Solar photosphere

The solar photosphere is the visible surface of the Sun. Because it is opaque, it is not possible to see the Sun’s interior layers directly. Convection currents transport energy from the interior through the photosphere, and this convection, along with magnetic activity, causes surface features such as granules and sunspots. Solar magnetic activity may affect Earth’s climate.

Overview

The Sun is a ball of gas and, as such, has no solid surface. The Sun’s photosphere approximates its surface, marking the region between the solar interior and the chromosphere (the region that approximates the solar “atmosphere”). Because the photosphere is relatively opaque, it blocks any view of the solar interior. As a result, most images of the solar disk show only the photosphere. The photosphere is also the coolest layer of the Sun, with a temperature of about 6,600 kelvins near its bottom and a temperature of about 4,400 kelvins near the chromosphere at the top; in the 400 kilometers between, the photosphere’s temperature gradually decreases with altitude above the solar interior. Astronomers know that the photosphere is relatively cool, because it displays absorption lines that are darker than the spectrum of the hot solar interior. The temperature of the overlying chromosphere is likewise hotter, climbing to about 30,000 kelvins as the distance from the photosphere increases. Where the chromosphere ends is the corona, and in the approximately 100-kilometer-thick transition between the chromosphere and corona, the temperature rapidly increases from about 6,000 kelvins to several hundred thousand kelvins. In the corona itself, the temperature can be millions of kelvins, but the gas is very thin.

Looking at a picture of the solar disk, one can notice that the edge of the disk appears darker than the central portions. This illusion is called limb darkening. In the central portion of the solar disk, one sees light from the deepest and hottest layer of the photosphere. Near the Sun’s edge—its limb—only the upper, cooler layers of the photosphere are visible because the very edge of the solar disk does not have the base of the photosphere in our line of sight behind the top layers. The cooler upper layers of the photosphere emit less energy than the hotter, deeper layers, so the limb of the solar disk appears darker. It is not really dark; it is just less bright. Limb darkening, therefore, indicates that the lower layers of the photosphere are hotter than the upper layers.

Because it is cooler than the hot, compressed gas of the solar interior, the Sun’s photosphere produces an absorption line spectrum (a continuous spectrum with dark absorption lines superimposed on it at certain wavelengths). In 1814, Joseph von Fraunhofer observed the solar spectrum and—because he was unable to identify the elements producing the various spectral absorption lines he observed—labeled the prominent lines using capital letters.

The most obvious features of the Sun’s photosphere are the granules, which give the photospheric surface a mottled light and dark appearance. Granules are bright regions that vary in size but are typically about 1,000 kilometers across. They also are temporary features, typically lasting from about five to thirty minutes. Granulation results from convection currents below the photosphere.

Nuclear reactions powering the Sun take place in the core, and the energy must be transferred from the core to the surface. In the deep interior of the Sun, the energy is transferred by radiation. In the upper portions of the interior, just below the photosphere, the energy is transferred by convection currents. The solar convection currents are similar to the convection currents that heat a room if there is a radiator on one side and no fan to blow the warm air to the other side. These solar convection currents form convection cells, regions where the hot gas from the interior flows up and then flows back down after it cools. The granules that we observe are the tops of these convection cells. They appear brighter because they are still hot from the Sun’s interior energy.

Supergranules are larger versions of granules. They might typically be about 35,000 kilometers across. A typical supergranule might last one or two days, much longer than the typical lifetime of a granule. Rather than being observed directly, like granules, supergranules are typically observed from Doppler maps of the Sun’s surface. The upward-moving material is moving toward Earth, and hence toward the observer, so the wavelengths of common spectral lines are blueshifted to slightly shorter (higher-energy) wavelengths and hence appear brighter.

Granules are part of what astronomers call the “quiet Sun,” which comprises the solar features that are always present. Features that are present or more common only during the maximum of the solar magnetic activity cycle comprise the “active Sun.” Features of the active Sun found in the photosphere are sunspots and faculae.

