Astronomy Jargon 101: Van Allen Belts

In this series we are exploring the weird and wonderful world of astronomy jargon! You’ll have to carefully navigate around today’s topic: the Van Allen Belts!

The Van Allen belts are regions containing highly energetic subatomic particles trapped by the magnetic field of a planet or moon. They get their name from the space scientist James Van Allen, who discovered them.

If a planet or moon has a strong enough magnetic field, it has the potential of forming Van Allen belts. The belts contain charged particles, like electrons and protons, that constantly swim throughout the solar system. Most of these particles come from the solar wind, which is the steady rain of high-energy electrons emitted by the Sun. Other particles come from well outside the solar system, like distant supernovae or star-forming regions.

When these particles encounter a planet, they begin to interact with its magnetic field. Some of the particles barrel on through, while others just bounce right off. But if the particles have just the right energy, they become trapped. They follow the magnetic field lines around and around the planet, constantly circling it.

Around the Earth, the Van Allen belts split into two distinct regions. The inner radiation belt extends from 1,000 km in altitude to 12,000 km. It contains mostly electrons and some protons. While the energies of these particles are relatively low, their density is much higher in this region.

The outer belt extends from 12,000 km all the way up to 60,000 km. The particles here have much more energy than the ones in the inner belt, but they are much more diffuse. The outer belt is also much looser, and tends to fluctuate depending on solar weather and changes in the Earth’s magnetic field.

Jupiter has by far the strongest magnetic field in the solar system, and its system of Van Allen belts is millions of times more intense than the Earth’s. This poses a significant hazard to any spacecraft visiting the Jovian system.