🔭 astrophysics

Unusual Circumstances of the 2024 June 8 GLE

This paper reports that the June 8, 2024 GLE event is unique among such phenomena for lacking associated sustained gamma-ray emission despite having all typical precursors, a discrepancy the authors attribute to east-west asymmetry in the energetic particle flow from the shock.

Original authors: Nat Gopalswamy, Pertti Makela, Hong Xie, Sachiko Akiyama, Seiji Yashiro, Atul Mohan

Published 2026-02-16

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CC BY 4.0

Original authors: Nat Gopalswamy, Pertti Makela, Hong Xie, Sachiko Akiyama, Seiji Yashiro, Atul Mohan

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 Mystery of the "Silent" Solar Storm

Imagine the Sun as a massive, chaotic fireworks factory. Usually, when it has a huge explosion (a solar flare and a Coronal Mass Ejection, or CME), it sends out two things at the same time:

A blast of high-speed particles (protons) that can hit Earth's atmosphere.
A loud "gamma-ray scream" (Sustained Gamma-Ray Emission, or SGRE) caused by those particles crashing into the Sun's surface.

For decades, scientists have noticed that these two things always go together. It's like a firework that always makes a loud bang when it explodes. If you see the particles, you expect to hear the bang.

The Puzzle:
On June 8, 2024, the Sun had a massive explosion. It sent out a huge wave of particles that hit Earth (a "Ground Level Enhancement" or GLE). The conditions were perfect for a "bang" (a gamma-ray scream). The explosion was fast, the magnetic fields were complex, and the radio waves were screaming.

But here's the twist: The "bang" never happened. The gamma-ray detectors on the Fermi satellite heard nothing. It was a silent explosion. This paper tries to figure out why.

The Investigation: What Happened?

The scientists (led by Nat Gopalswamy) looked at the event like detectives examining a crime scene. Here is what they found:

1. The Explosion Was Real and Big

The Sun didn't just sneeze; it roared.

The Trigger: It started with a smaller explosion (M3.3 flare) that broke a magnetic "rope" (a filament) on the Sun.
The Big Boom: This triggered a much bigger explosion (M9.7 flare) that launched a massive cloud of gas (CME) into space.
The Speed: This cloud was moving incredibly fast—about 1,500 kilometers per second (that's fast enough to circle the Earth in 27 seconds!). It kept speeding up as it went.

2. The Radio Waves Were Screaming

When a shockwave moves through the Sun's atmosphere, it creates radio waves, like the sound of a jet breaking the sound barrier.

The scientists saw a "Type II" radio burst that lasted for 9 hours.
This is a classic sign that a shockwave is powerful enough to create those high-energy protons needed for the "gamma-ray scream."
The Clue: Everything pointed to the fact that a scream should have happened.

3. The Missing Piece: The "Silent" Gamma Rays

The Fermi satellite, which is the world's best "ear" for hearing these gamma-ray screams, was watching.

There was a small gap in the data (about 51 minutes) right when the explosion peaked.
However, the scientists calculated that if a scream happened, it should have lasted for 7 hours.
Even with the data gap, the satellite should have caught at least some of the scream later on. It didn't. The gamma-ray signal was flat.

The Solution: The "East-West" Traffic Jam

So, why did the particles hit Earth, but the gamma-ray scream didn't happen?

The scientists propose a clever theory involving direction.

Imagine the shockwave from the explosion as a giant, expanding balloon.

The Nose: The front of the balloon (facing the direction of travel) is where the particles get accelerated the most.
The Flanks: The sides of the balloon.

The Theory:
The explosion happened on the West side of the Sun (from our perspective).

The "Nose" of the shockwave was shooting high-energy particles toward the West (away from Earth, behind the Sun's limb).
Because the particles were heading West, they crashed into the Sun's surface behind the Sun. This would have created a gamma-ray scream, but we couldn't see it because the Sun was in the way.
The "Flanks" (sides) of the shockwave were what reached Earth. They sent enough particles to cause the GLE (the 3% increase in radiation), but they weren't hitting the Sun's surface hard enough to create the gamma-ray scream.

In simple terms: It's like a car crash.

Usually, a crash sends debris flying everywhere (particles) and makes a loud noise (gamma rays).
In this case, the car was driving away from us. The debris (particles) that flew toward us was just a few pieces of dust. But the loud crash (gamma rays) happened on the other side of the car, where the main impact was. We saw the dust, but we didn't hear the crash.

The Conclusion

This event (GLE 75) is special because it breaks the usual rule: "No particles without a gamma-ray scream."

The scientists conclude that the high-energy particles were asymmetric. They flowed mostly to the West (away from us), missing the Sun's surface on our side. Therefore, no gamma-ray scream was produced on the side of the Sun facing Earth.

It's a reminder that space weather is complex, and sometimes, the loudest explosions happen in places we can't see.

1. Problem Statement

Ground Level Enhancements (GLEs) are rare solar energetic particle (SEP) events where protons are accelerated to GeV energies. Historically, since the 1980s, all observed GLEs have been associated with Sustained Gamma-Ray Emission (SGRE) from the Sun. SGRE requires protons with energies $>300$ MeV to interact with the solar atmosphere and produce gamma rays via pion decay.

