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

This paper reports the first multi-spacecraft observation of sunward streaming 3He-rich solar energetic particles by Solar Orbiter and Parker Solar Probe, revealing that these particles traveled significantly longer paths than expected due to redirection by a slow coronal mass ejection originating from active region 13615.

The Gist

The Big Picture: A Solar "U-Turn"

Imagine the Sun as a giant lighthouse constantly beaming out particles (like tiny, high-speed marbles) into space. Usually, when a burst of these particles happens, they shoot straight out from the Sun, like water from a garden hose. Scientists call these bursts "Solar Energetic Particle" (SEP) events.

Usually, if you are floating in space, you see these particles coming toward you from the Sun. But in this study, scientists using two special spacecraft—Solar Orbiter (SO) and Parker Solar Probe (PSP)—saw something weird happen in April 2024.

Instead of seeing particles flying away from the Sun, they saw them flying back toward the Sun. It was like watching a car drive down a highway, hit a massive detour sign, and suddenly drive all the way back to the starting line.

The Mystery: Two Strange Clues

The researchers found two very strange things about these specific particle bursts (which were rich in a rare type of helium called Helium-3):

1. The "U-Turn" (Sunward Streaming): The particles were moving toward the Sun, not away from it. This is very rare for this type of particle.
2. The "Long Way Home" (Path Length): To get to the spacecraft, the particles had to travel a path that was 2 to 8 times longer than the straight-line distance from the Sun. Imagine trying to walk from your house to the grocery store, but instead of walking 1 mile, you had to walk 5 miles because you were forced to take a giant detour around a construction zone.

The Detective Work: Finding the Culprit

The scientists asked: What could force these particles to take such a long, backward path?

They looked at the Sun's history. They found a "slow-motion" explosion (a Coronal Mass Ejection, or CME) that happened two days before the particles arrived. Think of this CME not as a fast bullet, but as a slow-moving, giant cloud of magnetic force expanding outward from the Sun.

The Analogy:
Imagine the Sun is a factory, and the magnetic field lines are train tracks.

• Normally, the tracks are straight lines leading away from the factory.
• The slow CME is like a giant, slow-moving train that got stuck on the tracks.
• The particles (the new trains) tried to leave the factory, but they hit the back of the stuck train.
• Instead of stopping, they were forced to wrap around the outside of the stuck train to get past it.

Because they had to wrap around this giant magnetic cloud, their path became incredibly long. And because the cloud was moving slowly, the particles ended up getting pushed back toward the Sun as they tried to navigate around it.

The Evidence: A Multi-Spacecraft View

The scientists had a unique advantage: they had two "cameras" in space at different distances.

• Solar Orbiter was about 30% of the way from the Sun to Earth.
• Parker Solar Probe was much closer, about 16% of the way.

Usually, the closer spacecraft sees the particles first. But in this case, Solar Orbiter saw them first, and Parker Solar Probe saw them later. This proved the particles were indeed moving backward toward the Sun. If they were moving away, the closer probe would have seen them first.

Why Does This Matter?

The paper suggests a few interesting things about how the Sun works:

1. The "Seed" Effect: These particles didn't just disappear. They were likely pushed back toward the Sun's surface (the corona). The scientists think these particles might get stuck there, acting like "seeds" that can be re-accelerated later to create even bigger solar storms.
2. Widespread Storms: Sometimes, these particle storms are seen over huge areas of space (hundreds of degrees wide). The paper suggests that if a particle storm wraps around a giant magnetic cloud (like the CME in this study), it can spread out over a very wide area, explaining why we sometimes see these storms everywhere at once.

Summary

In short, this paper describes a rare event where solar particles got caught in a magnetic "traffic jam" caused by a slow-moving solar cloud. This forced the particles to take a massive detour and travel backward toward the Sun. It's the first time scientists have seen this happen with two spacecraft at once, giving us a new understanding of how solar particles can get lost, redirected, and potentially recycled in our solar system.

Technical Summary

Technical Summary: Sunward Streaming 3He-rich SEPs Observed by Solar Orbiter and Parker Solar Probe

Problem Statement
Solar Energetic Particle (SEP) events, particularly those rich in Helium-3 ($^3$He), serve as critical probes for understanding particle acceleration in the lower solar corona and transport processes in the heliosphere. While $^3$He-rich events are typically characterized by extreme isotopic enrichment and limited longitudinal spread (typically 20–30°), significant gaps remain in understanding their origins, acceleration mechanisms, and transport dynamics. Specifically, previous observations have noted anomalously long particle travel path lengths (2–4 times the nominal Parker spiral) and rare instances of widespread distribution. Furthermore, the phenomenon of "sunward streaming"—where SEPs flow toward the Sun rather than away from it—is well-documented in shock-accelerated events but remains unexplained for $^3$He-rich events, which are accelerated entirely within the solar corona. This study addresses the anomalous observations of two $^3$He-rich SEP events that exhibited both sunward streaming and exceptionally long path lengths during the April 1–4, 2024 conjunction of the Solar Orbiter (SO) and Parker Solar Probe (PSP).

