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Taking the Temperature of the Corona

"tte Sun's enormous gravity would hold a relatively cool gas close to the photosphere, just as the Earth's gravity binds its atmosphere into a thin shell. But this is inconsistent with the larger extent of the corona seen during a total solar eclipse. A temperature of a million degrees is required to keep the corona extended; the motion of the heated gas supports it against the Sun's gravitational pull. At cooler temperatures, the atoms would have to be much lighter than hydrogen to move fast enough and extend the corona out so far, but hydrogen is the lightest element there is.

"tte detailed character of spectral lines can be used to infer the temperature and motion of the solar gas. "tte hotter the gas, the faster the motions, and the greater the wavelength shifts from the Doppler effect, "ttese displacements widen the spectral lines, and can be used as a thermometer to measure the corona's temperature. In 1941, for example, the Swedish astronomer Bengt Edlen (1900-1993) noticed that the observed widths of the emission lines in the corona indicate a temperature of about two million degrees.

In 1946, the Russian astrophysicist Vitalii L. Ginzburg (1916- ) and the Australian radio astronomers David F. Martyn (1906-1970) and Joseph L. Pawsey (1908-1962) independently used observations ofthe Sun's meter-wavelength radio radiation to confirm the existence of a million-degree solar corona by measuring the temperature of the radio emission.

tte free electrons in the corona, which are not attached to anything, scatter sunlight from the photosphere and illuminate the corona, tte electrons bend small amounts of the sunlight into our line of vision, just as tiny dust particles illuminate a sunbeam in your room and air molecules scatter sunlight to make the sky blue. But the coronal electrons are so rarefied that most of the light from the photosphere passes right through the low-density corona, ttat is the reason the dim corona is a million times fainter than the photosphere at visible wavelengths.

ttere are about a billion electrons and protons per cubic centimeter (109 cm~3) at the base of the corona, which sounds like a lot, but the low corona is one hundred times more tenuous than the chromosphere (1011 cm~3) and an additional million times more rarefied than the photosphere (1017 cm 3). And even the photosphere is far more rarefied than our air.

tte average drop in density from the photosphere to the bottom of the corona is matched by an overall increase in temperature from about 5,780 kelvin in the photosphere to almost 10,000 degrees in the chromosphere and a million degrees at the base of the corona (Fig. 6.7). A very thin transition region, less than 100 kilometers thick,

Images Temp Chromosphere

FIG. 6.7 Transition region The temperature of the solar atmosphere decreases from values near 6,000 kelvin at the visible photosphere to a minimum value of roughly 4,400 kelvin at the base of the chromosphere, about 500 kilometers higher in the atmosphere. The temperature increases with height, slowly at first, then extremely rapidly in the narrow transition region between the chromosphere and corona, from about 10,000 kelvin to about a million kelvin. Thousands of needle-like spicules, each lasting about five minutes, are detected shooting up from the chromosphere into the transition region, so this static, layered model of the transition region does not completely portray its dynamic, ever-changing aspect. The height is in kilometers, abbreviated km, the temperature in degrees kelvin, denoted K, and the mass density in kilograms per cubic meter, abbreviated kg m~3. (Courtesy of Eugene Avrett, Smithsonian Astrophysical Observatory.)

FIG. 6.7 Transition region The temperature of the solar atmosphere decreases from values near 6,000 kelvin at the visible photosphere to a minimum value of roughly 4,400 kelvin at the base of the chromosphere, about 500 kilometers higher in the atmosphere. The temperature increases with height, slowly at first, then extremely rapidly in the narrow transition region between the chromosphere and corona, from about 10,000 kelvin to about a million kelvin. Thousands of needle-like spicules, each lasting about five minutes, are detected shooting up from the chromosphere into the transition region, so this static, layered model of the transition region does not completely portray its dynamic, ever-changing aspect. The height is in kilometers, abbreviated km, the temperature in degrees kelvin, denoted K, and the mass density in kilograms per cubic meter, abbreviated kg m~3. (Courtesy of Eugene Avrett, Smithsonian Astrophysical Observatory.)

lies between the chromosphere and corona. Both the density and temperature change abruptly in the transition region; the density decreases as the temperature increases in such a way to keep the gas pressure spatially constant in this region of transition.

tte corona then slowly cools with increasing distance from the Sun, slightly decreasing in temperature to about 100,000 kelvin at the Earth's orbit, tte corona also thins out as it expands into the increasing volume of space, reaching a density of only about 5 electrons and 5 protons per cubic centimeter at the Earth's orbit.

Although the electrified coronal particles move about at great speed, there are so few of them that the total energy in the corona is quite low. Only about a millionth of the Sun's total energy output is required to heat the corona. And even though the free electrons are extremely hot, they are so scarce and widely separated that an astronaut or a satellite will not burn up when immersed in the rarefied corona just outside the Earth. But if the corona were as dense as the center of the Sun, at a temperature of a million degrees it would contain enough energy to vaporize our planet.

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