Parker Solar Probe observations of solar energetic particle (SEP) events with inverse velocity arrival (IVA) features

TL;DR

The study addresses how solar energetic particle events can exhibit inverse velocity arrival (IVA), a departure from standard velocity dispersion, by analyzing Parker Solar Probe data with a novel contour-line method that unifies multi-instrument measurements. It systematically identifies 14 IVA events from 2018–2024 and classifies SEP onsets into three types: VD, nose-only IVA, and mixed IVA/VD, revealing a two-population scenario where an initial VD population is followed by a slower, later-accelerated nose population. Most IVA noses occur in the 0.5–5 MeV range and are associated with CME-driven shocks, with average shock angles around 48 degrees and shock speeds near 1000 km/s, though the sample size is limited. The work highlights inner-heliospheric shock acceleration as a key driver of IVA, provides a quantitative framework for comparing IVA across missions, and motivates further cross-mission analyses to unravel the balance between acceleration, transport, and connectivity in SEP events.

Abstract

In SEP events, velocity dispersion (VD) is characterized by the earlier arrival of faster, higher-energy particles relative to slower ones, assuming negligible acceleration time and transport effects. The "Labor Day event" at Parker Solar Probe (PSP) on 2022 September 5 provided a unique arrival profile, in which the medium energy (~ few MeV) particles arrive earlier than both lower and higher energy particles. This created a so-called "nose" structure in the intensity spectrogram formed by measurements from the two energetic particle instruments, EPI-Lo and EPI-Hi, of the Integrated Science Investigation of the Sun (ISOIS) suite. Unlike typical VD, the delayed arrival of higher energy particles compared to medium energy particles, i.e., the "inverse velocity arrival" (IVA), could be caused by various acceleration, transport, and instrumental effects, including shock acceleration. By applying a new method based on the contour-line of the intensity, we found 14 IVA events in the ISOIS observations up to the end of 2024. Several parameters that may modify velocity dispersion characteristics are further explored including the spacecraft radial distance, the speed of corresponding CMEs and shocks, the angle between the shock normal and the upstream magnetic field, and the spacecraft magnetic footpoint longitudinal separation from the flare location. The energy of the early arriving particles, i.e., the nose energy, can be grouped into low (L, <0.5 MeV), medium(M, 0.5 - 5 MeV), and high(H, >5 MeV) categories. Most (11/14) of the IVA events have medium nose energies. This SEP list provides ingredients for examination of shock acceleration in the inner heliosphere, and the existence of IVA events sheds new light on the acceleration and propagation of SEPs.

Figures (5)

• Figure 1: (a): PSP observations of the energetic particles from EPI-Lo, EPI-Hi/LET + HET, the magnetic field variation and the radio emission by FIELDS bale_fields_2016 before and during the Labor Day event on 2022 September 5. Three vertical lines, from left to right, mark the flare eruption time, SEP event onset time cohen_observations_2024, and the shock arrival time, respectively. (b): The relative positions of PSP and Solar Orbiter to the flare region during the event Gieseler2023FrASS. The Parker spiral lines are calculated using the averaged solar wind velocities measured at PSP before the start of the SEP events.
• Figure 2: Three types of SEP events (top to bottom): (a): VD event - Spiky event with normal velocity dispersion; (b): Nose-only event - SEP event with IVA features; (c): Mixed event - a mixture of the above two types with clear separation between the first arriving VD and the later inverse features. The intensity unit is $\mathrm{count}/(\mathrm{s\cdot sr\cdot MeV \cdot cm^2)}$. These plots combine measurements from EPI-Hi/LET-A and HET-A, and the mean intensity averaged over all EPI-Lo apertures. The contour lines are calculated between the two vertical dashed lines.
• Figure 3: The dynamic spectrograms of the other 12 IVA events. The contour lines are drawn between two vertical dashed lines.
• Figure 4: The distribution of the IVA events along the radial distance (A), versus the longitudinal separation from the flare (B), the $\theta_{Bn}$ of the shock (C), and the speed of the shock when it passes PSP and the speed of CME (D). The left sub-panels display the distribution in the order of time, and the right sub-panels present histograms of the distribution. The blue colored stars and histogram represent results for all 14 cases, while the orange colored ones are for events with shocks measured locally at PSP. The vertical dashed and the dotted lines indicate the corresponding mean and median values in the given panels. Specifically, red stars and histogram in panel (D) indicate the speed of local shock that arrived at PSP, and
• Figure 5: The relationship between the speed of the local shock (V$_{sh}$ ) / CME (V$_{cme}$) and the longitudinal separation from the spacecraft's magnetic footpoint to the flare, and the radial distance of the spacecraft, and