Solar chromosphere
The solar chromosphere is a thin and dynamic layer of the Sun's atmosphere located directly above the photosphere, extending only a few thousand kilometers in thickness. It plays a crucial role in solar phenomena, exhibiting higher temperatures than the cooler photosphere, with temperatures rising from about 4,400 kelvins at the top of the photosphere to approximately 6,000 kelvins as one moves outward. Although the chromosphere is less dense than the photosphere, its intricate features, such as spicules and plages, are essential for understanding solar magnetic activity. Spicules, for instance, are vertical jets of material from the chromosphere, contributing significantly to the mass of the corona above.
The chromosphere is not typically visible due to the overwhelming brightness of the photosphere, but it can be observed during total solar eclipses or through specialized instruments like coronagraphs. This layer exhibits distinctive emission lines, particularly the H-alpha line, which gives it a reddish color. Observations of the chromosphere also provide insight into the Sun's magnetic activity, which can influence Earth's climate by affecting solar luminosity. Understanding the chromosphere is vital for comprehending the interconnected nature of solar layers and their impact on both solar and terrestrial phenomena.
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Solar chromosphere
The solar chromosphere is the layer of the Sun’s atmosphere, a few thousand kilometers thick, immediately above the photosphere. The gas is warmer, thinner, and more transparent than in the photosphere. The chromosphere is most directly visible from Earth as a layer of color during a total solar eclipse. Its spectrum shows bright emission lines, unlike the dark absorption lines of the photosphere. The element helium was first discovered in the Sun’s chromosphere during a total solar eclipse.
Overview
The Sun is a ball of gas without a solid surface. The Sun’s photosphere, however, is usually considered to mark the surface of the Sun. Relatively opaque, the photosphere blocks our view of the solar interior. It is also the coolest layer of the Sun. When astronomers observe the solar spectrum, they see dark absorption lines from the photosphere because the photosphere is cooler than the hot solar interior.
The chromosphere is the layer of the Sun’s atmosphere directly above the photosphere. The chromosphere is only a few thousand kilometers thick. The temperature of the gas in the chromosphere is slightly higher than that in the photosphere. The bottom layer of the photosphere is at 6,600 kelvins, but the coolest level at the top of the photosphere is at 4,400 kelvins. For reasons that are not completely understood, the temperature in the chromosphere climbs from this low to about 6,000 kelvins as the distance from the photosphere increases. The chromosphere has a lower density than the photosphere; its density decreases with height above the photosphere from nearly 10-5 kilograms/meter3 at the photosphere-chromosphere boundary to about 10-10 kilograms/meter3 in the transition region between the chromosphere and the corona.
The layer of the Sun’s atmosphere above the chromosphere is the corona. In the approximately 100-kilometer-thick transition region between the chromosphere and corona, the temperature rapidly increases with height above the photosphere, from about 6,000 kelvins to a few hundred thousand kelvins. The temperature in the corona can be millions of kelvins, but the gas is very thin. In the transition region, the density drops rapidly to about 10-12 kilograms/meter3.
Because the chromosphere is so thin and transparent compared to the photosphere, it cannot be seen when the photosphere is visible. The much brighter photosphere overwhelms the fainter chromosphere. The chromosphere is briefly visible during total solar eclipses. When the Moon blocks light from the photosphere, the chromosphere shines briefly until the Moon also blocks the chromosphere. The chromosphere is visible only briefly, just before and just after totality. It is also possible to see the chromosphere using a coronograph, which is a disk in the focal plane of the telescope that blocks the photosphere to reveal the chromosphere and corona.
A dark, or absorption line, spectrum is produced when light from a hot, compressed gas passes through a cooler, thin gas. Therefore, the Sun’s photosphere produces an absorption line spectrum. A bright, or emission line, spectrum results from a hot, thin gas. When astronomers observe the Sun’s chromospheric spectrum, they are observing the chromosphere at the edge of the Sun, so the hotter interior is not directly behind the chromosphere in their line of sight. Hence, they observe emission lines. These emission lines flash into view for a short time just before and after the total portion of the eclipse. The chromospheric spectrum thus revealed is therefore called the flash spectrum.
The Sun is mostly hydrogen, and the brightest emission line in the chromospheric spectrum is the red hydrogen H-alpha line. This red emission line gives the chromosphere its reddish-pinkish color. The color also gives the chromosphere its name; chromo comes from the Greek word for color. Two spectral lines from the element calcium, the H and K lines, are also prominent in the chromospheric spectrum.
Astronomers use the H-alpha line and the H and K lines of calcium to observe the surface of the chromosphere. The chromosphere is relatively transparent at most wavelengths, but it is fairly opaque at the wavelengths of these spectral lines. Hence, observing the Sun at these wavelengths allows astronomers to image the chromosphere rather than the photosphere. The light at these wavelengths originates from different depths in the chromosphere, so comparing images made at the different wavelengths gives astronomers a three-dimensional image of the Sun’s surface. Lyot filters made of calcite crystals are best for this purpose, but they are very expensive. Interference filters are less expensive, and H-alpha interference filters are available at a reasonable cost to allow amateur astronomers to observe the Sun safely with small telescopes.
