(Illustration) At the highest electron energies measured — above 1 megaelectron volt (Mev) — researchers saw electrons in the outer belt only. Credits: NASA Goddard/Duberstein
600 miles from Earth’s surface is the 1st of 2 donut-shaped electron swarms, Van Allen Belts, or radiation belts. Understanding the shape and size of the belts, which shrink and swell in response to incoming radiation from the sun, is crucial for protecting technology in space. Scientists wish to know just which orbits could be jeopardized in different situations.
“The shape of the belts is actually quite different depending on what type of electron you’re looking at,” said Geoff Reeves . “Electrons at different energy levels are distributed differently in these regions.” Rather than the classic picture of the radiation belts – small inner belt, empty slot region and larger outer belt – new analysis reveals the shape can vary from a single, continuous belt with no slot region, to a larger inner belt with a smaller outer belt, to no inner belt at all. Many of the differences are accounted for by considering electrons at different energy levels separately.
(Illustration) The radiation belts look much different at the lowest electron energy levels measured, about 0.1 MeV. Here, the inner belt is 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 in these lower electron energies. Credits: NASA Goddard/Duberstein
They found the inner belt is much larger than the outer belt when observing electrons with low energies, while the outer belt is larger when observing electrons at higher energies. At the very highest energies, the inner belt structure is missing completely. So, depending on what one focuses on, the radiation belts can appear to have very different structures simultaneously.
Illustration) During geomagnetic storms, the empty region between the two belts can fill in completely with lower-energy electrons. Traditionally, scientists thought this slot region filled in only during the most extreme geomagnetic storms happening about once every 10 years. However, new data shows it’s not uncommon for lower-energy electrons — up to 0.8 MeV — to fill this space during almost all geomagnetic storms. Credits: NASA Goddard/Duberstein
These structures are further altered by geomagnetic storms. When fast-moving magnetic material from the sun – in the form of high-speed solar wind streams or coronal mass ejections – collide with Earth’s magnetic field, they send it oscillating, creating a geomagnetic storm. Geomagnetic storms can increase or decrease the number of energetic electrons in the radiation belts temporarily, though the belts return to their normal configuration after a time. “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.”
Often, the outer electron belt expands inwards toward the inner belt during geomagnetic storms, completely filling in the slot region with lower-energy electrons and forming one huge radiation belt. At lower energies, the slot forms further from Earth, producing an inner belt that is bigger than the outer belt. At higher energies, the slot forms closer to Earth, reversing the comparative sizes.
(Illustration) The traditional idea of the radiation belts includes a larger, more dynamic outer belt and a smaller, more stable inner belt with an empty slot region separating the two. However, a new study based on data from NASA’s Van Allen Probes shows that all three regions — the inner belt, slot region and outer belt — can appear different depending on the energy of electrons considered and general conditions in the magnetosphere. Credits: NASA Goddard/Duberstein
Besides studying extremely high-energy electrons – carrying millions of electron volts – that had been studied before, the Van Allen Probes can capture information on lower-energy electrons that contain only a few thousand electron volts. Additionally, spacecraft measure radiation belt electrons at a greater number of distinct energies than was previously possible. “Previous instruments would only measure 5 or 10 energy levels at a time,” said Reeves. “But the Van Allen Probes measure hundreds.”
Van Allen Probes data found that these lower-energy electrons circulate much closer to Earth than previously thought. Precise observations from hundreds of energy levels, rather than just a few, will allow scientists to create a more precise and rigorous model of what, exactly, is going on in the radiation belts, both during geomagnetic storms and during periods of relative calm.
http://www.nasa.gov/feature/goddard/2016/nasa-s-van-allen-probes-revolutionize-view-of-radiation-belts
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