Probing Solar Wind Structures with Solar Energetic Particle Observations from Solar Orbiter

The transport of solar energetic particles (SEPs) through the heliosphere is governed by the interplanetary magnetic field (IMF) embedded in the solar wind plasma. Large-scale fluctuations in the IMF give rise to the transient variation in SEP intensities. Here, we present Solar Orbiter (SolO) observations of a distinct class of SEP flux variations: short-timescale (~1 hr) directional flux reversals (DFRs). Data from the Energetic Particle Detector (EPD) reveal that these reversals are a common feature in SEP events, occurring simultaneously across a wide energy range (keV to tens of MeV) and exhibiting steep intensity gradients. Unlike classic 'dropout' events—where intensity decreases isotropically—DFRs display a asymmetric signature where a intensity drop in one direction coincides with a spike in another. These intermittent variations are associated with flux-rope magnetic structures and distinct solar wind properties, which serves as direct evidence that the spacecraft has encountered a new solar wind stream with a different magnetic field connectivity. These observations demonstrate that SEPs act as an effective probe of solar wind structures, providing new insights into their nature.