Catalogue and statistics of greater than 100 MeV solar proton events during solar cycles 23-25 from SOHO-ERNE observations

This paper presents a comprehensive catalogue and statistical analysis of greater than 100 MeV solar proton events from May 1996 to August 2024, derived from SOHO-ERNE observations and cross-calibrated with multi-instrument data to establish a unified framework for understanding high-energy particle acceleration across solar cycles 23–25.

The Gist

Imagine the Sun as a massive, chaotic fireworks factory. Sometimes, it doesn't just shoot off a few sparks; it launches entire factories of high-speed particles into space. These are called Solar Energetic Particles (SEPs). Most of the time, these particles are like gentle rain, but occasionally, the Sun fires a "cannonball" of protons so powerful they can travel at nearly the speed of light, reaching Earth with energies over 100 million electron volts. These are the "heavy hitters" of space weather, capable of frying satellites and posing risks to astronauts.

For decades, scientists have been trying to figure out exactly how and when the Sun fires these cannonballs. Is it the solar flare (the explosion on the surface) that launches them, or is it the Coronal Mass Ejection (CME)—a giant cloud of magnetic gas that acts like a snowplow pushing particles ahead of it?

This paper is the result of a massive detective project called SPEARHEAD. The team has spent years building the ultimate "crime scene report" for these high-energy events.

Here is the breakdown of their work in simple terms:

1. The Detective Work: Building the "Black Book"

The researchers used a space telescope called SOHO (which has been watching the Sun since 1996) to create a massive catalogue. Think of this catalogue as a giant logbook or a "Wanted Poster" database.

• The Timeframe: They looked at data from Solar Cycles 23, 24, and the current Cycle 25 (roughly 1996 to 2024).
• The Target: They specifically hunted for protons with energy above 100 MeV. This is like looking for the "heavyweight champions" of solar particles, not the lightweight ones.
• The Method: They didn't just look at the particles; they cross-referenced them with everything else happening on the Sun at that exact moment. Did a flare happen? Was there a radio burst? Did a CME launch?

The Result: They found 172 of these high-energy events. This is the most complete list of its kind ever made.

2. The Suspects: What Causes the Cannonballs?

To understand the crime, you need to identify the suspects. The team checked four main "suspects" for every event:

• The Flare (The Spark): A sudden flash of X-rays on the Sun's surface.
• The CME (The Snowplow): A massive bubble of magnetic gas ejected into space.
• The Radio Bursts (The Sirens): When particles zoom through space, they scream in radio waves. The team looked for three types of "sirens":
  • Type III: Fast, high-pitched screams from electrons racing away.
  • Type II: A slow, deep rumble indicating a shockwave (like a sonic boom).
  • Type IV: A long, continuous hum from particles trapped in magnetic loops.
• The Gamma Rays (The Smoking Gun): High-energy light produced when protons smash into the Sun's atmosphere.

The Findings:

• 96% of the time, a CME was present.
• 76% of the time, a Flare was present.
• 100% of the time, there was a Type III radio burst (the fast scream).
• 95% had a Type II burst (the shockwave).

3. The Timing: Who Fired First?

One of the biggest questions in solar physics is: Does the flare launch the particles, or does the CME shockwave do it?

The team used a "stopwatch" approach. They compared the exact second the particles arrived at Earth with the exact second the solar events started.

• The Verdict: It's a team effort. The particles usually arrive very shortly after the flare starts and the CME launches. It's like a synchronized dance.
• The Twist: Sometimes, the particles arrive before the CME shockwave fully forms, suggesting the flare itself gave them an initial boost. Other times, the CME shockwave takes over and accelerates them further. It's not just one or the other; it's a relay race where both the flare and the CME pass the baton.

4. The "Heavy Hitters" Connection (GLEs)

Some of these events are so powerful that the particles don't just hit satellites; they punch through Earth's atmosphere and hit the ground. These are called Ground-Level Enhancements (GLEs).

• The study found a very strong link between the space-based measurements and the ground-based detectors.
• Analogy: If the space telescope sees a "tsunami" of particles, the ground detectors feel the "flood." The bigger the wave in space, the bigger the flood on the ground.

