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Evidence that SOL2012-06-03 Late Phase $γ$ Rays are Produced by $>$ 300 MeV Protons from CME-Shock Acceleration of Suprathermals from the Flare

This paper argues that the late-phase gamma-ray emission observed in the SOL2012-06-03 event is caused by CME-shock acceleration of flare-produced suprathermal protons, evidenced by the emission's timing and high ion-to-electron ratio which align with gradual shock-accelerated events rather than impulsive flare processes.

Original authors: Gerald H. Share, Ronald J. Murphy

Published 2026-02-13

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CC BY 4.0

Original authors: Gerald H. Share, Ronald J. Murphy

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Solar "Echo" That Wasn't Just a Reflection

Imagine the Sun is like a massive, chaotic concert hall. Usually, when a solar flare happens (a giant explosion on the Sun's surface), it's like a drummer hitting a cymbal: a loud, sharp CRASH followed by a quick fade-out. Scientists call this the "impulsive phase."

For a long time, when scientists saw a second burst of energy from the Sun, they thought it was just a weird echo or a second drum hit from the same drummer.

But a new paper by Gerald Share and Ronald Murphy argues that for a specific solar explosion on June 3, 2012, that second burst wasn't a second drum hit. It was a completely different musician joining the band late to the show.

Here is the simple breakdown of their discovery:

1. The Two-Act Play

Most solar flares are a one-act play:

Act 1 (The Impulse): A sudden, violent explosion that shoots out X-rays and gamma rays. It's fast, hot, and over quickly.
The "Echo" (The Late Phase): Sometimes, minutes later, there is a second wave of high-energy gamma rays.

In the past, some scientists thought the 2012 event was unique because it showed two distinct peaks. Share and Murphy say, "Actually, we've seen this movie before!" They point out that this "Late Phase" has been happening since 1982. They call it LPGRE (Late Phase Gamma-Ray Emission).

2. The Race Track Analogy: Who is the Runner?

The big question is: What is causing this second wave?

The Old Theory (The Flare): The second wave is just leftover energy from the initial explosion.
The New Evidence (The CME Shock): The authors argue the second wave is caused by a Coronal Mass Ejection (CME). Think of a CME as a massive, invisible shockwave or a "snowplow" of solar material rushing away from the Sun at 900 km per second.

Here is the race track analogy:

The Start: The solar flare happens first. It releases a group of "suprathermal" protons (fast-moving particles). Let's call them The Flare Runners.
The Chase: The CME shockwave (The Snowplow) starts moving away from the Sun.
The Overtake: The Flare Runners are fast, but the Snowplow is huge and moving fast too. The runners have to catch up to the Snowplow.
The Boost: Once the runners catch the Snowplow, they get a massive "boost" from its shockwave. This accelerates them to incredible speeds (over 300 MeV).
The Return: These super-charged particles are then funneled back down magnetic tracks to the Sun's surface, creating that second burst of gamma rays we see.

3. The Timing Clue

The paper uses a stopwatch to prove this.

The initial explosion happened at a specific time.
The second gamma-ray peak happened 17 seconds later.
Share and Murphy did the math: If you take the speed of the CME and the speed of the protons, it takes exactly about 17 seconds for the protons to catch up to the shockwave, get accelerated, and send the signal back.

If the second peak were just a leftover from the first explosion, it wouldn't have that specific 17-second delay. The delay is the "travel time" for the particles to run the race.

4. The "Crowd" Ratio

There is one more piece of evidence: The mix of particles.

In a normal solar flare (the "drummer"), you get a mix of protons and electrons, but there are way more electrons.
In this "Late Phase" event, the mix is totally different. There are way more protons than electrons.

This is like walking into a concert and finding that instead of a mix of instruments, it's only heavy metal guitars. This specific "heavy metal" ratio is exactly what you see in other solar events caused by shockwaves (CMEs), but not in events caused by simple magnetic reconnection (flares).

The Bottom Line

The paper concludes that the 2012 solar explosion wasn't a unique "two-phase" event. Instead, it was a classic example of a Late Phase Gamma-Ray Emission.

The Sun didn't just explode once; it launched a shockwave that acted like a cosmic particle accelerator, catching up to particles it had already released, giving them a super-boost, and sending them back to create a second, distinct burst of energy. It's not a second explosion; it's the shockwave doing the heavy lifting.

1. Problem Statement

Recent literature (Pesce-Rollins et al., 2025) identified the solar eruption SOL2012-06-03 as the first event to exhibit two distinct phases of $>100$ MeV $\gamma$ -ray emission, suggesting separate acceleration mechanisms. However, the authors argue that this interpretation overlooks historical precedents. The paper addresses the need to re-evaluate the nature of the second emission phase in SOL2012-06-03, proposing that it is not a unique, isolated phenomenon but rather the onset of Late Phase Gamma-Ray Emission (LPGRE), a well-documented class of solar events first observed in 1982. The core problem is distinguishing between impulsive flare acceleration and shock acceleration mechanisms in high-energy solar emissions.

