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Where do the charged particles in the Van Allen radiation belts come from?

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Diagram of Earth's atmosphere layers-troposphere, stratosphere, mesosphere, and thermosphere-showing how temperature and air pressure change with increasing altitude, with key features like the ozone layer and pressure percentages at various heights. — Ionosphere and magnetosphere The layers of Earth's atmosphere, with a yellow line showing the air temperature at various heights.

The charged particles trapped within the Van Allen radiation belts originate from several distinct sources, depending on the specific region of the belts.123

Inner Belt

The high-energy protons found in the inner belt are primarily produced through the interaction of cosmic rays with Earth's atmosphere. When these energetic rays strike atmospheric atoms and molecules, they generate neutrons. Some of these neutrons are ejected upward and, during their short lifespan, decay into protons and electrons, which then become trapped by Earth's magnetic field.123

Outer Belt

The outer belt contains a more diverse population of particles. In addition to those of atmospheric origin, a significant portion of the ionization is derived from the solar wind—a steady stream of particles emanating from the Sun. This includes helium ions, which are absent in the inner belt. Particles from Earth's ionosphere also contribute to the population of the magnetosphere and these radiation zones.123