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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