Sunspots are dark areas on the Sun’s photosphere. Sunspots appear dark because they are about 2,000 kelvins cooler than the rest of the photosphere. Although this temperature is still quite hot, sunspots are nevertheless cool when compared to the background of the rest of the photosphere. Large sunspots can extend to tens of thousands of kilometers, larger than Earth but still very small compared to the Sun’s size. Sunspots will typically cover less than about 1 percent of the Sun’s photosphere.

Magnetograms of the solar surface measure the magnetic field strength at points across the Sun’s surface. Magnetograms show that sunspots are regions of intense magnetic fields, with magnetic field strengths up to a few thousand times stronger than the rest of the Sun’s surface. Sunspots are darker and cooler than the rest of the photosphere because these strong magnetic fields conspire to deflect the convection currents, bringing heat energy from the interior to the photosphere. The reduced heat flowing from the interior causes lower temperatures.

Faculae are regions on the photosphere that are hotter and, therefore, brighter than the rest of the photosphere. They might be thought of as the opposite of sunspots; like sunspots, faculae are regions of strong magnetic fields, but for faculae, the magnetic fields concentrate, rather than deflect, the energy from the interior. Faculae form in regions surrounding sunspots and in lower boundaries between the elevated granules. When faculae extend up into the chromosphere, they are called plages.

The numbers of both sunspots and granules wax and wane in an eleven-year sunspot cycle. This eleven-year cycle in the number of spots is actually half of the Sun’s twenty-two-year magnetic activity cycle. Sunspots form in groups that contain leading and following spots. The magnetic polarities (north or south magnetic poles) of the leading and following spots in a group interchange each eleven-year cycle to produce a twenty-two-year magnetic activity cycle. The last sunspot maximum occurred around 2013 during solar cycle 24.

Knowledge Gained

The Sun’s total energy output, or luminosity, is largely the energy from the photosphere. Hence, if the photosphere’s brightness changes, the Sun’s total energy output and the solar radiation reaching Earth also change. Variations in the Sun’s luminosity could cause, and very likely have caused, climate changes on Earth. Changes in the Sun’s luminosity are so small that, from using ground-based measurements, it is difficult (but not impossible) to measure the Sun’s luminosity accurately enough to measure those variations. With the advent of space-based observatories—which eliminated the need to correct for the amount of light absorbed by Earth’s atmosphere—it became possible to measure the Sun’s luminosity more easily and accurately. Beginning in the late 1970s, satellite data show that when there is a maximum number of sunspots, the Sun is a very small amount more luminous than at the sunspot minimum.

These data, when combined with the long-term sunspot record, suggest that changes in the Sun’s energy output may have caused the Little Ice Age in the seventeenth century. The Little Ice Age was a period of more than two centuries that was colder than normal, and that coincided with a prolonged period of virtually no sunspot activity from about 1645 to about 1715 CE. There was also a prolonged medieval grand maximum in sunspot activity from about 1100 to about 1250 CE. This period was the warmest of the past millennium. It is likely, therefore, that changes in the amount of magnetic activity have affected Earth’s climate at other times in the past. It is also possible that such changes may be a contributing factor to global climate change. The Sun has been fairly active since the mid-1700s, except for minor, brief decreases in sunspots in the early 1800s and in the late 1800s.

Context

The Sun’s energy is crucial to life on Earth. Without it, we would not survive. Hence, we want to understand the Sun and all its components, including the photosphere. As the visible disk of the Sun that we see, the photosphere is the immediate source of this energy. Changes in the photospheric energy output could have drastic effects on life on Earth and may even affect climate change.

Visible light, ultraviolet light, infrared light, radio waves, X-rays, and gamma rays are all forms of electromagnetic waves. The significance of visible light is that it is the region of the electromagnetic spectrum that is detectable by the human eye. This range is determined by the Sun’s photosphere. With a temperature of 5,800 kelvins, the photosphere is brightest at the wavelength region from red to blue, peaking at yellow. This is also the wavelength of peak sensitivity of the human eye and most animal eyes. Our eyes evolved to detect the wavelength region of the electromagnetic spectrum that is most plentiful. Scientists continued to study the Sun and its layers into the twenty-first century to provide new information regarding its outer layers, such as the photosphere, as well as its interior composition. Launched in 2018, the Parker Solar Probe flew through the Sun’s outer atmosphere in 2021 providing scientists with new data.

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