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The photosphere is the Sun's visible surface, the region with which we are most familiar. This gaseous layer is only about 100 km deep – extremely thin compared to the 700,000 km radius of the Sun. It appears darker towards the limb, or edge of the visible disk. This limb darkening is the result of looking at the cooler and dimmer regions of the solar disk, whereas the observer looks straight down toward the hotter, brighter regions in the center of the disk.

With a temperature of only 5,700°C, the photosphere is cool enough for molecules to form, so it mostly consists of neutral (not ionized) gas. Most of the spectral Fraunhofer lines are formed here.

The most easily recognized features in the photosphere are dark sunspots, which occur singly or in groups. They grow and then fade over days or weeks. However, for reasons not yet understood, the sunspots increase in overall number, decrease, and then return to a peak every 11 years or so. This is called the sunspot cycle.

Convectional cells, found all over the photosphere, are marked by small, cell‐like granules. About 1,000 km across, they show where gas is rising, cooling, and then sinking – similar to motions in a pan of boiling water. The granules are short‐lived, lasting for only about 20 minutes before they are replaced by new, upwelling cells. The flow of gas within the granules can reach speeds exceeding 7 km/s, producing sonic “booms” and other noise that generates surface waves.

By measuring the Doppler effect in the solar spectrum, it is possible to measure the motion of material in the photosphere. The measurements show that much larger scale convectional motion occurs over the entire photosphere, creating supergranules about 35,000 km across. Individual supergranules survive for a day or two and have flow speeds of about 0.5 km/s.

Figure 2.14 A detailed view of convection cells, or granulation, in the photosphere, taken by Japan's Hinode spacecraft. The lighter areas show where gas is rising from below, while the darker, intergranular lanes reveal where cooler gases are sinking.

(JAXA/NASA/PPARC)

Also visible are small, bright regions, known as faculae, which are often associated with sunspots. Faculae occur where strong magnetic fields greatly reduce the local density of the gas. The low density makes it nearly transparent, so the lower levels of granules are more easily visible. At these deeper layers, the gas is hotter and radiates more strongly, explaining the brightening.