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Ion Pickup

Ion Pickup. One of the fundamental processes in space plasma physics. What is “ ion pickup” ?. The original issue is: “ Can newly created ions be picked up by a moving background plasma in the absence of Coulomb collisions?”.

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Ion Pickup

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  1. Ion Pickup One of the fundamental processes in space plasma physics

  2. What is “ ion pickup”? The original issue is: “ Can newly created ions be picked up by a moving background plasma in the absence of Coulomb collisions?”

  3. Observation of helium ions in the solar wind in 1970 led to intense scientific debate in space physics comunity: Can the solar wind pick up newly born ions?

  4. Most scientists thought “Momentum and energy transfer definitely require collisions.”

  5. My hypothetical notion is: Wave-particle interactions might gave rise to pickup. But I did not have a quantitative theory to support this idea.

  6. Continuous ionization leads to newborn ions with random gyro phases.

  7. The ring-beam distribution function • The beam velocity is about equal to the solar wind speed which is about ten times the Alfven speed. • The newborn ions can excite numerous wave modes such as Alfven waves, ion cyclotron waves, Langmuir waves and whistlers, etc.

  8. A Simple Model • Excluding electrostatic waves • Electromagnetic waves. • Parallel propagation • Cold thermal and beam ions • Alfven waves, ion cyclotron waves, and whistlers.

  9. Dispersion Equation • For the case of a ring beam • An approximate solution

  10. Solutions are discussed in several papers • Wu and Davidson (1972) JGR, 77, 5399 • Wu and Hartle (1974) JGR,79, 283 • Winske et al. (1984) JGR, 89, 7327 • Winske et al. (1985) JGR, 90, 2713

  11. Concerning instabilities we must remember: • Growth rate is not the most important consideration. • Saturation level is essential.

  12. Personal Efforts (1) • 1972-1975 Study of instabilities • 1975 Research came to a stop • 1983 AMPTE program (failure) • 1983-1984 Numerical simulations • 1985 Alfven waves • 1986-1992 Study of cometary ions

  13. A breakthrough • In 1983 a hybrid-code simulation demonstrated the pickup process • The physics seemed to be simple.

  14. Alfven Waves • A unique feature: existence of a constant phase velocity. • Existence of a wave frame • Cannot resonate with thermal ions • High saturation level • High wave energy density

  15. Pitch-angle scattering • In the wave frame of reference there is no wave electric field. • The kinetic energy of a particle is conserved. • Particles are accelerated in the pitch-angle direction.

  16. Principle of Pitch-Angle Scattering

  17. Concept of Trapping • Oscillatory particle motion in the presence of Alfven waves • Acceleration and deceleration

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