OVSA Science Highlight No. 2: Two Phases of Impulsive SEP Acceleration
Contributed by Meiqi Wang (Center for Solar-Terrestrial Research, New Jersey Institute of Technology, 323 Martin Luther King Jr Blvd., Newark, NJ 07102-1982, USA); Edited by Bin Chen; Posted on August 19, 2025.
A class of solar energetic particle (SEP) events that feature a short duration of a few hours is sometimes referred to as "impulsive" SEP events (Reames 1999). These events are usually accompanied by enrichment in the 3He isotope (Lin et al. 1996, Reames 2021) and have been argued to have a close association with reconnection processes in solar flares. Questions regarding how these energetic particles are produced by magnetic-reconnection-driven processes during solar flares remain outstanding.
Timing analysis of the release time of the SEPs observed in situ, when compared with remote-sensing observations of their emission signatures, provides important information about their origin. Recently, systematic energy-dependent delays of energetic electrons have been reported by using in situ measurements (e.g., Li et al. 2021). The origin of such energy-dependent delays may have important implications for the acceleration and/or transport processes of the SEPs. Li et al. proposed that these delays arise because outward-propagating electrons undergo a longer acceleration process than downward-propagating electrons.
This study investigates a SEP event associated with an X1.5-class solar flare on 2021 July 3, using combined in situ measurements and multi-wavelength remote-sensing observations, including hard X-ray (HXR) data made by Fermi/GBM and microwave data obtained by the Expanded Owens Valley Solar Array (see Figure 1). The HXR flux exhibits two impulsive periods during the main energy release phase: the first period is characterized by a single peak with a rapid rise and decay, while the second displays a more gradual light curve with a harder spectrum. Timing analysis based on in situ measurements of energetic electrons by the Parker Solar Probe (PSP) shows that these two HXR emission periods coincide with an energy-dependent release of the in situ energetic electrons. In particular, the more energetic electrons and protons appear to be released during the second impulsive episode (Figure 2).
The main results of this study are depicted in the following scenario (Figure 3): During the first impulsive period when the magnetic flux rope eruption is initiated, the magnetic energy release likely occurs in a relatively compact, localized reconnection region. In the second impulsive period, with the presence of a well-developed, large-scale reconnection current sheet trailing the erupting flux rope, more energetic electrons and ions are accelerated in the region above the loop-top. Finally, reconnection between the erupting flux rope and the ambient open field may provide a possible ‘’pathway’’ for energetic particles to escape into the interplanetary space, some of which reach the well-connected PSP spacecraft and are detected as in situ SEPs.
Based on the recent paper by Wang, M., Chen, B., Knuth, T., Cohen, C., Lee, J., Wang, H., & Yu, S. (2025), "Two Phases of Particle Acceleration of a Solar Flare Associated with In Situ Energetic Particles," The Astrophysical Journal, DOI: 10.3847/1538-4357/adbdd0
References
- Reames, D. V. 1999, SSRv, 90, 413
- Lin, R. P., Larson, D., McFadden, J., et al. 1996, GeoRL, 23, 1211
- Reames, D. V. 2021, Solar Energetic Particles. A Modern Primer on Understanding Sources, Acceleration and Propagation, Lecture Notes in Physics, vol. 978 (Berlin: Springer)
- Li, G., Wu, X., Effenberger, F., et al. 2021, GeoRL, 48, e95138