Sunward Streaming 3He-rich SEP Events Observed by Solar Orbiter and Parker Solar Probe during Perihelion Passage

We report on two 3He-rich solar energetic particle (SEP) events observed by Solar Orbiter (SO) and Parker Solar Probe (PSP) during the April 1 - 4, 2024 conjunction when both spacecraft are within 0.3 AU near their respective perihelion passage. The two 3He-rich SEP events, originating from active region (AR) 13615, exhibit two key anomalies: (1) sunward streaming SEPs and (2) SEP travel path lengths exceeding 2 - 8 times the nominal Parker spiral expectations. Remote observations suggest these SEPs have been redirected by a slow coronal mass ejection (CME) that also originated from AR 13615 two days prior on March 30, 2024 at 21:04 UT. Using the near-Sun CME speed and width measurements, we estimate its size and location at the onset time of the first 3He-rich SEP event. Based on our estimates, SEPs propagating around the ICME front have travelled between 0.76 - 0.95 AU when they arrive at SO, increasing to 0.94 - 1.1 AU at PSP, consistent with our observed SEP arrival times. These findings constitute the first multi-spacecraft observation of sunward streaming 3He-rich SEPs. We discuss the implications of this phenomenon on 3He-rich seed material and the rare widespread 3He-rich SEP events (see Section 5.4).

💡 Research Summary

This paper presents a detailed study of two ³He‑rich solar energetic particle (SEP) events observed simultaneously by Solar Orbiter (SO) and Parker Solar Probe (PSP) during the 1–4 April 2024 conjunction when both spacecraft were within 0.3 AU of the Sun near perihelion. The events originated from active region (AR) 13615 and displayed two striking anomalies: (1) a bulk sunward streaming of the particles, i.e., the dominant flow was directed toward the Sun rather than away, and (2) exceptionally long particle travel path lengths, measured to be 2–8 times longer than the nominal Parker‑spiral magnetic field length.

In‑situ measurements were obtained with SO’s Solar Isotope Spectrometer (SIS) and PSP’s Energetic Particle Investigation (LET). SIS‑A (sunward‑facing) and SIS‑B (anti‑sunward) provided high‑resolution mass spectra from 0.04 to 10 MeV nucleon⁻¹, while LET covered 1–15 MeV nucleon⁻¹. Type III radio bursts recorded by PSP/FIELDS at 19:58 UT on 1 April and 02:05 UT on 3 April marked the solar release times. By aligning these release times with the observed onsets in a common energy channel (1.47 MeV nucleon⁻¹ for SO, 1.68 MeV nucleon⁻¹ for PSP) the authors derived path lengths of 0.98 ± 0.05 AU for SO and 1.37 ± 0.05 AU for PSP—corresponding to 3.3× and 8.6× the expected Parker‑spiral distances at 0.3 AU and 0.16 AU, respectively. Velocity‑dispersion plots clearly favored the longer, “1‑AU‑equivalent” paths over the nominal 0.3 AU/0.16 AU curves.

Pitch‑angle distributions revealed that the sunward‑facing SIS‑A recorded higher fluxes than the anti‑sunward SIS‑B, confirming a net sunward anisotropy. This is unusual for ³He‑rich events, which are normally accelerated low in the corona and then stream outward. The authors propose that a slow coronal mass ejection (CME) launched from the same AR on 30 March 2024 21:04 UT (speed < 400 km s⁻¹, angular width ≈ 70°) acted as a magnetic “deflector.” Using the CME’s near‑Sun speed and expansion, they estimated the CME front’s position at the onset of the first SEP event to be 0.76–0.95 AU from the Sun for the SO line of sight and 0.94–1.10 AU for PSP. The SEPs are therefore thought to have wrapped around the CME’s leading edge, traveling a longer helical path before reaching the spacecraft. This geometry naturally explains both the elongated path lengths and the observed sunward streaming, as particles that have traversed the CME flank re‑enter the interplanetary magnetic field directed back toward the Sun.

The paper discusses broader implications. First, the CME‑mediated redirection offers a plausible mechanism for the rare “widespread” ³He‑rich SEP events that have been observed over >120° of heliolongitude; a single CME can connect distant magnetic footpoints, allowing the same seed population to be sampled by widely separated observers. Second, the presence of ³He‑rich material trapped or lingering on the CME front could serve as a pre‑accelerated seed for subsequent large‑scale gradual SEP events when a faster shock overtakes the structure. This bridges the long‑standing question of how ³He‑rich particles contribute to the seed pool for high‑energy events.

In conclusion, the authors provide (i) the first multi‑spacecraft observation of sunward‑streaming ³He‑rich SEPs, (ii) quantitative evidence that a slow CME can dramatically lengthen SEP travel paths by forcing particles to follow a wrapped trajectory around its front, and (iii) a discussion of how this process impacts our understanding of both widespread ³He‑rich events and the seed population for larger SEP storms. The work calls for high‑resolution MHD simulations and multi‑point particle tracing to further validate the proposed CME‑deflection scenario.