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. All radio frequencies come to us from above the photosphere this is the prime way of determining atmospheric temperatures. 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. 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. 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. Because the temperature is high, the density drops slowly, by a factor of e (2.718) every 50,000 kilometres. The density at the base of the corona is about 4 × 10 8 atoms per cubic centimetre, 10 13 times more tenuous than the atmosphere of Earth at its base. The ultraviolet portion of the spectrum is filled with strong spectral lines of the highly charged ions.
Hydrogen and helium are entirely ionized, and the other atoms are highly ionized. The brightest regions are the active regions surrounding sunspots. There are regions of little or no corona called coronal holes. Loops of bright material connect distant magnetic fields.
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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. SpaceNext50 Britannica presents SpaceNext50, From the race to the Moon to space stewardship, we explore a wide range of subjects that feed our curiosity about space!.Learn about the major environmental problems facing our planet and what can be done about them! Saving Earth Britannica Presents Earth’s To-Do List for the 21st Century.Britannica Beyond We’ve created a new place where questions are at the center of learning.100 Women Britannica celebrates the centennial of the Nineteenth Amendment, highlighting suffragists and history-making politicians.
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