🔭 astrophysics

Continued activity of the 25th cycle: largest in 20 years. Ground-level enhancement and Forbush decrease

This paper analyzes the contrasting spectral signatures of a Ground Level Enhancement (GLE77) and a strong Forbush decrease during Solar Cycle 25, demonstrating that combined neutron monitor and SEVAN observations provide robust, complementary diagnostics for understanding rigidity-dependent cosmic ray modulation during extreme heliospheric disturbances.

Original authors: B. Sargsyan, A. Chilingarian

Published 2026-01-28

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CC BY 4.0

Original authors: B. Sargsyan, A. Chilingarian

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 Big Picture: A Cosmic Weather Report

Imagine the Sun as a giant, sometimes temperamental neighbor. Every few years, it goes through a "mood swing" called a solar cycle. After a very quiet period (Cycle 24), the Sun is now waking up again (Cycle 25) and throwing some serious tantrums.

This paper is a report card on two specific "tantrums" that happened recently:

A Solar Explosion (GLE 77): On November 11, 2025, the Sun let out a massive burst of high-energy particles (like a firehose spraying water).
A Solar Shield (Forbush Decrease): On January 19–20, 2026, the Sun sent out a massive cloud of magnetic gas that acted like a shield, blocking the usual background radiation from deep space.

The scientists used special detectors in Armenia to watch how these events changed the "rain" of particles hitting the Earth.

The Tools: Two Different Types of Umbrellas

To understand the events, the scientists used two different types of "detectors" (or umbrellas) to catch the cosmic particles:

Neutron Monitors: Think of these as lightweight, sensitive raincoats. They are great at catching the "light rain" (lower-energy particles) that comes from deep space. They react quickly and strongly to small changes.
SEVAN Detectors: Think of these as heavy-duty, deep-sea diving gear. They are designed to catch the "heavy boulders" (high-energy particles) that punch through the atmosphere. They are less sensitive to the light stuff but are the only ones that can see the heavy stuff.

By using both, the scientists could see the whole picture: the light rain and the heavy boulders.

Event 1: The Firehose (GLE 77)

What happened: The Sun erupted, shooting a massive wave of solar particles toward Earth.
The Analogy: Imagine a sudden, powerful jet of water hitting a garden.
What the scientists saw:

The "lightweight raincoats" (neutron monitors) got soaked immediately. The signal was huge.
The "diving gear" (muon detectors) also felt the water, but the pattern was different.
Key Finding: This event showed that the Sun can shoot out particles that are incredibly powerful (hard to stop), reaching energies we haven't seen in 20 years. The "heavy boulders" were definitely there, proving the solar firehose was very strong.

Event 2: The Magnetic Shield (Forbush Decrease)

What happened: A giant cloud of magnetic gas (from a solar storm) swept past Earth.
The Analogy: Imagine a giant, invisible windbreaker jacket being put on the Earth. This jacket blocks the "background rain" (Galactic Cosmic Rays) that usually falls on us from deep space.
What the scientists saw:

The Light Rain Stopped: The "lightweight raincoats" (neutron monitors) showed a massive drop in activity. The shield blocked almost all the light rain.
The Heavy Boulders Kept Coming: The "diving gear" (muon detectors) showed a much smaller drop. The shield was good at stopping the light stuff, but the heavy, high-energy boulders punched right through it.
Key Finding: The shield is "rigidity-dependent." It's like a sieve: it catches the small pebbles (low energy) but lets the big rocks (high energy) pass through.

The "Missing Particles" Mystery

The scientists did something clever. Instead of just counting how many particles were there, they calculated the "Missing Particles."

The Analogy: Imagine you know exactly how many people usually walk through a park gate every hour. One day, you count them and find 20% fewer people. You then create a "Missing Person Profile" to see who is missing.
The Discovery:
- Who was missing? Mostly the "lightweight" people (low-energy particles).
- Who was still there? The "heavy-duty" people (high-energy particles).
- The Comparison: When they compared the "Missing Person Profile" (the Shield event) with the "Firehose Profile" (the Explosion event), they found they were mirror opposites.
  - The Shield blocked the weak stuff but let the strong stuff through.
  - The Firehose added a huge amount of strong stuff that wasn't there before.

A Curious "Bump" in the Data

The scientists noticed a strange little "bump" in the data for the heavy particles (muons) around a specific energy level (40–50 MeV).

The Analogy: It's like listening to a song and noticing a sudden, brief change in the melody that doesn't fit the smooth pattern of the rest of the song.
What it means: It suggests that at this specific energy level, the "heavy particles" are changing how they are made or behaving in the atmosphere. It's a detail that the "lightweight" detectors (neutrons) completely missed, proving that you need the "heavy-duty" detectors to see the full story.

The Bottom Line

This paper tells us that:

Solar Cycle 25 is active: We are seeing big, interesting events again.
One tool isn't enough: To understand space weather, you need to look at both the "light rain" (neutrons) and the "heavy boulders" (muons).
The Shield is selective: Solar storms block the weak cosmic rays but let the strong ones through.
The Explosion is powerful: When the Sun erupts, it sends out a mix of particles that includes some very heavy, high-energy ones.

By combining these two views, scientists get a much clearer, 3D picture of how the Sun and Earth interact during these extreme weather events.

