NASA's Van Allen probes revolutionize view of radiation belts
"The shape of the belts is actually quite different depending on what type of electron you're looking at," said Geoff Reeves from Los Alamos National Laboratory and the New Mexico Consortium in Los Alamos, New Mexico, lead author on the study published on Dec. 28, 2015, in the Journal of Geophysical Research. "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 -- this new analysis reveals that 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.
|At the highest electron energies measured — above 1 megaelectron volt (Mev) — researchers|
saw electrons in the outer belt only [Credit: NASA Goddard/Duberstein]
The researchers found that the inner belt -- the smaller belt in the classic picture of the belts -- 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.
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.
"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."
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.
"Previous instruments would only measure five or ten energy levels at a time," said Reeves. "But the Van Allen Probes measure hundreds."
Measuring the flux of electrons at these lower energies has proved difficult in the past because of the presence of protons in the radiation belt regions closest to Earth. These protons shoot through particle detectors, creating a noisy background from which the true electron measurements needed to be picked out. But the higher-resolution Van Allen Probes data found that these lower-energy electrons circulate much closer to Earth than previously thought.
"Despite the proton noise, the Van Allen Probes can unambiguously identify the energies of the electrons it's measuring," said Reeves.
Precise observations like this, 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.
"You can always tweak a few parameters of your theory to get it to match observations at two or three energy levels," said Reeves. "But having observations at hundreds of energies constrain the theories you can match to observations."
Source: NASA/Goddard Space Flight Center [January 19, 2016]