About 600 miles from Earth's surface is the first of two donut-shaped electron swarms, known as the Van Allen Belts, or the radiation belts. Understanding the shape and size of the belts, which can shrink and swell in response to incoming radiation from the sun, is crucial for protecting our technology in space. The harsh radiation isn't good for satellites' health, so scientists wish to know just which orbits could be jeopardized in different situations.
Since the 1950s, when scientists first began forming a picture of these rings of energetic particles, our understanding of their shape has largely remained unchanged -- a small, inner belt, a largely-empty space known as the slot region, and then the outer belt, which is dominated by electrons and which is the larger and more dynamic of the two. But a new study of data from NASA's Van Allen Probes reveals that the story may not be so simple.
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
"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.
"It's like listening to different parts of a song," said Reeves. "The bass line sounds different from the vocals, and the vocals are different from the drums, and so on."
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.
These storm-driven electron increases and decreases are currently unpredictable, without a clear pattern showing what type or strength of storm will yield what outcomes. There's a saying in the space physics community: if you've seen one geomagnetic storm, you've seen one geomagnetic storm. As it turns out, those observations have largely been based on electrons at only a few energy levels.
"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.
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 -- 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, the spacecraft measure radiation belt electrons at a greater number of distinct energies than was previously possible.
"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."
The Johns Hopkins Applied Physics Laboratory in Laurel, Maryland, built and operates the Van Allen Probes for NASA's Science Mission Directorate. The mission is the second mission in NASA's Living With a Star program, managed by NASA's Goddard Space Flight Center in Greenbelt, Maryland.
Karen Fox | EurekAlert!
New thruster design increases efficiency for future spaceflight
16.08.2017 | American Institute of Physics
Tracking a solar eruption through the solar system
16.08.2017 | American Geophysical Union
Physicists at the University of Bonn have managed to create optical hollows and more complex patterns into which the light of a Bose-Einstein condensate flows. The creation of such highly low-loss structures for light is a prerequisite for complex light circuits, such as for quantum information processing for a new generation of computers. The researchers are now presenting their results in the journal Nature Photonics.
Light particles (photons) occur as tiny, indivisible portions. Many thousands of these light portions can be merged to form a single super-photon if they are...
For the first time, scientists have shown that circular RNA is linked to brain function. When a RNA molecule called Cdr1as was deleted from the genome of mice, the animals had problems filtering out unnecessary information – like patients suffering from neuropsychiatric disorders.
While hundreds of circular RNAs (circRNAs) are abundant in mammalian brains, one big question has remained unanswered: What are they actually good for? In the...
An experimental small satellite has successfully collected and delivered data on a key measurement for predicting changes in Earth's climate.
The Radiometer Assessment using Vertically Aligned Nanotubes (RAVAN) CubeSat was launched into low-Earth orbit on Nov. 11, 2016, in order to test new...
A study led by scientists of the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) at the Center for Free-Electron Laser Science in Hamburg presents evidence of the coexistence of superconductivity and “charge-density-waves” in compounds of the poorly-studied family of bismuthates. This observation opens up new perspectives for a deeper understanding of the phenomenon of high-temperature superconductivity, a topic which is at the core of condensed matter research since more than 30 years. The paper by Nicoletti et al has been published in the PNAS.
Since the beginning of the 20th century, superconductivity had been observed in some metals at temperatures only a few degrees above the absolute zero (minus...
Researchers from the University of Miami (UM) Rosenstiel School of Marine and Atmospheric Science, the Italian Space Agency (ASI), and the Instituto Geofisico--Escuela Politecnica Nacional (IGEPN) of Ecuador, showed an increasing volcanic danger on Cotopaxi in Ecuador using a powerful technique known as Interferometric Synthetic Aperture Radar (InSAR).
The Andes region in which Cotopaxi volcano is located is known to contain some of the world's most serious volcanic hazard. A mid- to large-size eruption has...
16.08.2017 | Event News
04.08.2017 | Event News
26.07.2017 | Event News
16.08.2017 | Physics and Astronomy
16.08.2017 | Materials Sciences
16.08.2017 | Interdisciplinary Research