Published in Physical Review Research, the study, led by researchers at the University of Birmingham and the Czech Academy of Sciences, examines how spacecraft measurements can conceal the fine structure of particle motion. The international team argues that collisionless phase mixing—a process in which particles with slightly different velocities gradually separate into increasingly intricate patterns—can mimic the observational signature of diffusive transport, even when particles are not being randomly scattered.

Radiation belts are vast, doughnut-shaped regions surrounding planets with strong magnetic fields. Earth’s belts contain electrons and ions energetic enough to damage satellites, interfere with spacecraft electronics and threaten astronauts. Comparable particle environments have been identified around Jupiter, Saturn and Ganymede, and may also exist around ultracool brown dwarfs. Predicting how these particles accelerate and travel is therefore central to space-weather forecasting and the design of future missions.

The process described by the researchers begins with a localised injection of energetic particles. At first, the particles form a relatively coherent population. As they drift around the planet, however, they do not all move at exactly the same speed. Tiny differences in energy, pitch angle or position can cause some particles to move slightly ahead of others. Over time, the original population stretches into long, narrow filaments and eventually develops a complex, highly folded structure in phase space—the mathematical space describing both particle positions and velocities.

This evolution is deterministic rather than random. No wave-driven scattering or stochastic “jumps” are required to produce the increasingly dispersed appearance. Yet a spacecraft sampling the radiation belt does not observe the entire particle population at once. It records a limited measurement along its trajectory, with finite spatial, temporal and instrumental resolution. When the spacecraft crosses a region containing structures too fine to resolve, the detailed variations are effectively averaged out. The resulting signal becomes smooth, resembling the gradual spreading expected from diffusion.

Diffusion is a familiar concept in plasma physics. In conventional radiation-belt models, waves and other disturbances scatter particles, changing their energies and directions in a manner that can be described statistically. Over time, these random interactions spread a particle population through space or energy. The new study does not claim that diffusion is absent from radiation belts. Instead, it shows that the same broad observational pattern can arise from a fundamentally different mechanism, making it difficult to identify the underlying process from a single spacecraft record.

Lead author Adnane Osmane of the University of Helsinki says the distinction matters because models built on diffusion can produce confident predictions about particle lifetimes, acceleration and loss. If a smooth measurement is interpreted automatically as evidence of random scattering, researchers may estimate the strength of wave-particle interactions incorrectly. That could affect forecasts of when radiation levels will rise, how long hazardous particles will remain trapped and which satellites or missions are most vulnerable.

The study also exposes a broader problem in space physics: the difficulty of separating spatial structure from temporal change. A single spacecraft samples different locations at different moments, so it may be impossible to determine whether a measured variation reflects particles evolving with time or the spacecraft simply crossing a complicated pattern. Corresponding author Mirek Hanzelka of the Czech Academy of Sciences says this ambiguity is a major limitation of many past radiation-belt missions, because different physical processes can leave remarkably similar signatures in the data.

The researchers compare the effect to viewing a detailed Jackson Pollock painting from a great distance. The intricate lines and splashes do not become a Rothko-like field of colour, but the observer can no longer resolve the fine structure. In the same way, organised particle filaments do not transform into random motion; they are merely hidden by the measurement process. The team argues that future missions using constellations of spacecraft could help solve the problem by observing the same particle population simultaneously from multiple locations. Such multipoint measurements could reveal whether apparent diffusion is genuine scattering or the blurred signature of deterministic phase mixing. The result is a warning—and an opportunity—for space scientists: in the radiation belts, an image that looks smooth may conceal a system that is anything but random.

Subject of Research: Not applicable

Article Title: Collisionless phase mixing mimics diffusive transport in radiation belt observations

News Publication Date: 24 July 2026

Web References: https://journals.aps.org/prresearch/abstract/10.1103/5mmn-fm2p; https://teams.issibern.ch/beyonddiffusion/

References: Adnane Osmane et al., “Collisionless phase mixing mimics diffusive transport in radiation belt observations,” Physical Review Research, DOI: 10.1103/5mmn-fm2p

Image Credits: Adnane Osmane

Keywords

Radiation belts, space weather, particle diffusion, collisionless phase mixing, plasma physics, spacecraft observations, Earth’s magnetosphere, satellite safety, energetic particles, space science

In the latest study, researchers from the Institute of High Energy Physics at the Chinese Academy of Sciences alongside the National Institute of Natural Hazards employed an integrative analysis technique to decipher the underlying mechanisms steering the SAA’s evolution. Utilizing CSES’s sophisticated High Energy Particle Package (HEPP) and Magnetic Field Measurement Instrument (HPM), this study meticulously charted proton flux distributions ranging from 2 to 20 MeV energies and correlated those with magnetic field variations derived from the International Geomagnetic Reference Field (IGRF) model. The investigation focuses on contrasting differential proton flux maps spanning January 2019 through January 2024, thereby capturing the neighborhood of profound geomagnetic perturbations within the anomaly’s core.

Crucial to the study’s revelations is the observation of the SAA’s proton flux center exhibiting consistent drifts: westward and northward movements have been confirmed over the five-year monitoring interval. A double-Gaussian fitting approach allowed for high-precision quantification, revealing a daytime northward drift rate averaging 0.29 degrees per year, while westward drifts hovered around 0.35 degrees annually. Notably, the drift magnitude manifested an energy dependence, where lower-energy protons (2.0–10.0 MeV) responded more swiftly to geomagnetic irregularities than higher-energy counterparts (10.0–20.0 MeV). This suggests intricate interactions between particle energies and Earth’s magnetic field topology, emphasizing the influence of magnetic field inhomogeneities in shaping particle transport dynamics.