Technical Summary: Coherent Observation of GLE 77 and the January 2026 Forbush Decrease via Neutron Monitors and SEVAN Spectrometry

Problem Statement
Ground Level Enhancements (GLEs) and Forbush Decreases (FDs) represent two distinct manifestations of solar-terrestrial interactions: the injection of high-energy solar protons and the suppression of galactic cosmic rays (GCRs) by interplanetary magnetic structures, respectively. While both phenomena are routinely detected by global neutron monitor (NM) networks, their energy-dependent characteristics and their specific manifestations across different secondary particle channels (neutrons vs. muons) remain insufficiently explored. Standard NM observations integrate over broad primary rigidity ranges, making it difficult to disentangle whether modulation effects are dominated by low-rigidity particles or extend uniformly to higher rigidities. This study addresses the need for a more differential view of cosmic-ray modulation by comparing the spectral behavior of "missing" particles during an FD with the "excess" particles during a GLE, utilizing simultaneous observations from NMs and the SEVAN detector network.

Methodology
The authors analyzed data from two major events in Solar Cycle 25:

GLE 77 (11 November 2025): Triggered by an X5.1-class solar flare, observed by neutron monitors at Aragats (3200 m) and Nor Amberd (2000 m), and the SEVAN Light detector.
Forbush Decrease (19–20 January 2026): Associated with an X1.95 flare and a full-halo coronal mass ejection (CME) that caused a severe geomagnetic storm (G4/G3 levels).

The study leverages the SEVAN detector concept, which combines neutron- and muon-sensitive channels at single locations. Specifically, the analysis utilizes:

Neutron Monitors (NM): Sensitive primarily to lower-rigidity primaries (nucleonic component).
SEVAN "010" Channel: Dominated by secondary neutrons with limited muon admixture, comparable to NM response.
SEVAN Muon Channels: Sensitive to higher-rigidity primaries responsible for ground-level muons.

The core methodology involves reconstructing energy-dependent spectra of secondary particles. For the FD, the authors define "missing particle spectra" by calculating the difference between the quiet-time secondary particle spectra and those measured during the main phase of the disturbance. These deficit spectra are compared against the enhancement spectra observed during GLE 77. The analysis focuses on the spectral shape, steepness, and amplitude differences between neutron and muon channels to infer the rigidity dependence of the modulation.

Key Contributions and Results

Coherent Multi-Instrument Detection: The January 2026 FD was coherently registered by the global NM network and SEVAN detectors across multiple altitudes. The temporal sequence confirmed a causal chain: CME-driven shock arrival $\rightarrow$ magnetospheric compression (geomagnetic minimum) $\rightarrow$ cosmic-ray suppression.
Rigidity-Dependent Suppression: The study confirms that FDs selectively suppress the pre-existing galactic cosmic-ray population based on rigidity.
- Neutron vs. Muon Amplitude: The FD amplitude was significantly larger for neutrons (15–20% suppression) than for muons. This demonstrates that low-rigidity primaries are modulated more strongly than the higher-energy primaries responsible for ground-level muons.
- Spectral Steepness: The "missing" neutron spectrum showed a moderate steepening with energy, whereas the muon deficit spectrum was substantially softer (steeper indices), indicating that the FD influence diminishes rapidly at higher energies.
Intrinsic Asymmetry between FD and GLE: A comparison of the FD deficit spectra with GLE 77 enhancement spectra revealed a fundamental asymmetry:
- GLEs introduce an additional, harder particle component that remains substantial at high deposited energies, particularly in the muon channel.
- FDs represent a suppression that steepens and fades toward higher energies.
- The high-energy tail is far more robust for muons during a GLE than the deficit is during an FD.
Spectral Structure in Muon Channels: The analysis identified a localized "bump" (local hardening) in the muon spectrum near 40–50 MeV deposited energy during the FD. This feature, absent in the neutron spectrum, suggests a transition region in the effective parent population or atmospheric cascade contribution, highlighting that muon measurements carry spectral information not inferable from neutron data alone.
Altitude and Anisotropy Effects: The study noted that the Aragats Neutron Monitor (higher altitude) exhibited significantly larger $\sigma$ -amplitudes and more structured temporal peaks compared to the lower-altitude Nor Amberd monitor during GLE 77, underscoring the role of atmospheric depth and geomagnetic transmission in resolving weak GLEs.

Significance
The paper claims that the combined use of neutron monitors and SEVAN spectrometry provides a robust, complementary diagnostic of rigidity-dependent cosmic-ray modulation during extreme heliospheric disturbances. By reconstructing energy-dependent "missing particle" spectra, the study moves beyond integral count-rate measurements to characterize how different rigidity ranges are suppressed during ICME-magnetosphere interactions.

The authors emphasize that while neutron monitors remain the primary tool for detecting FDs, SEVAN measurements add essential spectral discrimination, enabling the separation of low- and high-energy responses. This approach allows for a more precise understanding of the asymmetry between cosmic-ray suppression (FD) and enhancement (GLE), particularly regarding the differential behavior of neutrons and muons at higher energies. The results are presented as a valuable tool for characterizing severe space weather events in Solar Cycle 25 and beyond, without redefining established geomagnetic storm severity metrics but rather providing a physics-driven perspective on particle access to the atmosphere.