Evidence of flare and coronal mass ejection-driven shock acceleration of relativistic protons on 21 November 2024

Abstract

On 21 November 2024 at 17:55 UTC, a solar flare erupted from far-side active region (AR) 13892, with enhanced hard X-ray flux measured by Solar Orbiter (SO). It was associated with a coronal mass ejection (CME) travelling at 1436 km/s along the limb. This event produced a strong and widespread solar energetic particle (SEP) event at Earth, STEREO-A and SO, with proton energies exceeding 100 MeV. We investigate the relativistic particle acceleration during this event, focusing on separating flare- and shock-related contributions and linking spectral evolution to the CME-driven shock properties. We combine multi-spacecraft energetic-particle observations with velocity dispersion analysis (VDA), three-dimensional CME-driven shock reconstruction, and magnetic connectivity modelling. Radio observations and SEP composition provided independent diagnostics of shock- and flare-associated particle acceleration, respectively. VDA yields solar release times (SRTs) consistent with an early flare-accelerated particle population at all three observers, despite their broad range of helio-longitudes. At STEREO-A, the early release is followed immediately by magnetic connection to an efficient CME-driven shock. A 3D reconstruction of the type II radio burst location confirms a connection between Earth and an efficient shock-acceleration region several minutes later. Within the adopted models, the SO release precedes magnetic connection to the reconstructed front, disfavouring acceleration at the connected shock as the source of the earliest particles. Despite this diversity, STEREO-A and SO show similar spectral evolution, while the near-Earth spectrum extends to relativistic energies and exhibits a high-energy rollover. SEP abundances also show signatures of flare-related acceleration at all three observers. The analysis suggests multiple acceleration processes acting non-uniformly across observers.