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

TL;DR

This study reports the first ground-based gamma-ray detection of GLE#77 from Arctic and Antarctic stations, demonstrating the effectiveness of polar gamma-ray detectors in observing extreme solar energetic particle events.

Contribution

It introduces simultaneous gamma-ray observations at polar stations during a GLE, highlighting their potential to complement neutron monitor data in solar event analysis.

Findings

First ground-based gamma-ray detection of GLE#77 from polar regions.

Prompt and delayed responses observed with distinct timing and rigidity dependence.

Correlation between gamma-ray signatures and satellite proton flux measurements.

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.