2. Methodology

The authors employ a comparative analysis and physical modeling approach:

Historical Comparison: They compare the temporal and spectral characteristics of SOL2012-06-03 against a catalog of over 40 known LPGRE events, specifically referencing the prototype event SOL1982-06-03 and SOL1984-04-25.
Multi-wavelength Data Analysis: The study analyzes data from the Fermi Gamma-ray Burst Monitor (GBM) and Large Area Telescope (LAT), as well as historical data from the Solar Maximum Mission (SMM/GRS and HXRBS). Key metrics include:
- Hard X-ray (100–300 keV) flux rates.
- $\gamma$ -ray flux in 10–50 MeV and $>65$ MeV bands.
- Flux ratios ( $10\text{--}50$ MeV / $>65$ MeV) to identify pion ( $\pi$ ) decay signatures.
Spectral Fitting: The authors perform spectral fitting on the $\gamma$ -ray data to determine the power-law index of the accelerated protons during different phases of the event.
Kinematic Modeling: They calculate the time delay required for flare-produced suprathermal protons to overtake an expanding Coronal Mass Ejection (CME) and be accelerated by the CME-driven shock.

3. Key Contributions

Reclassification of SOL2012-06-03: The paper argues that the second $>100$ MeV peak in SOL2012-06-03 is the onset of LPGRE, characterized by a duration of $>8$ minutes, rather than a distinct, separate acceleration event.
Mechanism Identification: It provides evidence that the high-energy ions in LPGRE are accelerated by CME-driven shocks acting on suprathermal ions pre-existing from the initial flare, rather than by magnetic reconnection in the impulsive flare phase.
Electron-to-Proton Ratio Analysis: The authors highlight a critical diagnostic: the high ion-to-electron ratio in LPGRE events (consistent with gradual Solar Energetic Particle events) versus the low ratio in impulsive flares. This supports the shock acceleration hypothesis.
Temporal Consistency: The paper demonstrates that the observed delay between the impulsive X-ray peak and the second $\gamma$ -ray peak is physically consistent with the transit time of suprathermal protons overtaking the CME shock.

4. Results

Spectral Characteristics: The second phase of SOL2012-06-03 exhibits a spectrum dominated by pion decay, with a flux ratio of $(10\text{--}50 \text{ MeV})/(>65 \text{ MeV}) \gtrsim 7$ . The power-law index of the protons hardened from $5.6 \pm 1.4$ (first peak) to $4.3 \pm 1.0$ (second peak), aligning with values observed in the decay phases of other LPGRE events.
Temporal Dynamics:
- The event featured a $\sim1$ -minute impulsive phase followed by a second $>100$ MeV peak occurring $\sim17$ seconds after the hard X-ray peak, with no corresponding X-ray peak.
- Modeling indicates that for a CME speed of $\sim900$ km s $^{-1}$ , 75 keV suprathermal protons released at the X-ray peak would overtake the shock in $\sim5$ seconds. Including acceleration time ( $\sim10$ s) and return time to the Sun, this explains the observed $\sim17$ s delay.
Flux Ratios: If the second peak is treated as the onset of LPGRE, the late/impulsive phase flux ratio increases to $\sim15$ , further distinguishing it from standard impulsive flare behavior.
Particle Composition: Analysis of the behind-the-limb event SOL2014-09-01 (used as a proxy) suggests an electron-to-proton ratio of $\sim500$ for energies $>100$ MeV. This is two orders of magnitude lower than in impulsive events, strongly supporting a shock-acceleration origin for LPGRE.

5. Significance

Unified Understanding of Solar Eruptions: The paper reinforces the consensus that LPGRE is a distinct physical phenomenon driven by CME shocks, not merely a variation of impulsive flares. It corrects the interpretation of SOL2012-06-03, placing it within a broader context of over 40 similar events.
Acceleration Mechanism Validation: It provides robust evidence that CME-driven shocks are capable of accelerating suprathermal ions from the flare site to energies $>300$ MeV, contributing significantly to both LPGRE and gradual Solar Energetic Particle (SEP) events.
Diagnostic Utility: The study establishes the electron-to-proton ratio and the specific timing of the second $\gamma$ -ray peak relative to X-rays as key diagnostics for distinguishing between reconnection-driven (impulsive) and shock-driven (gradual/LPGRE) solar particle acceleration.
Implications for Space Weather: Understanding that high-energy protons can be sustained for hours via shock acceleration has implications for predicting radiation hazards associated with solar eruptions, particularly those occurring behind the solar limb or with delayed high-energy emission.