The 2024 June 8 GLE (GLE 75) presents a significant anomaly: despite exhibiting all the typical precursors of an SGRE event (a fast, wide Coronal Mass Ejection, a major solar flare, and an intense, long-duration Type II radio burst), no SGRE was detected by the Fermi Large Area Telescope (LAT). This event challenges the established correlation between GeV protons and SGRE, prompting an investigation into the physical mechanisms that prevented gamma-ray production despite the presence of high-energy particles.

2. Methodology

The authors employed a multi-instrument, multi-wavelength approach to characterize the event and test the SGRE hypothesis:

Solar Energetic Particle (SEP) Analysis: Utilized data from GOES satellites (12 MeV to >500 MeV channels) and ground-based Neutron Monitors (South Pole Bare, Oulu) to confirm the GLE and characterize the high-energy proton flux.
Solar Eruption Characterization: Analyzed SDO/AIA (1600 Å), SDO/HMI (magnetograms), and H-alpha data to study the complex magnetic topology, filament eruptions, and flare evolution in NOAA Active Region 13697.
CME Kinematics: Tracked the CME using SOHO/LASCO (C2/C3), STEREO-Ahead (EUVI/COR1), and deprojected the speeds to determine acceleration profiles and shock formation heights.
Radio Diagnostics: Combined dynamic spectra from the e-CALLISTO network (Australia) and STEREO/WAVES to analyze Type II radio bursts (metric to kilometric domains) to infer shock properties and duration.
Gamma-Ray Constraints: Examined Fermi/LAT and Fermi/GBM data to search for gamma-ray signatures. The authors accounted for a ~51-minute data gap in Fermi/LAT observations but used statistical relationships between Type II burst duration and expected SGRE duration to infer the likely absence of emission.

3. Key Results

A. Event Characteristics

GLE Intensity: A small event with a peak enhancement of ~3.2% above background.
Source Region: NOAA AR 13697 (located at ~S17W60). The eruption involved a complex magnetic structure with multiple neutral lines and two filaments (F1 and F2).
Flare Sequence: A precursor M3.3 flare triggered the eruption of filament F1, followed by a major M9.7 flare involving a second, fast filament fragment (Fragment 3).
CME Dynamics: The CME was a "full halo" event with a sky-plane speed of ~1408 km/s, accelerating to ~1570 km/s. The initial acceleration was ~1 km s⁻², typical for GLE-associated CMEs.
Radio Signatures: A complex Type II burst lasted ~~9 hours, extending from metric (~~220 MHz) to kilometric frequencies (~250 kHz). It featured distinct "flank" (Type II-F) and "nose" (Type II-N) components, indicating a strong, extended shock.

B. Particle Release and Shock Physics

Solar Particle Release (SPR): GeV particles arrived ~3 minutes after the electromagnetic emission, implying an SPR time of 01:52 UT.
Shock Height: The shock formed at ~1.55 Rs (deprojected) and the SPR occurred at ~4.0 Rs. These heights are consistent with empirical relations for GLE events.
Proton Flux: The event produced a high-intensity SEP event (~1000 pfu at >10 MeV) and significant flux up to >500 MeV, confirming the presence of the necessary GeV protons.

C. The SGRE Anomaly

Fermi Observations: Fermi/LAT data points taken before, during, and after the event showed flux levels at or below background.
Data Gap: There was a 51-minute gap in Fermi/LAT coverage immediately following the flare peak (01:49 UT to 02:40 UT), missing the impulsive phase.
Duration Discrepancy: Based on the 9-hour Type II burst duration, empirical models predict an SGRE duration of ~7.3 hours. The lack of detection over the observed timeframe, combined with the known correlation between Type II duration and SGRE, strongly suggests no SGRE occurred.

4. Proposed Explanation

The authors rule out magnetic mirroring (particles bouncing back before hitting the photosphere) as the primary cause, as the magnetic structure was not unusual. Instead, they propose an East-West Asymmetry in Particle Precipitation:

The shock nose likely directed high-energy particles predominantly toward the western side of the CME.
Since the source was located at W60 (near the limb), the western side of the CME was behind the solar limb.
Consequently, the high-energy protons precipitated onto the backside of the Sun, where they could not produce gamma rays observable from Earth. This explains the presence of GeV protons (detected at Earth via magnetic connection) without the associated SGRE (which requires interaction with the visible solar disk).

5. Significance and Contributions

First SGRE-Independent GLE: This paper documents the first confirmed GLE in Solar Cycle 25 (and potentially the first in the Fermi era) that lacks an associated SGRE, challenging the assumption that GeV protons always result in observable solar gamma rays.
Refinement of Shock Models: The findings support the hypothesis that the geometry of particle precipitation relative to the solar limb is a critical factor in SGRE detectability. It highlights that "behind-the-limb" precipitation can occur even for near-limb events due to shock asymmetry.
Implications for Space Weather: Understanding that high-energy particle acceleration can occur without the accompanying gamma-ray signature is crucial for space weather forecasting. It suggests that the absence of SGRE does not necessarily imply a lack of dangerous GeV protons, which are the primary drivers of radiation hazards for astronauts and aviation.
CME Interaction: The study reinforces the role of interacting CMEs (the M3.3 and M9.7 eruptions) in generating the high-intensity SEP fluxes typical of GLEs, even when the resulting gamma-ray emission is geometrically suppressed.