Delayed maximum energy solar energetic particle events

Statistical analysis from Solar Orbiter

¹ Southwest Research Institute San Antonio TX, USA
² Johns Hopkins Applied Physics Lab Laurel MD, USA
³ Institut für Experimentelle und Angewande Physik, Christian-Albrechts-Universität zu Kiel Kiel, Germany
⁴ The Johns Hopkins University Baltimore MD, USA
⁵ Space Research Group, Universidad de Alcalá Alcalá de Henares Madrid, Spain
⁶ Montana State University Bozeman MT, USA
⁷ California Institute of Technology Pasadena CA, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 2 September 2025
Accepted: 24 November 2025

Abstract

Investigations of solar energetic particles (SEPs) have long utilized the dispersive nature of onset times, as in, the earlier arrival of higher-energy particles compared to lower-energy particles, to infer information such as the path length to the acceleration site at the time of initial particle release. However, recent observations by Solar Orbiter and Parker Solar Probe have begun to characterize SEP events with an apparent delay in arrival times of the higher energy portion of the particle distribution, above a critical energy separating the delayed particles from that of the typical velocity dispersion signature at lower energies. Features of these delayed maximum energy (DME) SEP events, sometimes referred to as “inverse velocity dispersion” events, could provide new insight into the impacts of magnetic connectivity to locations along an expanding coronal mass ejection-driven (CME-driven) shock wave, variations of acceleration along the shock surface, and transport effects in the inner heliosphere. This study focuses on the occurrence rate and characteristics of DME events observed by Solar Orbiter relative to their footpoint locations with respect to the initial flare site. These DME events show a bias in occurrence rate towards events when the observer’s footpoints were westward of the associated flare location. Additionally, estimated locations at which the highest-energy particles of DME events are released into the flux tube suggest continued release of increasingly higher-energy particles from the CME-driven shock into the connected flux tube well into the inner heliosphere. This indicates that DME events could be attributed to inner heliospheric effects and are not actually coronal in origin. This finding is consistent with previous observations and interpretations of SEP events connected westward of the associated flare.