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

4. 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?”

5. 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?

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

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

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

12. 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.

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

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

15. 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

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

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

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

20. 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

21. 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.

23. Principle of Pitch-Angle Scattering

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