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NASA’s Van Allen Probes broaden view on the shape of Van Allen Belts


NASA's Van Allen Probes enabled the scientists to gather a more precise picture of the structure of Van Allen Belts that shield our planet from the solar radiation. New research shows their shape varies depending on the energy of charged particles constituting the belts, and the inner and outer radiation belts can simultaneously have very different structures.

The Van Allen Belts, the so-called radiation belts, consist of charged particles located approximately 965 km (600 miles) from our planet's surface, held together by the Earth's magnetic field. The belts respond to incoming solar radiation by shrinking or swelling. They are of utmost importance as they act as shields, protecting our technology in space.

The Van Allen Probes, Artist's conception. Image credit: NASA

The scientists began to study the radiation belts during the 1950s, and understanding of their shape has pretty much remained the same since. The structures comprising the belt were traditionally divided into the small, inner belt, the empty slot region, and the outer, more dynamic and larger belt, dominated by electrons. However, the new study, conducted by NASA's experts, indicates things might not be as simple as that, after all.

Illustration showing 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 can appear different depending on the energy of electrons considered and general conditions in the magnetosphere. Image credit: NASA Goddard/Duberstein

"The shape of the belts is actually quite different depending on what type of electron you’re looking at. Electrons at different energy levels are distributed differently in these regions," said Geoff Reeves from Los Alamos National Laboratory and the New Mexico Consortium in Los Alamos, New Mexico, lead author on the study on Van Allen Belts, published on December 28, 2015, in the Journal of Geophysical Research.

New research shows that the shape of the belts can change from a single, continuous belt with no slot region, to a larger inner belt with a smaller outer belt or to inner belt at all. Discovery was made by separately taking into account the electrons at different energy levels.

The illustration showing the belts at the highest electron energies measured (above 1 megaelectron volt (MeV)). Researchers saw electrons in the outer belt only. Image credit: NASA Goddard/Duberstein

When observing low-energy electrons, the inner belt is much larger than the outer belt, but at highest energies, the inner belt structure is completely missing. It appears that the Van Allen belts have very different structures simultaneously.

"It’s like listening to different parts of a song. The bass line sounds different from the vocals, and the vocals are different from the drums, and so on," Reeves explained.

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. Image credit: NASA Goddard/Duberstein

The belt structure is also altered by geomagnetic storms. The fast-streaming magnetic material from the Sun causes them to oscillate, thus producing geomagnetic storms, which can lead to decrease or increase of the number of energetic electrons temporarily, before the belts return to their usual configuration.

Storm-driven changes cannot be predicted for now, as the clear patterns showing the type and strength of the storm are missing. Basically, as the scientist put it: "If you've seen one geomagnetic storm, you've seen one geomagnetic storm". But these observations have only been made at few electron energy levels.

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. Image credit: NASA Goddard/Duberstein

"When we look across a broad range of energies, we start to see some consistencies in storm dynamics. 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."

It turns out that the outer electron belt often forms one huge radiation belt, as it expands toward the inner belt and fills in the slot region with low-energy electrons during geomagnetic storms. At lower energies, the slot forms further from Earth, and the inner belt is bigger than the outer belt. When energies are higher, the slot is situated closer to our planet, and an outer belt is bigger.

The twin Van Allen Probes satellites expand the range of energetic electron data we can capture. In addition to studying the extremely high-energy electrons which carry millions of electron volts, the Probes can also capture information on a few thousand electron volt energies.

"Previous instruments would only measure five or ten energy levels at a time. But the Van Allen Probes measure hundreds."

The lower energy electron fluxes couldn't be measured in the past because of the protons in the radiation belt regions closest to Earth which fly through particle detectors creating a noisy background. However, the Van Allen Probes are capable of collecting high-resolution data, which shows these electrons circulate much closer to our planet than previously assumed.

"Despite the proton noise, the Van Allen Probes can unambiguously identify the energies of the electrons it’s measuring."

Such precise observations should allow scientists to obtain more rigorous model of the behavior in the radiation belts during geomagnetic storms and during relatively calm periods.

“You can always tweak a few parameters of your theory to get it to match observations at two or three energy levels. But having observations at hundreds of energies constrain the theories you can match to observations,” Reeves concluded.

Featured image: The Van Allen Probes, Artist's conception. Image credit: NASA

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