SOLAR INFLUENCE ON EARTH — EVANS 199 



A prominence is a great cloud of hydrogen and helium with small 

 impurities of iron, sodium, magnesium, etc. It is usually between 

 20,000 and 100,000 miles high, with exceptional specimens that rise 

 to a million miles. The variety and complexity of their structure and 

 motions defy description, although there are a few characteristic 

 patterns that serve to classify them. They are too faint to be seen in 

 an ordinary telescope because the glare of scattered light in our atmos- 

 phere next to the edge of the sun is so intense that it drowns out the 

 prominences. Fortunately the prominences resemble radio stations in 

 emitting only light of a specific wavelength, while the scattered light 

 is composed of all wavelengths. Just as we can pick out one radio 

 station by tuning to its wavelength and rejecting all others, we can 

 see the prominences through an appropriate filter tuned to their wave- 

 length. In rejecting the other wavelengths, we reject 99.9 percent 

 of the scattered light, and the prominences become visible. The par- 

 ticular red wavelength we usually use is that emitted by hydrogen 

 atoms, so, strictly speaking, we see only the hydrogen in the 

 prominences. 



The most puzzling thing about the prominences is their motion, or 

 sometimes their lack of motion. At first sight you may not be sur- 

 prised to see a motionless prominence apparently floating above the 

 solar surface. You are used to seeing clouds floating around overhead 

 without feeling impelled to report them to the newspapers. The 

 startling thing about the prominences is that there is no air for them 

 to float in. They appear to stay there with nothing whatever to hold 

 them up against the sun's gravity, which is 27 times stronger than 

 terrestrial gravity. The prominences have every appearance of com- 

 plete ignorance of gravity, whether they are stationary clouds or 

 rapidly moving streamers. We can measure prominence motions quite 

 accurately, and we can calculate how the material ought to move in 

 the sun's gravitational field. Almost invariably the observed motion 

 has no resemblance whatever to the calculated motion. 



For explanation we have two choices. We either sacrifice the uni- 

 versal validity of Newton's law of gravitation, or we assume that 

 there are other forces on the sun that oppose gravitation, and some- 

 times exactly balance it. The first alternative is extremely repug- 

 nant. It would mean that in prominences we have found the only 

 example of matter in the known universe that is not subject to New- 

 ton's law that every particle attracts every other particle in the uni- 

 verse. The second alternative seems difficult, but we think we are 

 making progress. 



The matter composing prominences is highly ionized. Not just 1 

 atom out of every 10,000, as in our ionosphere, but more like 99 out of 

 every 100. From where we sit on the earth we may regard this as a 



326511—55 14 



