature, and gas pressure all decrease until, in a thin layer (only 400 kilometers thick), the material gradually changes from being completely opaque (light cannot pass through it) to being completely transparent.

Close inspection of the photosphere shows that it is not a uniformly bright layer, but has variations. It has a mottled or granular appearance, granulation, due to small regions of warmer and cooler material. The brighter regions are about 100 K hotter than surrounding regions and are rising at 2–3 km/s. These regions last only a few minutes before they dissipate and are replaced with other rising masses of warmer gas. Meanwhile, the surrounding cooler and darker regions are sinking back into the solar interior. This is direct observational evidence that the photosphere is also the top of a convective zone in the Sun (like the mass motion in a pot of boiling water that carries heat from the bottom to the top of the water). In the deeper interior, photons are able to move energy outward by slow diffusion in a radiative zone. Convection is far more efficient at moving energy, with energy passing outward through the solar convective zone (the outer 15% of the Sun) in a matter of hours. Photons, however, endure enormous numbers of collisions with nuclei and electrons and may take tens of thousands of years to work their way into the outer regions of the Sun. Larger areas of the photosphere may appear slightly brighter (warmer) or fainter (cooler) than other regions. This supergranulation indicates the existence of larger convective regions.

Within this layer also occur the sunspots, the most visible of photospheric features. Sunspots are temporarily cooler regions appearing as dark spots on the surface of the Sun. Sunspots appear dark only in contrast to surrounding hotter regions. In their centers, the temperature is lower by about 1,200 K, and hence their surface brightness is only one‐third that of nearby regions. A typical sunspot appears to have a dark center ( umbra) and lighter outer annulus ( penumbra) with radial striations caused by the magnetic field associated with the sunspot material. Direct measurement of surface magnetic fields shows that sunspot magnetism is stronger than the surroundings. Sunspots tend to appear in groups (up to 200,000 kilometers in diameter), usually developing rapidly in pairs, then fading away slowly over a few weeks. The leading sunspot in a pair has a magnetic polarity opposite to the trailing spot, with Southern Hemisphere pairs showing the north‐south polarity opposite to that of the Northern Hemisphere pairs.

Convection is a turbulent process and produces faculae, short‐lived hot jets of material that spurt outward into the layers outside the photosphere. Flares and eruptions, or brightenings of large regions lasting from minutes to hours, are associated with sunspots and again are a result of the convection. Such flares spew out large quantities of charged particles which, if they encounter the Earth four days later, cause distortions in the Earth's magnetic field (a geomagnetic disturbance).