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)

The coexistence of the velocity dispersion (VD), where the high energy particles arrive earlier and inverse velocity dispersion (IVD), where the lower energy particles arrive earlier in one solar energetic particle (SEP) event was first reported from PSP/ISOIS. The mechanism that generated this structure in the SEP event is still unknown. One theory proposes that the IVD structure is related to the acceleration process of energy particles by the CME-driven shock and the release and transport of these particles from the shock region to the observer when the observer, i.e. PSP, is close to the accelerator.

A key feature of the IVD structure is the location of the nose, which indicates the first arrival of medium energy particles at the observer. What is this energy associated with, and how does the nose energy change? To answer these questions requires the observation of more IVD events, which is what we aim to find with the PSP measurements.

One potential candidate that exhibits IVD feature on PSP/Epi-Hi is the SEP event that occurred on 2023-12-31 when PSP was about 0.17 au from the Sun. In this work, we will investigate this event to understand the generation of the IVD in this energy range.

Intriguingly, from the proton dynamic spectrum of SolO/EPD a potential IVD feature is also found. However, further analysis confirmed that the “IVD” in SolO is due to multiple SEP events that happened at similar times and is not what we are looking for.