Multi-spacecraft observations of large solar energetic particle events with an inner-heliospheric spacecraft fleet

Abstract

Solar energetic particle (SEP) events are major manifestations of solar activity and present significant radiation hazards to space-based technology and human exploration. Despite extensive study, key questions persist regarding the contributions of flare- and shock-related acceleration, and the temporal evolution of SEP intensity profiles in the heliosphere. This thesis employs novel multi-spacecraft observations from early solar cycle 25 to advance understanding of SEP origin, acceleration, and transport, providing key insights for future space weather forecasting. The work is based on SEP events observed between November 2020 and May 2023 by Solar Orbiter, Parker Solar Probe, STEREO A, BepiColombo, and near-Earth spacecraft (SOHO and Wind). A new multi-spacecraft SEP event catalog was developed during the SERPENTINE project providing event parameters for ∼1 MeV and ∼100 keV electrons and ∼25 MeV protons, as well as for associated solar flares and type II radio bursts. The catalog comprises 45 independent multi-spacecraft events corresponding to ∼150 single-spacecraft observations, and was further extended during the thesis to include 98 events through December 2023. Electron and proton peak in tensities were correlated to probe the roles of flares and shocks in SEP acceleration. Significant intensity correlations are found across the sample. Events from poorly connected locations form a single SEP population, consistent with acceleration by spatially extended CME-driven shocks. Conversely, well-connected locations show two populations: one with a clear correlation between electron and proton intensities, linked to shock acceleration, and another, enriched in electrons, indicating a mixture of flare and shock-related acceleration. A superposed epoch analysis of normalized SEP intensity–time profiles for electrons and protons shows similar trends, generally well described by power-law or exponential functions. The similarity of these profiles, along with observed onset delays, supports the value of relativistic electrons as early indicators for forecasting later-arriving protons.