Van Allen Radiation Belts

Zones of energetic charged particles — primarily from the solar wind — trapped and held around Earth by its magnetosphere. Part of geospace. Named after James Van Allen, whose instrument on Explorer 1 confirmed their existence in 1958. Earth has two main belts, spanning 640–58,000 km altitude, with radiation levels varying throughout.

By trapping solar wind particles, the belts protect Earth's atmosphere from erosion — a function Mars lacks due to its absent magnetosphere.

The Two Main Belts

Inner Van Allen Belt (1,000–12,000 km altitude; L=1.2–3)

• Dominated by high-energy protons (>100 MeV) and electrons (hundreds of keV)
• Protons originate primarily from cosmic ray collisions with the upper atmosphere producing neutrons, which decay (beta decay) into protons
• More stable than the outer belt
• Makes closest approach to Earth's surface at the South Atlantic Anomaly, where Earth's magnetic field is weakest

Outer Van Allen Belt (13,000–60,000 km altitude; ~3–10 Earth radii)

• Dominated by high-energy electrons (0.1–10 MeV)
• More variable — strongly influenced by solar activity and geomagnetic storms
• Electrons injected from the geomagnetic tail during storms; energized by wave-particle interactions with whistler-mode plasma waves
• Contains a mix of protons, alpha particles, and O⁺ oxygen ions from the ionosphere

The "safe slot" (gap between the belts) — Located at medium Earth orbit altitudes. Maintained by VLF radio waves (likely generated by lightning) that scatter particles in pitch angle. Solar outbursts can temporarily fill it.

Dynamics

Inner belt: particles spiral along magnetic field lines, bouncing between Earth's poles. Electrons drift eastward; protons westward.

Outer belt: highly variable. Geomagnetic storms inject particles from the magnetotail and accelerate them via wave-particle interactions. Particle fluxes can change dramatically within hours.

In 2013, Van Allen Probes detected a transient third radiation belt of ultra-relativistic particles, persisting for four weeks following a coronal mass ejection.

Hazards

• Energetic electrons can permanently damage satellite electronics
• Satellites must have shielded components if operating in or near the belts
• Apollo astronauts passed through quickly and received low, harmless doses
• Inner edge of the outer belt (>5 MeV electrons) acts as a sharp shield — the reason is not well understood