Shifting Shape Of Van Allen Belts Discovered

An artistic impression of what the outer Van Allen belt looks like. NASA Goddard

Earth's magnetic field acts as a shield against dangerous cosmic rays and powerful solar winds. Some of the particles hitting the magnetic field become trapped and form large swarms of charged particles, mostly electrons and protons, arranged in radiation belts, known as the Van Allen belts.

To protect satellites and space missions, it is important to understand exactly how the belts are shaped and how particles in them move. NASA has two probes studying the belts, and the latest results have been published in the Journal of Geophysical Research.

The belts were discovered in the 1950s and they were pictured as two concentric donuts, a small stable one between 1,000 and 6,000 kilometers (600 and 3,700 miles) and a large varying one between 13,000 and 60,000 kilometers (8,100 and 37,300 miles). But that no longer seems to be the case.

“The shape of the belts is actually quite different depending on what type of electron you’re looking at,” said Geoff Reeves, lead author of the study, in a statement. “Electrons at different energy levels are distributed differently in these regions.”

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1. The traditional picture of the radiation belts includes a larger, more dynamic outer belt and a smaller, more stable inner belt 2. Highest energy electrons form the outer belt only. 3. The lowest energy electrons have an inner belt much larger than in the traditional picture, expanding into the region that has long been considered part of the empty slot region. The outer belt is diminished and doesn’t expand as far. 4. During geomagnetic storms, the empty region between the two belts can fill in completely with lower-energy electrons. NASA Goddard/Duberstein

The new analysis focuses on electron distribution through the belts. NASA's Van Allen Probes were able to measure electrons at hundreds of different energies and map how the belts change.

According to the study, low-energy electrons form a thicker inner belt while the outer belt remains very thin. On the other hand, the highest energy electrons are only found on the outer belt and they cannot penetrate the magnetic field to populate an inner belt.

But something peculiar can happen during geomagnetic storms. The belts are expanded and compressed, and when considering only lower energy electrons, the slot between the two belts disappears completely.

The changes in the belts due to space weather are unpredictable so far, but the Van Allen Probes are the right tools to finally see the detailed effect of solar storms on the magnetic field.

“When we look across a broad range of energies, we start to see some consistencies in storm dynamics,” said Reeves. “The electron response at different energy levels differs in the details, but there is some common behavior. For example, we found that electrons fade from the slot regions quickly after a geomagnetic storm, but the location of the slot region depends on the energy of the electrons.”

By modeling the evolution of the belts during calm and active periods, scientists will be able to protect both satellites and future astronauts during high-altitude missions.

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