Just below the photosphere, the Sun transfers energy from the interior via convection currents. These convection currents produce structures on the surface of the photosphere, called granules, that are the tops of the convection current cells. Faculae are bright areas on the solar photosphere that are often found in the lower areas, marking the boundaries between the photospheric granules. When faculae extend upward from the photosphere into the chromosphere, they are called plages. Plages are, therefore, brighter than normal regions of the Sun’s chromosphere. Plage regions can have opposite magnetic polarities, so magnetic field lines flow outward from one plage and connect to another plage where they flow back into the Sun’s interior. Solar material often streams along these magnetic field lines to produce prominences. Prominences begin and end in the chromosphere but extend well out into the corona.
Perhaps the most important feature of the chromosphere is the spicule. The chromosphere is covered with spicules. They are vertical streams of chromospheric material moving upward into the corona. The total mass of material moving upward into the corona from spicules is approximately the mass of the entire corona every few minutes. Because the mass of the corona is not rapidly increasing and the solar wind does not have that much mass, astronomers know that most of this material must also be falling back into the chromosphere. However, astronomers are not able to observe the falling material. These spicules form a chromospheric bright network along the boundaries of large supergranules on the Sun’s photosphere that is related to the Sun’s magnetic activity.
The Sun’s bright magnetic network may play a role in climate changes on Earth. When the Sun is at the maximum of its sunspot cycle and has the largest portion of its surface covered by sunspots, it has a very slightly higher energy output than when it is at the sunspot minimum. Even though sunspots tend to reduce the Sun’s energy output, brighter areas such as faculae (which become plages when they extend to the chromosphere) and the bright chromospheric magnetic network more than compensate for the energy loss caused by the darker sunspots. During the prolonged Maunder minimum of sunspot activity in the late seventeenth century, Earth’s climate experienced its coldest period of the last millennium. From about 1100 to 1250, there was a medieval grand maximum of sunspot activity, corresponding to a warmer-than-normal period on Earth. Historical climate evidence suggests that solar luminosity variations caused by the Sun’s magnetic activity affect Earth’s climate. Sunspots and faculae on the photosphere and plages and the bright magnetic spicule network on the photosphere all play a role in changing the Sun’s energy output.
Knowledge Gained
During a solar eclipse in 1686, not long after the invention of spectroscopy, Jules Janssen, working in France, observed bright emission lines from the chromosphere. One line was very close to the wavelength of a well-known pair of yellow emission lines from the element sodium. This line was so bright that Janssen was able to study it in detail even when there was no eclipse. At about the same time, Norman Lockyer, from Britain, also observed this bright line outside eclipses. Further detailed study revealed that this newly discovered spectral line was not quite the right wavelength to be sodium. Furthermore, it did not match the wavelengths of lines from any known element at the time. This newly discovered element was named helium after the word helios, for Sun. In 1895, helium was finally isolated on Earth.
As the closest star to us, our Sun is the best-studied star and the only star for which it is possible to study surface details. Studying the solar chromosphere as well as the Sun’s other regions in detail reveals much about the atmospheres of other stars. A more complete understanding of stellar atmospheres, in turn, helps astronomers fine-tune their stellar models and improve their understanding of all aspects of stars.
One of the enduring mysteries about the Sun is why the temperature increases outward in the chromosphere and the corona. The Solar and Heliospheric Observatory (SOHO), launched in 1995, shed light on that question when it observed a magnetic carpet on the Sun’s surface. About four thousand magnetic field lines loop up daily from the Sun’s interior and then back down into the interior. Because these loops resemble the loops in a carpet, this phenomenon has been called the Sun’s magnetic carpet. When these loops burst from the Sun’s turbulence, they release energy to heat the outer layers of the Sun’s atmosphere.
Study of the Sun’s chromosphere continued in the twenty-first century. In June 2022, the Daniel K. Inouye Solar Telescope, located on the summit of Haleakala, Maui, in Hawai'i, took new and exciting images of the Sun’s chromosphere that are believed to provide enough data to usher in a new era of solar discovery and advance scientists’ understanding of the Sun.
Context
Although the chromosphere is a relatively thin layer of the Sun’s atmosphere, it is quite important to us. It is not possible to understand the photosphere below the chromosphere or the corona above it without understanding the chromosphere. For example, plages in the chromosphere are directly related to faculae in the photosphere. Spicules in the chromosphere extend vertically upward into the lower portions of the corona. Hence, photospheric, chromospheric, and coronal phenomena are all interconnected.
The chromosphere plays a role in solar magnetic activity. Interactions between solar magnetic storms and Earth’s magnetosphere can affect auroral activity on Earth and, in the case of strong magnetic storms, can affect radio communications on Earth.
The chromosphere and chromospheric phenomena play an important role in the Sun’s magnetic activity cycle. If the Sun’s luminosity changes with its activity cycle, the chromosphere likely played a role in past climate changes on Earth. It has not been proven, but some scientists think that solar variations may also play a contributing role in Earth’s warming. To understand this possible role in Earth’s climate fully, scientists must have a fuller understanding of the Sun’s chromosphere.
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