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