Spacecraft observations may conceal how particles really move through near-Earth space

High-energy particles within Earth's radiation belts can seem to disperse randomly, even when they are following a predictable path. This phenomenon arises because observations from spacecraft naturally obscure the intricate details of the particle population, according to a new study published in Physical Review Research. The International Space Science Institute (ISSI) research team, consisting of scientists from the University of Birmingham and the Czech Academy of Sciences, demonstrates that particles moving in a predetermined manner can produce patterns that closely resemble random motion, or diffusion, when viewed by a spacecraft.

The study emphasizes that intricate particle motion can generate spacecraft observations that closely resemble those typically attributed to diffusion. Radiation belts, doughnut-shaped regions filled with high-energy particles, are influenced by a planet's magnetic field and exist around various celestial bodies, including Earth, Saturn, Jupiter, and potentially ultracool brown dwarfs.

Grasping the behavior of particles within these environments is crucial since they pose risks to satellites, communications, and space missions. Complex structures of particles in predictable or energetic states can evolve into increasingly elaborate patterns as they navigate magnetic fields within the radiation belts. However, when these intricate structures are captured by spacecraft with limited resolution, they may present a smooth and diffusive appearance, despite the absence of actual diffusion.

Dr. Adnane Osmane, the lead author from the University of Helsinki, notes that for over six decades, spacecraft observations have been interpreted using diffusion-based models. The study's findings suggest that some observations could alternatively be explained by an entirely different process. The key takeaway is not that diffusion does not occur, but rather that observations alone may not always differentiate between diffusive and non-diffusive transport.

This has significant implications for interpreting spacecraft data and developing models of hazardous space environments around Earth and other celestial bodies. As a spacecraft moves within the radiation belts, it encounters particles traveling at varying speeds. Due to these speed discrepancies, well-organized particle structures can closely mimic the expected patterns resulting from random scattering by waves, even when no such scattering is taking place.

Co-author Dr. Oliver Allanson from the University of Birmingham highlights that for over six decades, scientists have interpreted radiation belt observations as evidence of random, diffusive particle movement through space. The study challenges these assumptions by suggesting that we may need to reconsider how we model and predict hazardous space environments, both around Earth and other planets, as well as in distant brown dwarfs.

The researchers draw an artistic analogy to illustrate this concept. A Jackson Pollock painting, rich with intricate lines, splatters, and filaments, resembles a Mark Rothko painting, which appears as expansive, smooth regions of color. The critical aspect is not the paintings themselves but what happens when fine details become indiscernible.

While a Pollock does not transform into a Rothko, some of its intricate structure becomes obscured, similar to how the complex filamentary structure created by particle motion becomes inaccessible through measurement, leading to highly structured dynamics appearing smooth and diffusion-like. The research underscores a limitation of many past radiation-belt missions, where a single spacecraft often struggles to distinguish spatial structure from temporal evolution, potentially leading to similar observational signatures for vastly different physical processes.

This underscores the need for future missions utilizing constellations of scientific satellites that can simultaneously observe the same particle populations from multiple locations. The study represents one of the first scientific publications to emerge from the ISSI International Team collaboration, highlighting the benefits of uniting experts in spacecraft observations, theoretical physics, and computational modeling to address longstanding questions in space science.