Solar Cycle Variation of Sustained Gamma Ray Emission from the Sun

Despite a technical malfunction limiting initial observations, analysis of Fermi LAT data and associated solar phenomena indicates that Solar Cycle 25 is stronger than Solar Cycle 24, with an estimated total of approximately 43 to 50 sustained gamma-ray emission events.

The Gist

Imagine the Sun as a massive, temperamental factory that occasionally explodes with energy. Sometimes, these explosions are just a quick spark (a flare), but other times, they are massive, sustained blasts of radiation that can last for hours. Scientists call these long-lasting blasts Sustained Gamma-Ray Emissions (SGRE). They are like the "after-shocks" of a solar explosion, where particles get accelerated to incredible speeds and crash into the Sun's atmosphere, creating a glow of high-energy light.

For decades, scientists have been trying to count how often these "after-shocks" happen to understand how strong the Sun's activity cycles are. The Sun works in roughly 11-year cycles, like a heartbeat that speeds up and slows down.

Here is the story of what this paper discovered, explained simply:

1. The Broken Camera Problem

The scientists used a powerful space telescope called Fermi to watch the Sun. It's like having a high-definition security camera pointed at the factory.

However, in 2018, the camera's "solar panel arm" (the Solar Array Drive Assembly) got stuck. It couldn't rotate to keep the sun shining on it perfectly. To keep the camera alive, the operators had to point it toward the edge of the sun's view rather than the center.

• The Analogy: Imagine trying to watch a baseball game through a window, but the window is stuck halfway open. You can only see the game for a few minutes every hour, and then the view is blocked.
• The Result: Because of this "stuck window," the camera missed a lot of the action during the current solar cycle (Cycle 25). When they counted the events they did see, it looked like the Sun was actually getting calmer compared to the previous cycle. But the scientists knew something was wrong with the count.

2. The "Clues" Left Behind

Even though the main camera (Fermi/LAT) was missing the action, the scientists realized they could find the explosions by looking for clues left behind by other instruments that didn't have a broken arm.

They knew that every time a big SGRE explosion happens, two other things almost always happen:

1. A Fast, Wide Cloud: A massive cloud of solar gas (a Coronal Mass Ejection or CME) shoots out at breakneck speeds.
2. A Radio Scream: The shockwave from that cloud creates a specific type of radio noise (a Type II burst).
3. The "Hard X-Ray" Signature: Most importantly, there is a specific type of intense, long-lasting flash of X-rays (lasting more than 5 minutes) that acts like a "smoking gun."

The Analogy: Imagine a burglar breaking into a house. The main security camera (Fermi) is broken and misses the break-in. But, you hear the alarm (Radio Burst), you see the getaway car speeding away (Fast CME), and you find a specific type of muddy footprint (Long X-ray burst) near the door. Even without seeing the burglar on video, you know a break-in happened.

3. Solving the Mystery

The scientists went back and looked at the "stuck window" periods. They found 79 instances where the "Radio Scream" and "Fast Cloud" happened. But they needed to know which ones were the big, sustained explosions (SGRE) and which were just small sparks.

They applied the "Smoking Gun" rule: If the radio scream was accompanied by a long X-ray flash (over 5 minutes), it was almost certainly an SGRE event.

• They found that 27 of the missed events had this "smoking gun."
• They added these 27 hidden events to the 16 they actually saw.
• The New Count: Instead of 16 events, there were likely 43 events in the current cycle.

4. The Big Conclusion: The Sun is Getting Stronger

When they first looked at the raw numbers, it seemed like the Sun was getting weaker in Cycle 25 compared to Cycle 24. But once they fixed the "broken camera" math, the truth came out:

• Cycle 25 is actually stronger than Cycle 24.
• The Sun is producing more fast clouds, more radio screams, and more intense magnetic storms.
• The only reason it looked weaker was because the camera was broken.

Why Does This Matter?

Think of the Sun's activity like the weather. If you are a pilot, you need to know if a storm is coming. These "Sustained Gamma-Ray" events are the most dangerous storms in space. They can knock out satellites, disrupt GPS, and even be dangerous to astronauts.

By figuring out that the Sun is actually getting more active, this paper helps us prepare for a busier, more energetic space weather future. It's like realizing the hurricane season is getting worse, not better, so we need to build stronger roofs and have better evacuation plans.

In short: The Sun isn't sleeping; it's just been hiding from our broken camera. Once we looked for the footprints it left behind, we realized the current solar cycle is a powerhouse, stronger than the last one.

Technical Summary

Here is a detailed technical summary of the paper "Solar Cycle Variation of Sustained Gamma Ray Emission from the Sun" by Gopalswamy et al.

1. Problem Statement

The primary objective of this study is to investigate the occurrence rate of Sustained Gamma-Ray Emission (SGRE) events from the Sun during Solar Cycle (SC) 25 and compare them with SC 24.

• The Anomaly: Preliminary data suggested a paradox: while SC 25 is stronger than SC 24 (higher sunspot numbers, more fast CMEs), the observed number of SGRE events in the first 61 months of SC 25 (16 events) was significantly lower than in the corresponding epoch of SC 24 (27 events).
• The Cause: This discrepancy is attributed to a mechanical failure of the Solar Array Drive Assembly (SADA) on the Fermi satellite starting in March 2018. The malfunction forced the satellite to keep the solar array at the edge of the Large Area Telescope (LAT) field of view, resulting in large observational gaps (1–3 weeks at a time) and reduced solar exposure by approximately 30%.
• The Challenge: The authors needed to estimate the number of "missing" SGRE events that occurred during these LAT data gaps to determine if SC 25 is indeed stronger than SC 24, as indicated by other energetic phenomena.

