National Geographic : 1957 Apr
to earth without an atmosphere. The ionosphere receives the sun's direct radiation. Our rocket would penetrate to that raw sunlight, to measure invisible ultraviolet light and X rays. The ions that give the ionosphere its name are electrically charged atoms and molecules from which electrons have been stripped by X rays and by a particular part of the ultraviolet light known as Lyman alpha radiation (named for Prof. Theodore Lyman of Harvard Uni versity, who discovered it). With a quiet, normal sun, Lyman alpha penetrates much deeper into the atmosphere than X rays. The rocket would measure X rays and Lyman alpha radiation. Its observations would add to our un derstanding of the origin of the free electrons that serve as tiny radio sta tions in the ionosphere. Radio sig nals bounce back and forth between the ionosphere and the ground, skipping from one continent to an other. X rays and ultraviolet rays produce these radio-reflecting elec trons at altitudes above 60 miles and are absorbed in the process. Why Radio Fades During unusual solar activity the number of free electrons increases. But, surprisingly, at such times the ionosphere's reflectivity drops off instead of improving, and our world news roundup announcer may say, "I'm sorry, but our signal is not coming through from Cairo." This is called radio fade-out. Extreme cases are followed a day later by magnetic disturbances that cause compasses to waver over the entire earth. Our rocket, now lazily riding the balloon's tail, was approaching 80,000 feet. In it were sensitive electronic instruments designed to detect Lyman alpha and X rays. and report what they learned back to our ship by radio. Lashed to the deck stood a truck trailer that looked from the outside like those that haul peaches from 565 Rohert F. Sisson Balloon Takes Shape as Tanks Below Deck Valve Helium into the Bag Expanded, the bag holds 150,000 cubic feet of gas. In the rare air at 80,000 feet its diameter swells to 72 feet. Men here pull down an outer sleeve that keeps uninflated folds from billowing.