National Geographic : 1953 Jan
Antarctica, and great est at the geomagnetic equator halfway be tween.* Our planet's mag netic field thus serves as a sort of filter, which permits only the strongest cosmic rays to come through where its deflecting power is greatest, while weaker and weaker rays are able to penetrate as the field's influence dimin- > ' ishes toward the north and south. It is like a steel plate that is thick in the center but grows thinner and thinner to ward the edges, and at which are being fired bullets traveling at many different speeds. Only the fastest bullets can penetrate the plate in the center, but those with less and less speed are able to go through as they strike nearer and nearer the edges. Many television pic ture tubes make use of this same principle by employing a magnetic field to control the paths of electrons in 103 the electron beam that Northern Sk produces the picture. Turning the focusing Over Hudson Bay th Turning the focusing low energy they could knob on a set of this Cosmic rays of this type type regulates the mag- netic poles. None but netic field. Because of this filtering effect of the earth's magnetic field, more rays penetrate it at the latitude of Swarthmore, Pennsylvania, our headquarters at the Bartol Research Founda tion, than farther south at the geomagnetic equator. Now we wanted to see if still more rays were coming through farther north at Churchill. On the Trail of a Discovery If we found more rays coming into the at mosphere here, the additional ones would be weak rays that could penetrate the earth's magnetic field only at higher latitudes. Should we find these weak rays coming in, we would have an important clue to where some of the cosmic rays come from and to conditions out in space controlling their flight toward the earth. This very first balloon flight might give us the answers we were seeking. I listened in- Drawn by Robert Northrop and Irvin E. Alleman ties Gave Clues to Cosmic-ray Origins e author discovered rays traveling earthward with such have come from no source farther away than the sun. e can penetrate earth's magnetic field only near geomag high-energy rays pass through at geomagnetic equator. tently to the buzzing sounds coming sharp and clear in my earphones. "The signal strength seems remarkably high," I said to my assistant, Bob Kerr, at the radio receiver controls. "What was the last indication of the balloon train's altitude?" "About 60,000 feet," he said, "and rising 250 feet a minute." "Let's make another check on the number of signals per minute," I said. "If particles too weak to enter the atmosphere at Swarth more are getting through here, some of them should be penetrating down to where the bal loon train is now." After listening for about 10 minutes and marking down each signal as it came, I began to realize with rising excitement that the buzzing sounds were coming about 20 percent * See diagram on page 680 in "Unlocking Secrets of the Northern Lights," by Carl W. Gartlein, NATIONAL GEOGRAPHIC MAGAZINE, November, 1947.