Probes monitoring the radiation belts surrounding Earth, have revealed that the geometry of these donut-shaped electron swarms, known as the Van Allen Belts, are more complicated that originally thought.
First detected by James Van Allen in 1958, The Van Allen belts were the first discovery of the space age, and our understanding of their shape has largely remained unchanged since then. The belts consist of a small, inner belt, a largely-empty space known as the slot region, and then a large, dynamic outer belt, that is dominated by electrons. Now however, over three years after NASA’s Van Allen Probes were launched from the Cape Canaveral Air Force Station in Florida, the twin spacecraft continue to push the boundaries of what is known about the space above our world to reveal new insights about the structure of the belts.
Above 1 megaelectron volt (Mev), the highest electron energies measured, researchers saw electrons in the outer belt only.
New analysis reveals that rather than the classic picture of the radiation belts – a small inner belt, empty slot region and larger outer belt – the shape can vary from a larger inner belt with a smaller outer belt, to a single, continuous belt with no slot region, to no inner belt at all. "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 in the Journal of Geophysical Research. "Electrons at different energy levels are distributed differently in these regions."
Analysis of the electron energy levels found that electrons in low energy states produced an inner belt that is much larger than the outer belt, while conversely, the outer belt is larger when observing electrons at higher energies. When the electrons are at their highest energy levels, the inner belt structure is missing completely. Therefore, the radiation belts can appear to have very different structures simultaneously, depending on what aspect is being focused on. "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."
At the lowest electron energy levels measured, about 0.1 MeV, the radiation belts look much different. 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.
These structures are also prone to alteration when they interact with high-speed solar wind streams or coronal mass ejections emanating from the Sun. As the fast-moving magnetic material collides with Earth's magnetic field, they send it oscillating, creating a geomagnetic storm that can increase or decrease the number of energetic electrons in the radiation belts. This effect is temporary however and the belts return to their normal configuration after a time.
Currently, these storm-driven electron increases and decreases are unpredictable, as previous observations have largely been based on electrons at only a few energy levels. Consequently, it has been difficult to assess without a clear pattern showing, what type or strength of storm will yield what outcomes. "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 behaviour. 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."
The scope of energetic electron data that can be captured from the twin Van Allen Probes satellites is considerably larger than previous missions and ranges from extremely high-energy electrons carrying millions of electron volts, to information on lower-energy electrons that contain only a few thousand electron volts. Additionally, the spacecraft measures 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."
During geomagnetic storms, the empty region between the two belts can fill in completely with lower-energy electrons. Nnew data shows it’s not uncommon for lower-energy electrons — up to 0.8 MeV — to fill this space during almost all geomagnetic storms and not just during the most extreme geomagnetic storms that happen about once every 10 years, as previously thought.
The higher-resolution data generated by the probes also has benefits when measuring the flux of electrons at lower energies compared with past observations. Protons in the radiation belt regions closest to Earth are prone to passing through particle detectors, creating a noisy background from which the true electron measurements needed to be picked out. Now, however it is known 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.
As the declining phase of the solar cycle approaches, researchers have learned that this is often when the biggest geomagnetic storms occur. “The Van Allen Probes extended mission will help us understand how radiation belts evolve during extreme space weather events,” said project scientist Sasha Ukhorskiy, in an interview last year while discussing the success of the Van Allen probe mission. The mission recently received the highest rating for scientific achievement in the NASA Heliophysics Division bi-annual senior review of operating missions, and was granted approval for a two-year extension.