Inverse Velocity Dispersion in Solar Energetic Particle Events Observed by Solar Orbiter

Inverse velocity dispersion (IVD) in solar energetic particle (SEP) events, where higher-energy particles arrive later than lower-energy particles, is increasingly observed by spacecraft such as Parker Solar Probe (PSP) and Solar Orbiter (SolO). However, the underlying mechanisms driving IVD events are not well understood. This study examines the physical processes responsible for long-duration IVD events by analyzing the SEP event detected by SolO on June 7, 2022. The event displayed a clear and prolonged IVD signature across proton energies ranging from 1 to 20 MeV, with heavy ions exhibiting varying nose energies. Simulations indicate that evolving shock connectivity plays a crucial role in shaping the IVD signature, as SolO’s connection shifts from the shock flank to the nose over time, resulting in a gradual increase in the maximum particle energy along the field line. Furthermore, model results show that limited cross-field diffusion affects both the nose energy and the duration of the IVD event. This study highlights that long-lasting IVD events are primarily driven by evolving shock connectivity to the observer, with connections to more efficient acceleration sites at larger solar distances.