5. Why Does This Matter?

You might ask, "Why do we care about a logbook of solar explosions?"

• Space Weather Forecasting: Just as meteorologists need to know how hurricanes form to predict storms, space weather forecasters need to understand how these particle cannons work to protect our technology.
• Protecting Astronauts: If we know the "recipe" for a high-energy event, we can better predict when it's safe for astronauts to go outside their ships and when they need to hide in a shielded bunker.
• The "Big Flare Syndrome": The study confirmed that big explosions usually come with big side-effects. If you see a massive flare and a fast CME, you can bet there will be a dangerous particle storm.

The Bottom Line

This paper is the encyclopedia of the Sun's most dangerous particle storms. By organizing 28 years of data into one clear, unified list, the authors have given scientists a powerful new tool. They've shown us that these high-energy events are complex, involving both solar flares and CMEs working together, and that they are almost always accompanied by specific radio and gamma-ray signatures.

It's like finally having the complete rulebook for the Sun's most violent games, helping us prepare for the next time the Sun decides to throw a punch.

Technical Summary

Here is a detailed technical summary of the paper "Catalogue and statistics of >100 MeV solar proton events during solar cycles 23–25 from SOHO/ERNE observations."

1. Problem Statement and Motivation

Solar Energetic Particle (SEP) events, particularly those involving protons with energies exceeding 100 MeV, pose significant hazards to space-borne and ground-based technologies due to their ability to penetrate Earth's atmosphere. While lower-energy SEPs are well-studied, the acceleration mechanisms, release timing, and transport of high-energy (>100 MeV) particles remain uncertain. Specifically, the relative contributions of flare-associated reconnection versus CME-driven shock acceleration are debated.

Existing datasets often lack long-term continuity or comprehensive cross-calibration for energies above 100 MeV. The SPEARHEAD project (Specification, Analysis, and Re-calibration of High-energy particle data) aims to refine instrument response functions and cross-calibrate datasets to improve the accuracy of high-energy particle measurements. This paper addresses the need for a unified, comprehensive catalogue of >100 MeV proton events spanning Solar Cycles 23, 24, and 25 to facilitate in-depth analysis of extreme particle acceleration.

2. Methodology

Data Sources and Event Detection

• Primary Instrument: The study utilizes data from the SOHO/ERNE (Energetic and Relativistic Nuclei and Electron) High Energy Detector (HED). Specifically, it analyzes the anti-coincidence channel (AC1), which detects particles penetrating the entire instrument stack. This channel had not been previously used for scientific studies but offers high-energy sensitivity (effective energy ~130 MeV).
• Time Coverage: May 1996 to August 2024 (covering Solar Cycles 23, 24, and the ongoing 25).
• Detection Algorithm: A systematic scan of AC1 count rates was performed using the Poisson–CUSUM method to identify statistically significant deviations from the background.
• Validation: To confirm detections and reduce false positives (e.g., caused by Forbush decreases), events were cross-checked against the D3 scintillator layer of ERNE, requiring a corresponding enhancement with a higher peak rate.
• Flux and Fluence Derivation: Since the ERNE penetrating-particle counter lacks a fully validated response function for direct flux conversion, peak fluxes and fluences were derived using SOHO/EPHIN (Electron Proton Helium Instrument) data. EPHIN data was recalibrated (extending up to ~500 MeV) to provide reliable energy-resolved spectra. The "bow-tie method" was applied to define an effective energy response, treating the >100 MeV channel as a differential proxy.

Solar Association Identification

Each detected SEP event was systematically associated with solar phenomena using multi-instrument observations:

• Soft X-ray (SXR) Flares: Identified using GOES/XRS data. The Solar Particle Release (SPR) time was estimated using the IRAP Propagation Tool to locate the source region.
• Hard X-ray (HXR) Flares: Cross-referenced with RHESSI (2002–2018) and Solar Orbiter/STIX (2020–present) data.
• Coronal Mass Ejections (CMEs): Mapped using SOHO/LASCO (C2/C3) coronagraphs. Linear speeds and angular widths were derived from height-time measurements.
• Radio Bursts:
  • Type III: Identified via Wind/WAVES, STEREO/WAVES, and ground-based observatories (10 MHz–2.5 GHz).
  • Type II: Identified via visual inspection of dynamic spectra for slow-drifting emissions indicating shock waves.
  • Type IV: Identified as broadband continuum emissions.
• Gamma-ray Emissions: Analyzed using Fermi/LAT (Large Area Telescope) and Fermi/GBM data for events occurring after 2011.
• Ground-Level Enhancements (GLEs): Cross-referenced with the neutron monitor network (GLE.oulu.fi) to identify relativistic proton events.

