Analyzing Aurora

For many years astronomers have been pretty certain that the Aurora Borealis is caused by something, presumably hydrogen, that is shot out of sunspots. For one thing, bright auroras generally appear about 15 hours after a sunspot has moved to a point on the sun directly opposite the earth. But proof was lacking; even the brightest auroras were too faint and diffuse to be studied spectrographically by existing instruments. Last week the proof was in hand: a few smudged bands on two photographic plates.

For four years Astronomer Aden B. Meinel of the University of Chicago's Yerkes Observatory had worked on the problem. He devised a powerful spectrograph, built around a special Schmidt camera, which gathers light from a large area of the sky. Last month, when the sun was nicely spotted in the right places, he got his apparatus ready and pointed it in the direction from which he expected the hydrogen particles to come. Nicely on schedule, the night sky lit up with an "extreme aurora."

During the night Astronomer Meinel made two exposures. The first was enough: the plate showed bands representing the special wave lengths of light emitted by electrically excited hydrogen atoms. The big news was that the bands were not sharp, but smudged. This proved (by the "Doppler effect")*that some of the light was coming from fast-moving objects: i.e., hydrogen particles racing toward the earth from the sun. A rough calculation gave their speed as about 1,800 miles per second --about the right speed for a 15-hour journey from the sun to the earth's upper atmosphere.

When Meinel got his news, other astronomers swarmed in from all over the observatory. They congratulated him on a really important accomplishment, which opens a new branch of astrophysics by permitting a detailed study of material ejected by a typical star, the sun. They toasted him in hot cocoa. Then Astronomer Meinel, worn out with nervous excitement, went to bed with exhaustion and a virus infection.

*Light coming from a moving object has a slightly changed wave length and therefore makes a displaced mark on a spectrograph's plate.

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