The relationship of inverse velocity dispersion SEP event characteristics with footpoint location: A survey of Solar Orbiter observations

Investigations of Solar Energetic Particle (SEP) events have long utilized the dispersive nature of onset times, i.e., earlier arrival of higher energy particles compared to lower energy populations, to infer information such as path length to an acceleration site. However, recent observations by Solar Orbiter and Parker Solar Probe have begun to characterize SEP events with an apparent “inverse velocity dispersion” (IVD) at higher energies, above a critical energy separating the classic velocity dispersion signature. These “nose”-feature SEP events may provide new insight into the impacts of magnetic connectivity to locations along an expanding CME-driven shock wave, variations of acceleration along the shock surface, and transport effects in the inner heliosphere. This presentation focuses on a statistical analysis of the occurrence rate and characteristics of IVD events observed by Solar Orbiter relative to their footpoint locations with respect to the initial flare site. While SEP events without IVDs have a broad distribution in location relative to the flare site, IVD events show a clear bias in occurrence to events with footpoints westward of the associated flare location. Implications of this spatial biasing, and impacts on the characteristics, i.e., critical energy and dispersive slope of the IVD portion of the event, is discussed in relation to recent modeling work (Ding et al., 2025). Additionally, the IVD events observed by Solar Orbiter, captured over a wide range of radial distances, is compared to a published IVD event from Parker Solar Probe near the corona at 15 solar radii (e.g., Cohen et al., 2024). These results imply that magnetic connectivity plays an important role in IVD events, particularly for those observed at larger radial distances.