The Effect of the Ambient Solar Wind Medium on a CME-driven Shock and the Associated Gradual Solar Energetic Particle Event

Abstract

We present simulation results of a gradual solar energetic particle (SEP) event detected on 2021 October 9 by multiple spacecraft, including BepiColombo (Bepi) and near-Earth spacecraft such as the Advanced Composition Explorer (ACE). A peculiarity of this event is that the presence of a high-speed stream (HSS) affected the low-energy ion component (≲5 MeV) of the gradual SEP event at both Bepi and ACE, despite the HSS having only a modest solar wind speed increase. Using the EUHFORIA (European Heliospheric FORecasting Information Asset) magnetohydrodynamic model, we replicate the solar wind during the event and the coronal mass ejection (CME) that generated it. We then combine these results with the energetic particle transport model PARADISE (PArticle Radiation Asset Directed at Interplanetary Space Exploration). We find that the structure of the CME-driven shock was affected by the nonuniform solar wind, especially near the HSS, resulting in a shock wave front with strong variations in its properties such as its compression ratio and obliquity. By scaling the emission of energetic particles from the shock to the solar wind compression at the shock, an excellent match between the PARADISE simulation and in situ measurements of ≲5 MeV ions is obtained. Our modeling shows that the intricate intensity variations observed at both ACE and Bepi were influenced by the nonuniform emission of energetic particles from the deformed shock wave and demonstrates the influence of even modest background solar wind structures on the development of SEP events.

Improved modelling of SEP event onset within the WSA–Enlil–SEPMOD framework

“…At Bepi and Earth, all three SEPMOD simulation runs produce fluxes that are higher than observed. While at Bepi this could be due to the packed configuration of the instrument during its cruise phase, implying that the spacecraft itself may screen some particles (see also Appendix A in Palmerio et al, 2022), it is clear that at Earth this is owing to either a too-large SEP injection patch or the specific heliospheric context and Wijsen et al (2023), in fact, showed that there was a solar wind high-speed stream flowing between the CME source region and Earth's location, which may have resulted in particles being partially occulted and/or delayed.…”

“…Jebaraj et al (2023) examined the acceleration of electrons by both the solar flare temporally associated with the parent solar eruption and by the shock driven by the corresponding CME. Finally, Wijsen et al (2023) simulated the propagation of the CME-driven shock and the ESP event at two of the five observers using the European Heliospheric FORecasting Information Asset (EUHFORIA; Pomoell & Poedts, 2018) and PArticle Radiation Asset Directed at Interplanetary Space Exploration (PARADISE; Wijsen et al, 2019) models.…”

Acceleration and Release of Solar Energetic Particles Associated with a Coronal Shock on 2021 September 28 Observed by Four Spacecraft

“…We note that the release time of particles estimated at STA may be influenced by the in situ magnetic field direction (see Section 4.3 below). The effects of inhomogeneities in the ambient medium leading to different particle properties are not discussed here (e.g., Kouloumvakos et al 2022Kouloumvakos et al , 2023Wijsen et al 2023).…”

Cannibals in PARADISE: The Effect of Merging Interplanetary Shocks on Solar Energetic Particle Events

“…The modeling chain EUHFORIA-PARADISE has been used previously to successfully study and reproduce in situ observed gradual SEP events (e.g., Wijsen et al 2022Wijsen et al , 2023bNiemela et al 2023). In this work, we take a different approach and study a synthetic solar eruptive event that consists of two fast, interacting CMEs.…”