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Ground Level Enhancement (GLE#77) in the gamma-ray component: First observation from Arctic and Antarctic stations

Pranali Thakur, Geeta Vichare, Selvaraj Chelliah

TL;DR

This paper documents the first ground-based gamma-ray observation of a ground-level solar event (GLE#77) using polar detectors in the Arctic and Antarctic, synchronized with GOES-18 proton measurements and NM data. It analyzes a distinct two-peak structure: a prompt peak associated with high-energy protons and a delayed peak linked to lower-energy protons, with amplitudes strongly dependent on geomagnetic cutoff rigidity and only weakly on solar zenith angle. The study demonstrates that polar NaI(Tl) gamma-ray detectors can complement traditional NM networks, offering new insights into particle acceleration and transport during extreme solar proton events. The findings enhance understanding of how energy, geomagnetic shielding, and local atmospheric conditions shape ground-level signatures of solar activity.

Abstract

This article presents the observations of the extreme ground-level enhancement (GLE #77) of Solar Cycle 25 that occurred on 11 November 2025, using ground-based NaI(Tl) gamma-ray detectors deployed at Arctic and Antarctic stations, together with neutron monitor data and particle measurements from the GOES-18 satellite. The event was associated with an intense X-class solar flare and a strong solar energetic proton event. This paper reports the first ground-based detection of a GLE using gamma-ray detectors operating simultaneously in both polar regions, which are concurrent with increases in neutron monitor counts. Thus highlights the capability of polar gamma-ray detectors to complement traditional neutron monitor observations during extreme solar proton events. A detailed analysis revealed distinct prompt and delayed responses during the event evolution. Interestingly, the signature of the prompt peak of GLE#77 (at 10:38 UT) was observed up to high-rigidity neutron monitors (low latitudes). However, the delayed peak (at 13:08 UT) was not seen at the stations with rigidity > 6 GV. The timing of the prompt and delayed peaks coincided with the proton flux peaks observed by the GOES-18 satellite at energies > 150 MeV and 12-99 MeV, respectively. It is observed that the GLE amplitude has a strong dependence on geomagnetic cutoff rigidity and has a weak solar zenith angle dependence.

Ground Level Enhancement (GLE#77) in the gamma-ray component: First observation from Arctic and Antarctic stations

TL;DR

This paper documents the first ground-based gamma-ray observation of a ground-level solar event (GLE#77) using polar detectors in the Arctic and Antarctic, synchronized with GOES-18 proton measurements and NM data. It analyzes a distinct two-peak structure: a prompt peak associated with high-energy protons and a delayed peak linked to lower-energy protons, with amplitudes strongly dependent on geomagnetic cutoff rigidity and only weakly on solar zenith angle. The study demonstrates that polar NaI(Tl) gamma-ray detectors can complement traditional NM networks, offering new insights into particle acceleration and transport during extreme solar proton events. The findings enhance understanding of how energy, geomagnetic shielding, and local atmospheric conditions shape ground-level signatures of solar activity.

Abstract

This article presents the observations of the extreme ground-level enhancement (GLE #77) of Solar Cycle 25 that occurred on 11 November 2025, using ground-based NaI(Tl) gamma-ray detectors deployed at Arctic and Antarctic stations, together with neutron monitor data and particle measurements from the GOES-18 satellite. The event was associated with an intense X-class solar flare and a strong solar energetic proton event. This paper reports the first ground-based detection of a GLE using gamma-ray detectors operating simultaneously in both polar regions, which are concurrent with increases in neutron monitor counts. Thus highlights the capability of polar gamma-ray detectors to complement traditional neutron monitor observations during extreme solar proton events. A detailed analysis revealed distinct prompt and delayed responses during the event evolution. Interestingly, the signature of the prompt peak of GLE#77 (at 10:38 UT) was observed up to high-rigidity neutron monitors (low latitudes). However, the delayed peak (at 13:08 UT) was not seen at the stations with rigidity > 6 GV. The timing of the prompt and delayed peaks coincided with the proton flux peaks observed by the GOES-18 satellite at energies > 150 MeV and 12-99 MeV, respectively. It is observed that the GLE amplitude has a strong dependence on geomagnetic cutoff rigidity and has a weak solar zenith angle dependence.
Paper Structure (10 sections, 1 equation, 6 figures, 1 table)

This paper contains 10 sections, 1 equation, 6 figures, 1 table.

Table of Contents

  1. Introduction
  2. Data
  3. Observation of Ground Level Enhancement (GLE#77) on 11 November 2025
  4. Gamma-ray observations at Arctic
  5. Gamma-ray observations at Antarctica
  6. Neutron Monitor (NM) Observations
  7. Discussion and Results
  8. Variation of amplitudes of prompt and delayed peaks with geomagnetic rigidity
  9. Variation of amplitude with solar zenith angle
  10. Conclusions

Figures (6)

  • Figure 1: GOES-18 solar flare X-ray fluxes, SGPS proton fluxes, and neutron fluxes during GLE #77 on 11 November 2025.
  • Figure 2: Gamma-ray variations (300 keV to 7 MeV) and during 10-12 November 2025 (top panel); gamma-ray variation with energy 3-7 MeV during the same period (bottom panel)
  • Figure 3: Enhancement seen in gamma-ray flux with energy 3-7 MeV during 11 November 2025 at Maitri, Antarctica.
  • Figure 4: Normalized neutron flux ($\%$) variations recorded at 42 neutron monitor stations, arranged from top to bottom in decreasing order of geomagnetic rigidity, during 11 November 2025 (GLE $\#$77).
  • Figure 5: The amplitude of $\%$ increase during (a) Prompt peak at 10:38 UT, (b) Delayed peak at 13:08 UT, during GLE$\#$77 on 11 May 2026.
  • ...and 1 more figures