The lowest layer of the chromosphere is the "reversing layer." The base of this layer is the top of the photosphere and the top is approximately 500 miles above the photosphere. Calculations for determining the thickness of the revesing layer are obtained from studies of solar eclipses. The time it takes the Moon to cross that layer and the known value of the Moon's velocity are used in this computation.
The reversing layer is responsible for the many dark lines in the otherwise continuous spectrum of sunlight. The gases in thos layer absorb certain wavelengths of light leaving a spectrum that appears dark in the places usually occupied by these wavelengths. The wavelengths of the dark lines identify clearly the chemical composition of the reversing layer. Identification is made by comparing these dark line spectra with those bright line spectra produced by chemical elements in the laboratory. All of the elements identified as being present in this layer are found on Earth, such as, hydrogen, carbon, nitrogen, aluminum, iron, cobalt, lead, cadmium, and platinum. It is very evident that more elements will eventually be identified in this layer of the Sun's atmosphere. During a total eclipse, the background of brighter spectral lines is removed as the Moon covers the surface layers at the edge of the Sun. For an instant, these dark llines appear as they really are - brilliant - and for a split second the spectrum is reversed.
The remaining portion of the chromosphere extends upward to about 10,000 kilometers (6,000 miles). The name chromosphere comes from the bright orange-pink color of this layer caused by hydrogen gas. Much of the research on the chromosphere is conducted through the use of a spectroscope. The slit of the spectroscope is set tangent to the Sun's disk so that the sunlight entering the slit is dispersed by the prism and, therefore, greatly weakened. The orange-pink color is in one single wavelength and not dispersed. This causes light from the chromosphere to stand out in comparison with light from the rest of the Sun.
Studies indicate that the chromosphere is in constant turbulence, with great masses of gas thrown upward in all directions to tremendous heights. These disturbances may be either prominences or flares.
Prominences are loops or sheets of glowing gas ejected from an active region on the solar surface. These often move through the chromosphere and continue into the corona. These spectacular "storms" often occur in regions of sunspots and last from several days to months. Instabilities in the Sun's magnetic field may cause the prominences, but the details have yet to be worked out. The dimensions of prominences are gigantic. A typical loop or arch measures 100,000 kilometers, more than ten times the diameter of the Earth. Sometimes they may reach heights that may exceed the diameter of the Sun. The velocity of the gases in prominences is another outstanding feature because of its magnitude and the way changes occur. Speeds of 200 to 300 miles per second are commonly recorded. The change in speed may be abrupt with new speeds being multiples of the former. Thus, a prominence may be rising at a speed of 80 miles per second, continue for a period of time at that speed, and suddenly change speeds to 160 miles per second, and then start moving at 240 miles per second.
Intensely bright clouds called flares appear from time to
time in the solar atmosphere. These differ from prominences in
brilliance, size, and duration. Also a result of magnetic
instabilities, flares are even more violent (and even less
understood) than prominences. They often flash across a region
of the Sun in a matter of minutes, releasing tremendous amounts
of energy. Flaresa at their maximum intensity are easily the
brightest spots on the Sun, although they are a great deal
smaller than prominences. So energetic oare flares that they are
often compared to bombs exploding in the lower regions of the
Sun's atmosphere. The particles released by flares are so
energetic that the Sun's magnetic field is unable to contain
them. Instead, these particles are blasted into space by the
violent explosions. Within a matter of hours these particles may
bombard the Earth's atmosphere creating disturbances in radio
communications. Normal communications on the Earth may be
impossible for hours and at times even days depending upon the
violence and direction of the flare explosion. Because of the
vital nature of Earth's communications systems, there is much
scientific interest when flare periods occur.
Questions:
1. How would you describe the reversing layer of the Sun?
2. How do we know that similar elements found on Earth are in the reversing layer? Explain.
3. Why is this layer said to be reversing? How is information gained about this layer of the Sun?
4. How would you describe the Sun's atmosphere? What two disturbances are frequently observed in the atmosphere of the Sun?
5. Describe the velocity of a prominence. Where do prominences usually occur?
6. How do prominences and flares differ? Why are scientists interested in studying them?
7. How did the term "chromosphere" originate?
