Recent research has overturned long-held assumptions about how energetic particles move during solar eruptions. For decades, scientists believed that faster, higher-energy particles always arrive first at detectors, producing a velocity dispersion pattern common in solar energetic particle (SEP) events.
However, ESA's Solar Orbiter has now recorded instances where high-energy particles rea by Riko Seibo
Tokyo, Japan (SPX) Sep 07, 2025
Recent research has overturned long-held assumptions about how energetic particles move during solar eruptions. For decades, scientists believed that faster, higher-energy particles always arrive first at detectors, producing a velocity dispersion pattern common in solar energetic particle (SEP) events.
However, ESA's Solar Orbiter has now recorded instances where high-energy particles reach detectors later than their lower-energy counterparts. This phenomenon, called inverse velocity dispersion (IVD), challenges established models and has prompted new investigation.
An international team led by Professors Jingnan Guo and Yuming Wang of the University of Science and Technology of China, with collaborators from Graz University and Kiel University, examined 10 SEP events showing IVD patterns observed by Solar Orbiter's Energetic Particle Detector. The study focused not on the effect itself but on its physical causes.
Their analysis showed that within the framework of diffusive shock acceleration (DSA), particles take longer to gain higher energies. This leads to a delayed release for fast particles, producing the observed IVD signature. Using this model, the team reconstructed shock acceleration parameters and calculated energy-dependent acceleration timescales for different shock conditions.
By deriving acceleration times directly from observations, the researchers were able to estimate the mean free path of particles near shocks close to the Sun. This method provides a new way to probe shock conditions that are otherwise inaccessible to direct measurement.
The findings strengthen links between SEP observations and theoretical acceleration models, deepening understanding of how interplanetary shocks energize particles to tens of MeV. Such insights are crucial both for space plasma physics and for predicting space weather events.
Practically, the work also addresses a key hazard. Energetic particles from solar eruptions can damage spacecraft systems and endanger astronauts. Improved models of particle timing and acceleration conditions will enhance forecasts of the radiation environment in the inner solar system, aiding future crewed missions to the Moon, Mars, and beyond.
Research Report:The delayed arrival of faster solar energetic particles as a probe into the shock acceleration process
Related Links
University of Science and Technology of China
Solar Science News at SpaceDaily