Authors: Zigong Xu (California Institute of Technology,Pasadena), C.M.S. Cohen(California Institute of Technology,Pasadena), A.C. Cummings(California Institute of Technology,Pasadena), R.A. Leske(California Institute of Technology, Pasadena), G.D. Muro (California Institute of Technology, Pasadena), E.R. Christian(Goddard Space Flight Center, Greenbelt), D.J. McComas(Department of Astrophysical Sciences, Princeton University, Princeton), M.E. Wiedenbeck(Jet Propulsion Laboratory, California Institute of Technology,), N.A. Schwadron(iversity of New Hampshire, Durham)

Velocity dispersion (VD) in solar energetic particle (SEP) events indicates that the high energy particles with high speed arrive earlier than the slow speed particles, assuming that all the particles are released almost simultaneously. This characteristic allows for the determination of the particle release time near the Sun. However, the Labor Day event on Sep 5, 2022, has different observations. The medium energy particles arrive earlier than both lower and higher energy particles, creating a nose structure in the intensity spectrogram of EPI-Lo. We call the feature, where the higher energy particles arrive later than lower energy particles, inverse velocity arrival (IVA), to distinguish it from the typical VD. The mechanism that generated this structure in the SEP event remains unclear. One theory proposes that the later arrival of the higher energy particles is due to the time scale for acceleration and release of high energy particles by the CME-driven shock relative to the transport time of these particles from the shock region to the observer.  It is expected to be particularly evident when the observer, i.e., PSP, is close to the accelerator.

There are still many questions related to this feature that have not been answered. For instance, what is the typical energy value of the nose, and what governs its variability? What are the shock properties that contribute to the later arrival time of the higher energy SEPs? How common are these events in the PSP data? Answering these questions requires observing of many SEP events that have IVA features and a complete survey of our past observations.

One potential candidate that exhibits the IVA feature on PSP/EPI-Hi is the SEP event on 2023-12-31 when PSP was about 0.17 au from the Sun. In this work, we will concentrate on this event to begin to study the generation of IVA in this energy range.