Methodology
The study utilizes multi-spacecraft in situ data and remote sensing observations to characterize two distinct $^3$He-rich SEP events originating from Active Region (AR) 13615.

• In Situ Measurements: Energetic particle data were obtained from the Solar Isotope Spectrometer (SIS) on SO and the Energetic Particle Investigation (EPI-Hi/LET) on PSP. These instruments provided high-precision mass resolution for $^3$He, $^4$He, and heavy ions (C–Fe), as well as directional flux measurements via telescopes with varying look angles (sunward vs. antisunward). Solar wind plasma parameters and Interplanetary Magnetic Field (IMF) data were acquired from SWA (SO) and SWEAP (PSP) to characterize the local environment.
• Path Length Analysis: Particle travel path lengths were calculated by defining the solar release time via associated Type III radio bursts and measuring the arrival times of specific energy bins (1.47 MeV/nuc for SO, 1.68 MeV/nuc for PSP). Velocity dispersion profiles were compared against nominal Parker spiral expectations.
• Anisotropy Quantification: First-order anisotropy was calculated using flux ratios between opposing telescopes (e.g., SIS-A vs. SIS-B) to determine the bulk flow direction of the particles, corrected for the Compton-Getting effect to transform data into the solar wind frame.
• Remote Sensing & Modeling: Solar source regions were identified using SO/EUI (EUV) and SO/STIX (X-ray) data. A candidate Coronal Mass Ejection (CME) was identified in SOHO/LASCO white-light coronagraph images. The CME's kinematics (speed, acceleration) and geometry (eccentricity, width) were modeled using elliptical fits to the CME front in LASCO difference images. A geometric model was then constructed to simulate SEP propagation along magnetic field lines draped around the propagating ICME to estimate theoretical path lengths.

Key Results

1. Sunward Streaming Anisotropy: Both SO and PSP observed a distinct negative anisotropy (bulk flow toward the Sun) during the dispersive phase of both events. At SO, antisunward-facing telescopes (SIS-B) measured higher fluxes than sunward-facing telescopes (SIS-A), a reversal of the expected direction for coronal release. This was confirmed by PSP, where antisunward telescopes (LET-B, LET-C) recorded higher intensities than the sunward-facing LET-A.
2. Anomalously Long Path Lengths: The calculated travel path lengths were significantly longer than nominal Parker spiral values. For the first event, SO measured a path length of $0.98 \pm 0.05$ AU (3.3x nominal) and PSP measured $1.37 \pm 0.05$ AU (8.6x nominal). The second event showed a path length of $0.83 \pm 0.05$ AU at SO, still nearly three times the nominal value.
3. Temporal Arrival Sequence: Despite PSP being closer to the Sun (0.16 AU) than SO (0.3 AU), the SEPs arrived at SO first. This temporal inversion provides direct evidence of a sunward-directed particle population traveling from the outer heliosphere inward.
4. Remote Sensing of Source and CME: The events originated from AR 13615, associated with EUV jets and low C-class flares. A slow CME (speed $\approx 368$ km/s) originating from the same region approximately 48 hours prior to the SEP events was identified as the likely redirecting agent.
5. Model Validation: A model assuming SEPs injected onto field lines draping around the ICME front yielded path lengths of 0.76–0.95 AU for SO and 0.94–1.1 AU for PSP. While these modeled values are slightly shorter than observed, they are in reasonable agreement and support the hypothesis that particles traveled around the ICME limb.

Key Contributions

• First Multi-Spacecraft Observation: This work presents the first simultaneous, multi-spacecraft observation of sunward streaming $^3$He-rich SEPs.
• Mechanism for Path Length Elongation: The study provides evidence that inner heliospheric ICMEs can redirect SEPs, causing them to travel along elongated magnetic field lines draped around the ICME structure, rather than solely attributing long path lengths to field line meandering.
• Explanation for Widespread Distribution: The authors propose that diffusion of SEPs along the broad front of a slow ICME can explain rare, widespread $^3$He-rich SEP events that span large heliospheric longitudes, independent of the SEP event's own association with a CME.
• Seed Population Implications: The observation suggests a mechanism where $^3$He-rich material can be redirected back to the solar corona, potentially populating the region with pre-accelerated seed material for subsequent, larger gradual SEP events.

Significance and Claims
The authors claim that these findings constitute a novel mechanism for understanding $^3$He-rich SEP transport. By demonstrating that SEPs can be redirected by a slow, pre-existing ICME, the paper offers a plausible explanation for both the extreme path lengths and the counter-intuitive sunward flow observed in these events. The study highlights that such redirection may be a common occurrence during solar maximum, given the high frequency of ICMEs in the inner heliosphere. Furthermore, the work suggests that the "seed" population for gradual SEP events may be significantly enriched by remnant $^3$He-rich material returned to the corona, a process that would lack remote signatures (such as EUV jets) at the time of the subsequent acceleration. The authors remain modest regarding the frequency of this phenomenon, noting that while the ICME hypothesis fits the data, definitive in situ ICME signatures were not detected at the spacecraft locations, implying the spacecraft may have just missed the ICME core. Future multi-spacecraft observations are required to refine these interpretations and determine the prevalence of this transport mechanism.