During a total solar eclipse, most of the sun is blocked from view. The photosphere is an outer layer of the sun that is typically considered by scientists to be the surface. During a total eclipse, the photoshpere is not visible. Outside the photosphere is the sun layer called the chromosphere, which is occasionally visible as a thin red ring during an eclipse. The corona is outside the chromosphere. During an eclipse, it is visible as a white halo around the sun.
Chromosphere temperature is about 12,000 F. This is hotter than the photosphere, and temperatures continue to increase toward the corona, which is the opposite of how radiation behaves as the distance from the source increases. According to the Solar-Heliospheric Research Group at the University of Michigan, "Scientists believe that this source of heating deals with wave motions, specifically magnetohydrodynamic waves. They are created when a magnetic field line is displaced." The force of magnetism probably causes additional heating. Thickness of the chromosphere is 2,000 to 3,000 km. Hydrogen gas causes the red color.
Narrow columns of gas called spicules rise through the chromosphere. Spicules move upward at about 30 km/sec and last for 10 to 15 minutes. Solar flares shoot from the photosphere into the chromosphere. A solar flare is an explosion that includes bursts of particle acceleration and plasma heating. According to NASA's Goddard Spaceflight Center, "In general, a solar flare produces copious radiation across the full electromagnetic spectrum from the longest wavelength radio waves to the highest energy gamma rays."
The corona extends millions of kilometers into space. Temperatures in the corona are 2 million to 5 million F, and like the corona, magnetic fields are probably responsible for the high temperature. At high temperatures, the corona emits X-ray radiation. Since the corona's glow is so much less than the photosphere, it can only be seen during an eclipse or with a coronagraph, an instrument that blocks the disk of the sun.
Prominences are clouds of gas that erupt from the upper chromosphere and extend through the corona. They are usually less energetic and smaller than solar flares. The solar wind is another feature of the corona. According to the Department of Physics and Astronomy at the University of Tennessee, "The solar wind escapes primarily through coronal holes, which are found predominantly near the Sun's poles; in the equatorial plane the magnetic field lines of the Sun are more likely to close on themselves, particularly in periods of low solar activity."
- Contemporary Physics Education Project: Layers of the Sun
- Views of the Solar System: Sun
- Solar-Heliospheric Research Group at the University of Michigan: Chromosphere
- Department of Physics and Astronomy at the University of Tennessee: The Chromosphere of the Sun
- NASA Goddard Spaceflight Center: Overview of Solar Flares
- Department of Physics and Astronomy at the University of Tennessee: The Solar Corona
- Photo Credit Digital Vision./Digital Vision/Getty Images
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