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Van Allen Belt Trapping

Charged particles spiraling, bouncing, and drifting in Earth's magnetic field

Meridional Cross-Section

Field lines
Inner belt (protons)
Outer belt (electrons)
Particle

Pitch Angle & Loss Cone

Bounce Motion

Status: Trapped
Loss cone angle: -
Mirror latitude: -
Gyro period: -
Bounce period: -
Drift period: -

Trapped Particle Dynamics

Discovered by James Van Allen in 1958, Earth's radiation belts contain energetic charged particles trapped by the dipole magnetic field. Particles execute three types of periodic motion, each associated with an adiabatic invariant.

1. Gyration (μs)

Particles spiral around field lines due to the Lorentz force. Gyro-radius ∝ energy/B.

2. Bounce (0.1-1s)

Magnetic mirror effect reflects particles at high latitudes. Mirror point depends on pitch angle.

3. Drift (minutes-hours)

Gradient and curvature drifts cause azimuthal motion. Electrons drift east, protons west.

Loss Cone: Particles with pitch angles smaller than the loss cone angle (~few degrees at L=6) will mirror within the atmosphere and be lost through collisions. This creates the auroral precipitation that lights up the polar skies.