This energy-dependent spatial differentiation also illuminated distinct distribution morphology within the SAA region. Low-energy protons revealed a characteristic double-peak distribution, indicative of complex trapping and precipitating routes modulated by localized magnetic field gradients. Conversely, higher-energy protons tended to coalesce into a single-peak pattern, confirming compatibility with legacy observations acquired from NOAA’s Polar Orbiting Environmental Satellites (POES) and Magnetic Electron Proton Detector (MEPED). Such synergistic validation underscores the robustness of multi-instrument satellite analyses in capturing fine-scale space environment features.

Parallel to proton flux variations, global geomagnetic field analysis utilizing IGRF-13 model data revealed a pronounced hemispheric asymmetry in field strength evolution. The Eastern Hemisphere’s magnetic intensity exhibited a strengthening trend, while the Western Hemisphere, particularly over the SAA’s geographical locus, experienced marked field weakening. This magnetic drop-off undermines the effectiveness of Earth’s natural radiation shield by reducing geomagnetic rigidity thresholds, thereby permitting an enhanced charge particle influx into near-Earth environments. Consequently, the concurrent escalation of proton flux detected by CSES in the SAA core could be directly attributed to this deteriorating field configuration.

More detailed regional assessments disclosed divergent proton behavior within L-shell boundaries: the inner belt region with L = 1.2–1.5 displayed a significant surge in proton intensity, whereas outer belts (L > 1.5) saw a reduction during the same epoch. This phenomenon evidences the simultaneous operation of competing processes. Enhanced solar activity, validated by elevated F10.7 solar radio flux indices, appears to suppress proton populations in outer belts via magnetospheric scattering or loss mechanisms. However, localized geomagnetic attenuation within the SAA core facilitates proton acceleration and deeper penetration into lower L-shell domains. This dualistic interplay illustrates the delicately balanced effects of solar-terrestrial coupling on the radiation environment.

Moreover, through precise boundary delineation of the SAA proton flux region, the study documents a net contraction of approximately 6% in the anomaly’s spatial extent between 2019 and 2024. This shrinkage corresponds to an annual reduction rate of roughly 4.3 × 10^5 square kilometers, emphasizing significant morphodynamic adjustments possibly triggered by varying geomagnetic and solar inputs. Notably, the negative correlation between solar radio flux (F10.7) and SAA area implies that solar activity modulation is instrumental in accelerating these geomagnetic field evolutions and particle redistribution.

These discoveries leverage the unique capabilities of the CSES satellite’s integrated instrument suite, offering unparalleled precision in resolving high-energy particle signatures alongside local magnetic field measurements. The high-resolution data permit fine temporal and spatial tracking of the SAA’s transformations, surpassing prior studies constrained by coarser satellite datasets. Such insights are critical for the design and positioning of low-Earth orbit spacecraft, ensuring optimized orbit planning to mitigate radiation exposure risks that might jeopardize satellite functionality or astronaut health.

Beyond immediate space weather forecasting and operational hazard management, this research furnishes vital contributions to our fundamental understanding of Earth’s magnetospheric physics. It enriches theoretical models of radiation belt particle transport, magnetosphere-ionosphere coupling, and the long-term geomagnetic field evolution influenced by internal geodynamo fluctuations and external solar drivers. As the solar cycle continues its ascent, continuous monitoring of the SAA region gains paramount significance for both scientific inquiry and practical aerospace applications.

In synthesis, this investigation portrays the South Atlantic Anomaly not as a static anomaly but a dynamically evolving feature intricately coupled to solar cycle influences, geomagnetic field perturbations, and energetic particle behaviors. The documented westward and northward prograde drifts, spatial contraction, and energy-dependent response patterns highlight the complex magnetospheric processes modulated both internally by terrestrial magnetic variations and externally by solar activity fluxes. Future efforts incorporating long-term satellite missions and advanced modeling will be indispensable to deepen our predictive capabilities concerning the SAA’s evolution and mitigate associated technological and biological vulnerabilities in space.

By harnessing the CSES satellite’s high-fidelity instrumentation and sophisticated analytical approaches, this landmark study establishes a critical empirical foundation for the progressive refinement of space environment models. It underscores the necessity of continuous in-situ environmental monitoring in anticipating radiation hazard fluctuations, thereby serving as a cornerstone for future spacecraft mission design and astronaut safety protocols. Moreover, the documented correlation between solar radio flux and proton flux distributions within the SAA unveils prospective pathways to incorporate solar activity forecasting in radiation belt mitigation strategies, fostering a more resilient space weather preparedness paradigm.

Ultimately, advancing our grasp of the South Atlantic Anomaly’s shifting geomagnetic and particle landscape supports broader objectives across geophysics, planetary science, and aerospace engineering. This research reaffirms the intrinsic interconnectedness of solar and terrestrial phenomena and paves the way for continued multi-disciplinary collaborations to decode the complexities of Earth’s space environment under a changing solar regime.

Subject of Research: Spatiotemporal evolution of geomagnetic field intensity and high-energy proton flux in the South Atlantic Anomaly during Solar Cycle 25.

Article Title: Revealing Dynamic Variations and Solar Coupling in the South Atlantic Anomaly through Six Years of CSES Observations

Web References: 10.1007/s11430-025-1672-2

Image Credits: ©Science China Press

Keywords: South Atlantic Anomaly, geomagnetic field, proton flux, radiation belts, CSES satellite, Solar Cycle 25, high-energy particles, space weather, geomagnetic drift, solar-terrestrial coupling