Bidirectional anisotropic solar energetic particle events observed by Solar Orbiter

TL;DR

This study analyzes two rare bidirectional solar energetic particle events observed by Solar Orbiter, revealing how flux ropes influence particle trajectories and magnetic connectivity, with implications for understanding SEP transport in complex magnetic structures.

Contribution

It provides the first detailed analysis linking bidirectional SEP events to flux ropes and their impact on particle transport and magnetic topology.

Findings

Bidirectional SEP events show opposite anisotropies and velocity dispersions.

Flux ropes significantly alter particle path lengths and magnetic connectivity.

Different acceleration sources are identified for sunward and anti-sunward particles.

Abstract

Solar Energetic Particle (SEP) events are critical for understanding particle acceleration and transport in the heliosphere. While most SEP events involve outward streaming particles along open magnetic field lines, bidirectional events characterized by simultaneous sunward and anti-sunward particle flows offer unique insights into magnetic field topology and the interplay of multiple acceleration sources. We aim to investigate the origin and transport of energetic particles in two rare bidirectional anisotropic SEP events observed by Solar Orbiter. Both events showed two clear velocity dispersion signatures with opposite particle anisotropies during their onset phase. The sunward streaming protons, characterized by delayed release time, harder spectral index, and higher intensities, may be attributed to coronal mass ejection-driven shock acceleration, while the promptly released anti-sunward streaming protons are likely linked to flare acceleration. Notably, in both cases, small-scale flux ropes were identified in situ during the time intervals corresponding to the bidirectional particle streaming. Path lengths derived for sunward and anti-sunward injections were substantially greater than nominal values of the Parker field lines, further supporting the role of the flux rope in shaping particle trajectories. These observations demonstrate that magnetic flux rope could significantly affect magnetic connectivity to the source region and SEP propagation in the inner heliosphere, while simultaneous velocity dispersion from two distinct particle sources allows for direct constraints on the topology of the flux rope. Our results highlight the value of combining particle anisotropy, release time, source spectra, and magnetic structure diagnostics to unravel SEP transport in complex transient magnetic structures, and also present new challenges for the current SEP transport model.