Different processes can produce similar spacecraft observations, making it difficult to see how particles are transported through Earth's radiation belt.
High-energy particles in the Earth’s radiation belt can appear to spread out randomly, even when they are moving in a ‘predictable’ way. This is because spacecraft observations naturally blur the fine-scale structure of the particle population, a new study reveals.
Publishing their findings in Physical Review Research, the International Space Science Institute (ISSI) research team, led by the University of Birmingham and the Czech Academy of Science, shows that particles moving in a ‘predictable’ way can create patterns very similar to random movement or ‘diffusion’ when observed by a spacecraft.
The researchers show that highly structured particle motion can create spacecraft observations that look almost identical to those traditionally interpreted as diffusion. The findings highlight a fundamental challenge for space scientists: different physical processes can produce the same observational evidence.
Radiation belts are doughnut-shaped regions of high-energy particles trapped by a planet’s magnetic field. They also exist around Saturn, Jupiter and its moon Ganymede, as well as recently discovered ultracool brown dwarfs. Understanding how particles move through these environments is important because they can damage satellites, disrupt communications and affect space missions.
For over 60 years, scientists have interpreted radiation belt observations as evidence that particles spread through space in a random, diffusive way. Our findings challenge these assumptions about how radiation belts work - suggesting we may need to rethink how we model and predict hazardous space environments around Earth, other planets, and even distant brown dwarfs.
Localised populations of ‘predictable’ or energetic particles evolve into increasingly intricate structures as they drift through magnetic fields in the radiation belts. However, when these complex structures are sampled by a spacecraft with limited resolution, they can appear smooth and diffusive, even when no diffusion has occurred.
Co-author Dr Oliver Allanson, from the University of Birmingham, said: “For over 60 years, scientists have interpreted radiation belt observations as evidence that particles spread through space in a random, diffusive way. Our findings challenge these assumptions about how radiation belts work - suggesting we may need to rethink how we model and predict hazardous space environments around Earth, other planets, and even distant brown dwarfs.
“Radiation belts contain highly energetic particles that can damage satellites, disrupt communications and affect space missions. Our study suggests major implications for how scientists interpret spacecraft data and build models of particle acceleration and transport - challenging researchers to re-evaluate decades of interpretations.”
As a spacecraft travels through the radiation belts, it samples particles moving around the planet at slightly different speeds. Because of these speed differences, well-organised particle structures look very similar to what would be expected if particles were being randomly scattered by waves, even when no such scattering is taking place.
Lead author Dr Adnane Osmane, from the University of Helsinki, said: “Our results show that some observations may also be explained by a fundamentally different process. The key message is not that diffusion does not occur, but that observations alone may not always distinguish between diffusive and non-diffusive transport.
"This has important implications for how we interpret spacecraft data and develop models of hazardous space environments around Earth and other planets. If different physical processes can appear similar in the observations, we need to be careful about how we infer the underlying physics."
The researchers use a striking artistic analogy to explain the idea. A Jackson Pollock painting contains a rich web of intricate lines, splatters and filaments, while a Mark Rothko painting appears as large, smooth regions of colour.
What matters is not the paintings themselves, but what happens when fine detail can no longer be resolved. A Pollock does not transform into a Rothko; rather, some of its intricate structure becomes hidden from view. Similarly, the complex filamentary structure created by particle motion does not disappear, but becomes inaccessible through the measurement process, causing highly structured dynamics to appear smooth and diffusion-like.
Iconic artworks such as Pollock’s Alchemy (1947) and Rothko’s 1949 piece Untitled (Violet, Black, Orange, Yellow on White and Red) illustrate this analogy.
Corresponding author Dr Mirek Hanzelka, from the Czech Academy of Science, said: “Our research points to an important limitation of many past radiation-belt missions: with a single spacecraft, spatial structure and temporal evolution can be difficult to tell apart, so very different physical processes may leave remarkably similar observational signatures. This makes a strong case for future missions using constellations of scientific satellites that can observe the same particle populations simultaneously at multiple locations.”
The research is one of the first scientific publications to emerge from the ISSI International Team collaboration - demonstrating the value of bringing together experts in spacecraft observations, theoretical physics and computational modelling to tackle long-standing questions in space science.
For more information, please contact Tony Moran, International Communications Manager or +44 (0)7827 832312
‘Collisionless phase mixing mimics diffusive transport in radiation belt observations’ - Adnane Osmane, et al is published in Physical Review Research.
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Participating institutions: University of Birmingham, UK; University of Exeter, UK; University of Helsinki, Finland; University of California, USA; University of Texas, USA; Air Force Research Laboratory Space Vehicles Directorate, New Mexico, USA; and Institute of Atmospheric Physics of the Czech Academy of Sciences, Czech Republic.
Assistant Professor
Profile of Dr Oliver Allanson, Department of Electronic, Electrical and Systems Engineering, University of Birmingham
