Authors: Zigong Xu (caltech), C.M.S. Cohen(caltech), A.C. Cummings(caltech), R.A. Leske(caltech), G.D. Muro (caltech), E.R. Christian(Goddard Space Flight Center), D.J. McComas(Princeton), M.E. Wiedenbeck(Jet Propulsion Laboratory), N.A. Schwadron(Princeton University,University of New Hampshire) , R.L. McNutt(Applied Physics Laboratory ), D. Lario(Goddard Space Flight Center)

The inverse velocity arrival (IVA) feature of solar energetic particles (SEP) events, first observed in the Labor Day event on Sep 5, 2022, has been the subject of much discussion.  Unlike typical velocity dispersion (VD), where high-energy particles arrive earlier than lower energy particles due to their higher speed, the IVA event exhibits a unique characteristic where the medium energy particles with an energy of ~ few MeV arrive earlier than both lower ( ~hundreds of keV) and higher ( tens of MeV) energy particles, creating a nose structure in the intensity spectrogram. Instead of inverse velocity dispersion, we refer to this feature as inverse velocity arrival, which better reflects its complicated physical origin, and avoids misleading interpretations.

Several scenarios have been suggested to account for IVA, including the slow, ongoing acceleration at the flank of expanding shock, the young shock’s inability to generate high-energy particles, a particle transportation process involving the connectivity between the particle source and the observer, and the effect of energy dependent scattering and pitch angle distributions. The roles of these mechanisms in such events are still under debate. In this study, we propose a “contour-line” method to identify IVA events.

In addition to the Sep 5, 2022 SEP event, our study identifies 14 other SEP events that possibly exhibit IVA-like features based on the observations from EPiLo and EPiHi. In this study, we present the survey results and discuss the different scenarios that could lead to the IVA feature.