Van Allen Belти: Магнитне відбиття, Кут Нахилу та Втратна Контрольна Площа

Spiralling along field lines

A charged particle moving through a magnetic field feels the Lorentz force, which curves its path into a helix that spirals around a field line rather than crossing it. Earth’s magnetic field is approximately dipolar — about 80% of it can be described as a tilted dipole (roughly 11° from the rotation axis) generated by the geodynamo in the liquid outer core, with a dipole moment of about 8×10²² A·m² and a surface field strength ranging from roughly 25–65 μT depending on location.

Because dipole field lines arc from pole to pole, trapped particles get carried toward each pole in turn rather than escaping outward — the basic geometry that creates the Van Allen radiation belts: an inner belt (roughly 1–2 Earth radii, mostly protons from cosmic-ray interactions) and an outer belt (roughly 3–9 Earth radii, mostly electrons from the solar wind), part of the larger magnetosphere that shields Earth from the solar wind.

Magnetic mirroring: bouncing between hemispheres

As a particle spirals toward a pole, it moves into a region of stronger field — and because its magnetic moment is conserved, its perpendicular velocity grows while its parallel (along-the-field) velocity shrinks. If the field becomes strong enough before the particle actually reaches the pole, the parallel velocity hits zero, reverses, and the particle is "mirrored" back toward the opposite hemisphere. This is magnetic mirroring, and it's what lets particles bounce back and forth between the northern and southern hemispheres for days, weeks or longer, drifting slowly around the planet all the while.

Pitch angle and the loss cone

Whether a particle mirrors safely or is lost depends on its pitch angle — the angle between its velocity vector and the local field line. Particles with a large pitch angle (moving mostly perpendicular to the field) mirror high in the magnetosphere, well above the dense atmosphere, and bounce indefinitely. Particles with too small a pitch angle mirror at an altitude that's already deep inside the upper atmosphere, so they collide with air molecules before they can bounce back and are lost — precipitated out of the belt. The range of pitch angles that leads to this fate is the loss cone, and every field line has one.

Understanding the Risks for Spacecraft and Astronauts

The Van Allen belts pose a significant hazard to unprotected spacecraft electronics and astronauts, particularly during geomagnetic storms triggered by coronal mass ejections. These storms cause an increase in trapped particle populations within the belts. Satellites traversing the South Atlantic Anomaly — characterized by a region of weaker magnetic field allowing the inner belt to dip closer to Earth's surface — require specific shielding measures against elevated radiation exposure during each pass.

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