The Chromosphere

The chromosphere represents the dynamic transition between the cool temperature minimum of the outer photosphere and the diffuse million-degree corona above. It derives its name and pink colour from the red Ha line of hydrogen at 6562.8 angstroms (A); 1 A = 10-10 metre. Because this line is so strong, it is the best means for studying the chromosphere. For this reason special monochromators are widely used to study the Sun in a narrow wavelength band. Because density decreases with height more rapidly than magnetic field strength, the magnetic field dominates the chro-mospheric structure, which reflects the extension of the photospheric magnetic fields. The rules for this interplay are simple: Every point in the chromosphere where the magnetic field is strong and vertical is hot and hence bright, and every place where it is horizontal is dark. Supergranulation, which concentrates

Active region toward the limb of the Sun, with spicules (right) and some sunspots (upper left). Image captured on June 16, 2003, by the Swedish Solar Telescope, La Palma, Spain. Lockheed Martin/Solar and Astrophysics Lab the magnetic field on its edges, produces a chromospheric network of bright regions of enhanced magnetic fields.

The most prominent structures in the chromosphere, especially in the limb, are the clusters of jets, or streams, of plasma called spicules, which occur at the edges of the chromospheric network, where magnetic fields are stronger. Spicules extend up to 10,000 km (6,000 miles) above the surface of the Sun. They rise from the lower chromosphere at about 20 km/sec (12 miles/sec) to a height of several thousand kilometres, and then within 10-15 minutes they disperse or collapse. About 100,000 spicules are active on the Sun's surface at any given time. Although they are invisible in white light, early observers could see them in the hydrogen alpha (Ha) emission line with a spectrograph, comparing them to a "burning prairie."

Because it strongly emits the high-excitation lines of helium, the chromosphere was originally thought to be hot. But radio measurements, a particularly accurate means of measuring the temperature, show it to be only 8,000 K (7,700°C, 13,900°F), somewhat hotter than the photosphere. Detailed radio maps show that hotter regions coincide with stronger magnetic fields. Both hot and cold regions extend much higher than one might expect, tossed high above the surface by magnetic and convective action.

When astronomers observe the Sun from space at ultraviolet wavelengths, the chromosphere is found to emit lines formed at high temperatures, spanning the range from 10,000 to 1 million K (9,700 to 999,700°C, 17,500 to 1.8 million °F). The whole range of ionization of an atom can be found. For example, oxygen I (neutral) is found in the photosphere, oxygen II through VI (one to five electrons removed) in the chromosphere, and oxygen VII and VIII in the corona. This entire series occurs in a height range of about 5,000 km (3,000 miles). An image of the corona obtained at ultraviolet wavelengths has a much more diffuse appearance as compared with lower temperature regions, suggesting that the hot material in the magnetic elements spreads outward with height to occupy the entire coronal space. Interestingly, the emission of helium, which was the original clue that the temperature increased upward, is not patchy but uniform. This occurs because the helium atoms are excited by the more diffuse and uniform X-ray emission from the hot corona.

The structure of the chromosphere changes drastically with local magnetic conditions. At the network edges, clusters of spicules project from the clumps of magnetic field lines. Around sunspots, plages occur, where there are no spicules,

Revelations in the Dark

When flash spectra (spectra of the atmosphere during an eclipse) were first obtained, astronomers found several surprising features. First, instead of absorption lines they saw emission lines (bright lines at certain wavelengths with nothing between them). This effect arises because the chromosphere is transparent between the spectrum lines, and only the dark sky is seen. Second, they discovered that the strongest lines were due to hydrogen, yet they still did not appreciate its high abundance. Finally, the next brightest lines had never been seen before. Because they came from the Sun, the unknown source element came to be called helium. Later helium was found on Earth.

but where the chromosphere is generally hotter and denser. In the areas of prominences the magnetic field lines are horizontal and spicules are absent.

The Corona

Another important set of unknown lines, revealed during an eclipse, come from the corona, and so its source element was called coronium. In 1940 the source of the lines had been identified as weak magnetic dipole transitions in various highly ionized atoms such as iron X (iron with nine electrons missing), iron XIV, and calcium XV, which can exist only if the coronal temperature is about 1 million K. These lines can only be emitted in a high vacuum. The strongest are from iron, which had alerted investigators to its high abundance, nearly equal to that of oxygen. Later errors in prior photospheric determinations had been discovered.

While the corona is one million times fainter than the photosphere in visible light (about the same as the full Moon at its base and much fainter at greater heights), its high temperature makes it a powerful source of extreme ultraviolet and X-ray emission. Loops of bright material connect distant magnetic fields. There are regions of little or no corona called coronal holes. The brightest regions are the active regions surrounding sunspots. Hydrogen and helium are entirely ionized, and the other atoms are highly ionized. The ultraviolet portion of the spectrum is filled with strong spectral lines of the highly charged ions. The density at the base of the corona is about 4 * 108 atoms per cubic cm, 1013 times more tenuous than the atmosphere of Earth at sea level. Because the temperature is high, the density drops slowly, by a factor of 2.7 every 50,000 km (31,000 miles).

Radio telescopes are particularly valuable for studying the corona because radio waves will propagate only when their frequency exceeds the so-called plasma frequency of the local medium. The plasma frequency varies according to the density of the medium, and so measurements of each wavelength tell us the temperature at the corresponding density. At higher frequencies (above 1,000 MHz) electron absorption is the main factor, and at those frequencies the temperature is measured at the corresponding absorbing density. All radio frequencies come to us from above the photosphere; this is the prime way of determining atmospheric temperatures. Similarly, all of the ultraviolet and X-ray emission of the Sun comes from the chromosphere and corona, and the presence of such layers can be detected in stars by measuring their spectra at these wavelengths.

Since the discovery of the nature of the corona, such low-density, super-hot plasmas have been identified throughout the universe: in the atmospheres of other stars, in supernova remnants, and in the outer reaches of galaxies. Low-density plasmas radiate so little that they can reach and maintain high temperatures.

By detecting excess helium absorption or X-ray emission in stars like the Sun, researchers have found that coronas are quite common. Many stars have coronas far more extensive than that of the Sun.

It is speculated that the high coronal temperature results from boundary effects connected with the steeply decreasing density at the solar surface and the con-vective currents beneath it. Stars without convective activity do not exhibit coronas. The magnetic fields facilitate a "crack-of-the-whip" effect, in which the energy of many particles is concentrated in progressively smaller numbers of ions. The result is the production of the high temperature of the corona. The key factor is the extremely low density, which hampers heat loss. The corona is a much more tenuous vacuum than anything produced on Earth.

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