2. Methodology

The authors employed a multi-faceted approach to reconstruct the SGRE count in SC 25:

• Data Sources:

  • Fermi/LAT: For confirmed SGRE events.
  • Fermi/GBM (Gamma-ray Burst Monitor) & Wind/Konus: For Hard X-ray (HXR) burst data during LAT gaps.
  • SOHO/LASCO & STEREO: For CME catalogs (Halo, Fast and Wide - FW).
  • Wind/STEREO WAVES: For Decameter-Hectometric (DH) Type II radio bursts.
  • GOES: For Sunspot Numbers (SSN) and Soft X-ray flares.

• Key Analytical Strategy:

  1. Correlation Analysis: The study leverages the established physical link between SGREs, Fast and Wide (FW) CMEs, DH Type II bursts, and >100 keV Hard X-ray (HXR) bursts.
  2. The HXR Criterion: Based on Share et al. (2018), the authors utilize the finding that SGRE events are invariably associated with impulsive HXR bursts at energies >100 keV with durations > ~5 minutes. Short-duration HXR bursts (<2 min) are typically not associated with SGREs.
  3. Gap Filling:
     • Identify all DH Type II bursts occurring during LAT data gaps in SC 25 (Total: 79).
     • Cross-reference these with >100 keV HXR bursts observed by GBM/Konus.
     • Filter for HXR bursts with durations > ~5 minutes to identify "SGRE candidates" within the gaps.
  4. Statistical Estimation: Three independent methods were used to estimate the total SGRE count:
     • Method A (SSN Scaling): Scaling the SC 24 count by the ratio of average SSN (SC 25/SC 24).
     • Method B (FW CME & DH Type II Scaling): Scaling based on the overabundance of FW CMEs and DH Type II bursts relative to SSN.
     • Method C (Association Rates): Applying the SC 24 association rates (SGRE per FW CME and SGRE per DH Type II) to SC 25 totals.

3. Key Results

• Solar Cycle Strength:

  • SC 25 is confirmed to be stronger than SC 24. The average Sunspot Number (SSN) increased by 39% (56.9 to 79.0).
  • FW CMEs increased by 80% (raw) and 29% (normalized to SSN).
  • DH Type II bursts increased by 85% (raw) and 33% (normalized to SSN).
  • Halo CMEs showed a lower normalized abundance in SC 25, consistent with stronger cycles having fewer halo CMEs relative to SSN.
  • GLEs (Ground Level Enhancements) quadrupled in SC 25.

• SGRE Event Reconstruction:

  • Observed: 16 SGRE events in SC 25 (first 61 months).
  • Inferred from Gaps: Out of 79 DH Type II bursts occurring during LAT gaps, 27 were associated with >100 keV HXR bursts having durations > ~5 minutes. These are inferred to be SGRE events.
  • Total Estimated SGREs: 16 (observed) + 27 (inferred) = 43 events.

• Comparative Estimates:

  • SSN-based estimate: ~38 events.
  • FW CME-based estimate: ~48 events.
  • DH Type II-based estimate: ~50 events.
  • Association Rate-based estimate: ~48 (from FW CMEs) and ~49 (from DH Type II).
  • Conclusion: All methods converge on a total of ~43–50 SGRE events, significantly higher than the raw observed count of 16.

• HXR Duration Characteristics:

  • Statistical tests (Kolmogorov-Smirnov) confirmed that the duration distributions of >100 keV HXR bursts associated with SGREs in SC 24, SC 25, and the LAT-gap DH Type II bursts are not statistically different.
  • The median duration for all populations is ~11–17 minutes, reinforcing the >5-minute threshold as a robust indicator.

4. Key Contributions

1. Correction of SGRE Statistics: The paper successfully corrects the apparent decline in SGRE events in SC 25, demonstrating that the reduction was an observational artifact caused by the Fermi SADA anomaly, not a physical decrease in solar activity.
2. Validation of HXR Proxy: It confirms and refines the criterion that >100 keV HXR bursts with durations > ~5 minutes are a necessary condition for SGRE, providing a reliable proxy to identify SGREs even when gamma-ray data is missing.
3. Physical Connection Confirmation: The study reinforces the physical link between FW CMEs, DH Type II bursts, and SGREs, showing that their occurrence rates scale similarly with solar cycle strength.
4. Cycle Strength Assessment: It provides robust evidence that SC 25 is stronger than SC 24, contrary to initial raw data interpretations, based on multiple energetic indicators (SGREs, GLEs, FW CMEs, Type II bursts).

5. Significance

• Space Weather Forecasting: Understanding the true occurrence rate of SGREs is crucial for space weather modeling, as these events indicate the acceleration of protons to >300 MeV, which poses radiation hazards to astronauts and spacecraft.
• Solar Cycle Prediction: The study highlights that while SSN is a general indicator, specific energetic phenomena (like GLEs and SGREs) may show non-linear scaling or extreme sensitivity to cycle strength. The finding that SC 25 is stronger than SC 24 has implications for future space weather risk assessments.
• Methodological Advance: The approach of using HXR and radio burst data to fill gaps in gamma-ray observations offers a template for analyzing solar activity during periods of instrument downtime or limited coverage.

In conclusion, the paper resolves the "missing SGRE" paradox in Solar Cycle 25, confirming that the cycle is energetically stronger than its predecessor and that the observed deficit was purely due to instrumental limitations.