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Close-up measurements reveal two clear particle populations
Our Sun appears calm from Earth, but spacecraft that approach the star show a far more dynamic environment. New analysis of in situ measurements from the European Space Agency's Solar Orbiter demonstrates that energetic particles launched during solar eruptions fall into two distinct groups with different origins and travel behaviors. These findings improve our understanding of solar energetic electrons (SEEs), a particle population that contributes to space weather and can threaten satellites and astronauts.
"The Sun is the most energetic particle accelerator in the Solar System," the researchers note. Using Solar Orbiter data collected between 2020 and 2022, the team examined more than 300 SEE events and identified a consistent dichotomy. One class—known as impulsive events—is tightly linked to rapid, localized releases of particles during solar flares. The other—gradual events—correlates with larger, longer-lived coronal mass ejections (CMEs) that produce a broader, more extended swell of particles across wider angular ranges.
Mission profile and how Solar Orbiter made the difference
Solar Orbiter travels on an eccentric orbit that takes it as close as about 42 million kilometers to the Sun. This proximity allowed instruments on board to sample electron streams in a relatively pristine state before they are modified by long-distance propagation effects. By combining direct, in situ particle measurements with remote-sensing observations of the solar surface and corona, the team could link specific particle streams to the flare or CME that produced them.
An illustration of Solar Orbiter measuring different types of solar energetic electrons. (ESA & NASA/Solar Orbiter/STIX & EPD)
Lead author Alexander Warmuth (Leibniz Institute for Astrophysics Potsdam, AIP) explains that only Solar Orbiter's combination of distance, instrumentation, and repeated encounters with particle events allowed researchers to resolve the two groups clearly. Co-author Frederic Schuller adds that this is the first time scientists have directly observed such a robust connection between the particle populations measured in situ and their solar source events.
Propagation effects, timing puzzles, and practical implications
One long-standing problem in heliophysics has been mismatches between the timing of electromagnetic signatures at the Sun—such as light and radio bursts—and when SEEs are detected by spacecraft. The team finds that these apparent lags are often not delays in particle release but are caused by the way electrons travel through the turbulent heliosphere. "Electrons encounter turbulence, get scattered in different directions, and so on, so we don't spot them immediately," says co-author Laura Rodríguez-García. These scattering and transport processes accumulate with distance from the Sun and change the observed timing and angular spread of particle events.
An immense M7-class solar flare captured by NASA's Solar Dynamics Observatory on 19 July 2012. (NASA/Royal Observatory Belgium/SIDC)
Understanding the source, timing, and evolution of SEEs has direct operational value. Daniel Müller, ESA project scientist for Solar Orbiter, emphasizes that refined knowledge of energetic particle generation and transport helps improve space weather forecasting and spacecraft risk mitigation. In particular, distinguishing impulsive, flare-related electron bursts from broader CME-driven streams enables more targeted models of radiation exposure for satellites and human missions.
Expert Insight
Dr. Elena Park, solar physicist (University-style affiliation), comments: "This analysis leverages Solar Orbiter's unique vantage point. Measuring electrons close to their source reduces ambiguity introduced by heliospheric scattering. That clarity lets us tie particle signatures back to specific flare or CME dynamics—critical for improving predictive models of space weather that protect assets in orbit and future crewed missions."
Conclusion
Solar Orbiter's close-up observations have confirmed a clear split between impulsive, flare-driven energetic electrons and more gradual, CME-associated streams. By measuring SEEs in situ and correlating them with solar source events, researchers have improved our understanding of particle acceleration mechanisms and the transport processes that modify particle arrival at spacecraft. These insights refine space weather models and enhance our ability to protect satellites and human explorers from solar radiation hazards, while Solar Orbiter continues to probe the near-Sun environment for further discoveries.
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