image: Commonly seen velocity dispersion feature is marked with blue circles in lower energy range observed by Solar Orbiter, while the onset of inverse velocity particles is indicated with orange triangles in higher energy ranges.
Credit: ©Science China Press
The acceleration and transport of energetic particles during solar eruptions remain central and long-standing challenges in space plasma physics. A widely accepted picture holds that higher-energy particles released during a solar eruption are detected earlier than lower-energy ones, forming a so-called velocity dispersion (VD) pattern in the dynamic spectrum that is commonly observed in solar energetic particle (SEP) events.
Recent measurements by ESA’s Solar Orbiter have revealed cases where this expectation is reversed, with higher-energy particles arriving later than lower-energy ones. This counterintuitive behavior, known as inverse velocity dispersion (IVD), invokes a bunch of studies.
An international research team, led by Professors Jingnan Guo and Yuming Wang and composed of members from the University of Science and Technology of China, Graz University and Kiel University, analyzed 10 SEP events with clear IVD signatures observed by Solar Orbiter’s Energetic Particle Detector (EPD). Rather than focusing on the phenomenon itself, the study investigated the underlying mechanisms. The dominant one is explained within the diffusive shock acceleration (DSA) framework, where particles require progressively longer time to reach higher energies, resulting in energy-dependent release.
Using DSA model, the team went backwards to IVD’s physical origin, innovatively retrieved the parameters of the shock acceleration conditions and determined energy-dependent acceleration timescales under different shock conditions. Observational-derived energy-dependent acceleration time was then used to retrieve the theoretical mean free path of particles at the acceleration shock chose to the Sun.
The findings highlight how IVD structures can serve to derive the physical conditions at interplanetary shocks that cannot be observed directly. By linking observational features with theoretical acceleration models, the study advances our understanding of how shocks energize particles to tens of MeV, a process fundamental to both basic space plasma physics and space weather forecasting.
Beyond its scientific importance, the work also has practical implications. Energetic particles accelerated during solar eruptions pose significant hazards to spacecraft systems and astronaut health. Improved knowledge of the timing and conditions of particle acceleration will help refine models of the radiation environment in the inner heliosphere, supporting future missions to the Moon, Mars, and beyond.