Synopsis: Electron Bounce

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NASA/Tom Bridgman

Scattering of Magnetic Mirror Trapped Fast Electrons by a Shear Alfvén Wave

Yuhou Wang, Walter Gekelman, Patrick Pribyl, and Konstantinos Papadopoulos

Published March 8, 2012

Solar winds and cosmic rays continually feed charged particles into the radiation belts trapped by Earth’s magnetic field. These high-speed particles pose a hazard to space missions and orbiting satellites, so scientists are considering various schemes to drain or divert them. Now, experiments published in Physical Review Letters show that a type of magnetic wave that propagates in a plasma is unexpectedly effective at scattering trapped, energetic electrons.

Yuhou Wang at the University of California, Los Angeles, and her colleagues discovered the strong wave-particle interaction using the Large Plasma Device, a 20-meter-long cylindrical plasma chamber, housed on the UCLA campus. With microwaves, the researchers heat a fraction of the plasma electrons, which are then trapped in a magnetic potential well, mimicking that of the Earth. In this setup, the electrons, which have energies of up to 3 mega-electron-volts, are detected by x rays that they produce upon colliding with the chamber walls.

The magnetic field lines and surrounding plasma in the chamber are analogous to a massive string that can be “plucked” to produce waves by introducing an oscillating magnetic field. Wang et al. find that when they create such waves, called Alfvén waves, with a small radio-frequency antenna, they see a sizable burst of x rays, suggesting that the waves have strongly scattered the electrons.

For now, Wang et al.’s experiments provide a controlled environment in which to study wave-particle interactions in a plasma that could prove useful for diverting electrons in Earth’s radiation belts. – Jessica Thomas

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