3. Key Contributions

• Comprehensive Catalogue: The creation of the most extensive reference dataset to date for >100 MeV proton events, containing 172 confirmed events from 1996 to 2024.
• Novel Data Utilization: First scientific use of the SOHO/ERNE penetrating-particle counter (AC1) for high-energy event detection.
• Unified Framework: Integration of in-situ particle data with a complete suite of remote-sensing solar signatures (X-rays, radio, CMEs, gamma-rays) to provide a holistic view of SEP events.
• Public Availability: The full catalogue and an interactive version are made publicly available via Zenodo and the SPEARHEAD project hub.

4. Key Results and Statistics

Event Statistics

• Associations:
  • CMEs: 96% (165/172) of events were associated with CMEs. These were predominantly fast (median speed ~1345 km/s) and wide (median width ~360°, i.e., halo CMEs).
  • SXR Flares: 76% (131/172) were associated with SXR flares. Most were strong (>M-class).
  • Radio Bursts: 100% of events exhibited Type III bursts; 95% had Type II bursts; 53% had Type IV bursts.
  • Gamma-rays: 65 out of 87 events during the Fermi era showed associated >100 MeV gamma-ray emission.
  • GLEs: 22 events were linked to GLEs or sub-GLEs (including 18 confirmed GLEs).

Timing and Correlations

• Onset Timing: Most >100 MeV SEP releases occur shortly after X-ray onsets and CME first appearances. Approximately 21% of events occur during the impulsive flare phase (before SXR peak), while ~35–41% occur during the SXR decay phase, suggesting extended acceleration.
• SXR vs. HXR: SXR onsets generally precede HXR onsets, and peak intensities show moderate correlation (PCC ~0.50–0.68), supporting the Neupert effect (impulsive HXR driving subsequent thermal SXR).
• CME vs. SXR: CME speeds show a moderate correlation with SXR peak flux (PCC ~0.40–0.60), indicating that while related, CME kinematics are also governed by ambient coronal conditions and reconnection geometry.
• SEP vs. Solar Drivers:
  • SEP Intensity vs. SXR: Moderate correlation (PCC = 0.48), suggesting flare magnitude alone cannot explain SEP variability.
  • SEP Intensity vs. CME Speed: Stronger correlation (PCC = 0.54) than with SXR, but still moderate, implying a mix of flare and shock acceleration mechanisms.
  • SEP vs. GLE: A strong correlation exists between SEP fluence and GLE fluence (PCC = 0.76), confirming that space-based high-energy SEP measurements are direct counterparts to ground-level enhancements.

5. Significance and Conclusions

This work establishes a critical baseline for understanding the most energetic solar particle events. The results indicate that >100 MeV SEP events are typically associated with the most powerful solar signatures (fast halo CMEs and strong flares). The moderate correlations between SEP intensity and individual solar drivers (flare or CME) suggest that these events are the result of complex interactions involving:

1. Dual Acceleration Mechanisms: Contributions from both flare reconnection (low-coronal) and CME-driven shocks (high-coronal).
2. Magnetic Connectivity: The efficiency of particle transport to Earth is strongly modulated by the magnetic connection between the source and the observer.
3. Event Size (Big Flare Syndrome): The observed correlations partly reflect the overall magnitude of the eruptive event rather than a single causal link.

The catalogue provides an essential resource for future research into extreme space weather, particle acceleration physics, and the development of radiation hazard models for space exploration. By consolidating nearly three decades of observations into a uniformly processed dataset, it enables cross-calibration with other missions and supports the investigation of the "upper end" of the SEP energy distribution.