Three-Step Particle Acceleration in Dual-Shock Interactions: Insights from Numerical Simulations and Observations of Ground-Level Events

This study investigates particle acceleration mechanisms in a twin-flare-jet shock scenario, revealing a three-stage acceleration process driven by shock interactions. Initially, Shock No.1 generates a normal power-law spectrum, followed by re-acceleration from Shock No.2, which hardens the spectrum and introduces a concave feature between 1 MeV and 20 MeV. The merged dual-shock system further accelerates particles, producing a high-energy tail up to 30 MeV. These findings align with observations of Ground-Level Enhancement (GLE) events, such as GLE 72, where dual-plateau structures suggest multi-stage acceleration. The results highlight progressive spectral hardening, a unique pile-up effect, and the critical role of magnetic field amplification in particle re-acceleration. This research enhances our understanding of shock interactions and their implications for high-energy astrophysical phenomena, including solar energetic particle events and cosmic-ray dynamics, emphasizing the significance of multi-shock scenarios in particle acceleration.