High Voltage in the Sky

GEOPHYSICS

Ever since University of Iowa Physicist James Van Allen discovered the earth-circling belts of radiation that bear his name, scientists have been trying to answer a simple but perplexing question: How do the electrons and protons that are trapped in the outer belt get there in the first place? Now, summarizing data he and other scientists have obtained from a host of recent satellites, Van Allen himself has reported a possible answer. At a meeting of the American Physical Society in Manhattan, he suggested that the charged particles are drawn into the belt by a high voltage generated across the earth's magnetic field.

Scientists have been reasonably certain for years that the electrons and protons in the outer belt come from the solar wind, a stream of charged particles emitted continuously from the sun at velocities varying from 670,000 to 1,600,000 m.p.h. But at these speeds, which are relatively low in the world of high energy physics, the electrons and protons are traveling too slowly to penetrate through the earth's magnetic field and into the outer belt; by all rights, they should bounce off the magnetic lines of force and be deflected back into space.

What makes the difference, Van Allen says, is an electrical potential of 50,000 volts generated across the earth's comet-shaped magnetic field by a combination of two complex effects. As the solar wind blows by the earth, compressing the magnetic field into a rounded shell on the daylight side and sweeping it into a long tail on the night side (TIME, April 22, 1966), it produces friction on the outer boundary of the magnetic field. This friction generates a positive electrical charge on the morning side of the boundary, a negative charge on the opposite, or evening, side. The charge is supplemented by a dynamo effect caused by the rotation of the earth and its magnetic field.

As a result, Van Allen says, protons striking the morning side of the boundary are attracted into the field by the negative charge on the evening side. Similarly, electrons hitting the evening side of the boundary are pulled into the field by the positive charge on the morning side. This new theory may also explain why the auroral displays consist entirely of electrons or entirely of protons streaming down through the atmosphere. The solar particles, intermixed on their journey through space, have apparently been segregated by the high voltage in the sky.

Scientists can now turn their attention to solving another radiation-belt enigma: although one out of 20 particles in the solar wind is a positively charged helium nucleus, or alpha particle, only three can be found for every 10,000 particles in the outer belt. "For some reason," says Van Allen, "the process that pulls electrons and protons into the magnetic field seems to discriminate against alpha particles. We hope to find out why."

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