During a strong geomagnetic storm, electrons at relativistic energies, which are usually only found in the outer radiation belt, are pushed in close to Earth and populate the inner belt. While the electrons in the slot region quickly decay, the inner belt electrons can remain for many months. Credits: NASA's Goddard Space Flight Center/Mary Pat Hrybyk-Keith
The behavior of Earth’s radiation belts is yet to be completely understood even though these two doughnut-shaped regions of charged particles encompassing our planet were discovered more than 50 years ago.
Recently, NASA’s Van Allen Probes mission has made observations, which show that the most energetic, fastest electrons in the inner radiation belt are not present for most of the time based on pervious assumptions.
The Journal of Geophysical Research has published a paper presenting the results, which show that not much radiation is typically present in the inner belt as previously thought - this is good news for spacecraft flying in the region.
Earlier space missions have not succeeded in distinguishing electrons from high-energy protons in the inner radiation belt. But by using the Magnetic Electron and Ion Spectrometer (MagEIS), a special instrument, on the Van Allen Probes, the scientists were able to look at the particles individually for the very first time.
The element of surprise discovered by the scientists here was that usually none of these super-fast electrons, called relativistic electrons, are present in the inner belt, which is indeed is contrary to what was expected by the scientist.
We’ve known for a long time that there are these really energetic protons in there, which can contaminate the measurements, but we’ve never had a good way to remove them from the measurements until now.
Seth Claudepierre, Van Allen Probes Scientist, Aerospace Corporation
Since their discovery at the dawn of the Space Age, Earth's radiation belts continue to reveal new complex structures and behaviors. This visualization shows how the radiation belts change in response to the injection of electrons from a storm in late June 2015. Red colors indicate higher numbers of electrons. (Credits: NASA's Goddard Space Flight Center/Tom Bridgman)
Taking into consideration both the radiation belts, scientists have believed for a long time that the outer belt is the rowdy one. The charged particles from the sun dash across the solar system during geomagnetic storms, resulting in the dramatic pulsating of the outer radiation belt, which grows and shrinks in response to the pressure of the magnetic field and the solar particles.
In the meantime, a steady position is maintained by the inner belt above the surface of the Earth. However, the new results highlight that the composition of the inner belt is not as constant enough based on the expectations of the scientists.
Generally, the inner belt is made up of low-energy electrons and high-energy protons. However, relativistic electrons were pushed deep into the inner belt following a very severe geomagnetic storm in June 2015.
The design of the MagEIS enabled the findings to be visible. This instrument produces its own internal magnetic field, which enables it to classify particles based on their energy and charge. The scientists separated the electrons from the protons in order to become aware of which particles contributed to the population of particles present in the inner belt.
When we carefully process the data and remove the contamination, we can see things that we’ve never been able to see before. These results are totally changing the way we think about the radiation belt at these energies.
Seth Claudepierre, Van Allen Probes S cientist, Aerospace Corporation
Based on the infrequency of the storms, which can actually inject relativistic electrons into the inner belt, the scientists presently understand the typical existence of lower radiation levels there - a result that brings about implications for spacecraft flying in the region. Engineers and scientists will be able to design cost-effective and lighter satellites customized to withstand less intense radiation levels they will come across only by understanding how much radiation is actually present.
The findings make room for a new realm for scientists to study in the future besides offering a new outlook on spacecraft design.
“This opens up the possibility of doing science that previously was not possible,” said Shri Kanekal, Van Allen Probes deputy mission scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, not involved with the study. “For example, we can now investigate under what circumstances these electrons penetrate the inner region and see if more intense geomagnetic storms give electrons that are more intense or more energetic.”
The Van Allen Probes is considered to be the second mission in NASA’s Living with a Star Program and also one of the several heliophysics missions analyzing the near-Earth environment. In order to understand the physical processes that remove or add electrons to the region, the spacecraft plunges for five to six times a day through the radiation belts on a highly elliptical orbit.