Download - Ion Pickup
Ion Pickup
One of the fundamental processes in space plasma physics
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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?”
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?
Most scientists thought
“Momentum and energy transfer definitely require collisions.”
My hypothetical notion is:
Wave-particle interactions might gave rise to pickup. But I did not have a quantitative theory to support this idea.
Continuous ionization leads to newborn ions with random gyro phases.
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.
A Simple Model
• Excluding electrostatic waves
• Electromagnetic waves.
• Parallel propagation
• Cold thermal and beam ions
• Alfven waves, ion cyclotron waves, and whistlers.
Dispersion Equation• For the case of a ring beam
• An approximate solution
2 2 2 220 02 2
2 20 0
( )0
( ) 2( )p p
A i i
kv k vck c
v kv kv
1/3
0
bp
n
n
0
1bn
n 0 Av v
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
Concerning instabilities we must remember:
• Growth rate is not the most important consideration.
• Saturation level is essential.
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
A breakthrough
• In 1983 a hybrid-code simulation demonstrated the pickup process
• The physics seemed to be simple.
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
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.
Principle of Pitch-Angle Scattering
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Concept of Trapping
• Oscillatory particle motion in the presence of Alfven waves
• Acceleration and deceleration