The impact of large-scale transient structures on solar energetic particle transport

Solar energetic particle (SEP) events provide key constraints on particle acceleration and transport in the heliosphere. While most events are dominated by outward streaming particles along open magnetic field lines, rare bidirectional events showing concurrent sunward and anti-sunward flows, offer a unique probe of magnetic connectivity and the coupling of multiple acceleration sources. Here we analyze two uncommon bidirectional anisotropic SEP events observed by Solar Orbiter, focusing on their association with magnetic flux ropes. Both events exhibit two distinct velocity-dispersion signatures during the onset phase, accompanied by opposite anisotropies. The sunward-streaming protons show a delayed apparent release, a harder spectrum, and higher intensities, consistent with acceleration at a CME-driven shock and subsequent transport within transient magnetic structures. In contrast, the promptly released anti-sunward protons are more consistent with flare-related acceleration. These observations demonstrate the diagnostic power of jointly using anisotropy, inferred release times, source spectra, and in situ magnetic structure signatures to disentangle SEP transport in complex large-scale transients, and they pose new constraints for current SEP transport models.