The radiation belts that surround the Earth contain highly energetic particles that fly around at speed very close to the the speed of light, are sporadically energized and occasionally suddenly disappear. Scientists refer to these most energetic electrons in the near-Earth space as ultra-relativistic as the relativistic effects are very pronounced for these particles. A recent NASA's Van Allen probes mission (ended in 2019) has measured many of these phenomena in the Earth Van Allen Belts for the first time. Measurements showed very unusual rings at these very high energies that can persist in space for month and then suddenly disappear. Data analysis continued years after the mission ended. For example, detailed modeling and analysis by GFZ researcher Yuri Shprits' group revealed the physical mechanisms that cause these structures.

In a series of high-profile publications, the researchers explained that this behavior is due to very peculiar interactions between plasma waves, which are periodic fluctuations of the magnetic and electric fields. In particular, they showed that the dramatic acceleration is local and not due to the diffusion of particles from the outer region. Shprits explains, “these particles surf the plasma waves and gain energy, allowing near-Earth space to heat them locally to enormous energies so that they reach 0.999 of the speed of light.” Such interactions exist only in plasma, the fourth state of matter, he said, and are due to so-called collective behavior. He also said the sudden and dramatic loss in the heart of the radiation belts is due to interactions with waves that can scatter these particles into the atmosphere.

The determining factor for the interactions that lead to the acceleration as well as the disappearance of the particles is so-called cold plasma. Although the space plasma is so thin that the particles never collide with each other, the cold particles can collectively influence the surfing of the ultra-relativistic particles on the waves. Based on all these findings and the new physical insight, the authors, led by Yuri Shprits argue that the ultra-relativistic electrons in the radiation belts are a new population of particles. The work has been highlighted as a "editor’s highlight" by the new high-impact AGU journal Science Advances.

Original study: Shprits, Y. Y., Allison, H. J., Wang, D., Drozdov, A., Szabo-Roberts, M., Zhelavskaya, I., & Vasile, R. (2022). A new population of ultra-relativistic electrons in the outer radiation zone. Journal of Geophysical Research: Space Physics, 127, e2021JA030214. doi.org/10.1029/2021JA030214