Ground-Level Enhancement 77 of 11 November 2025: Relativistic Solar-Particle Acceleration, an Extreme Geomagnetic Storm, and Forbush Decreases Driven by Solar Active Region 4274

Abstract

The rising phase of Solar Cycle 25 has produced several extreme space-weather episodes driven by explosive magnetic reconnection in the solar atmosphere. This study presents an integrated, multi-parameter analysis of the eruptive sequence of 9–14 November 2025 originating from the magnetically complex active region NOAA AR 4274, focusing on the X5.1 flare of 11 November 2025 (peak 10:04 UT). This flare accelerated solar protons to relativistic (>1 GeV) energies and produced the Ground-Level Enhancement (GLE) catalogued as GLE #77, recorded by the global neutron-monitor network from ~10:15 UT and now established as the strongest GLE in 19 years. Using daily sunspot number and F10.7 cm flux (Royal Observatory of Belgium/SIDC and DRAO), hourly interplanetary magnetic field (IMF) and solar-wind plasma data (NASA OMNIWeb), geomagnetic indices (Kp, Dst, SYM-H), and pressure-corrected neutron-monitor count rates (Oulu and the global network via gle.oulu.fi and NMDB), this study traces the complete Sun–heliosphere–Earth chain over 8–17 November 2025. GLE #77 reached peak amplitudes of about 125% in standard neutron monitors and 165% in bare neutron monitors globally, with pronounced rigidity dependence (e.g., ~102% at South Pole, ~25% at high-latitude and ~9–21% at mid-latitude stations) and a complex, anisotropic, dual-peak time profile. The associated compound shock–ICME structure drove a severe geomagnetic storm (Kp = 9−; Dst ≈ −231 nT; SYM-H ≈ −254 nT), with the main phase occurring early on 12 November, accompanied by sustained southward IMF Bz and elevated solar-wind speed. The relativistic-particle enhancement was followed by Forbush decreases in galactic cosmic-ray intensity. Placed in the context of recent GLEs (GLE 69–GLE 73), GLE #77 emerges as the hardest and strongest relativistic solar-particle event since GLE 70 (2006). The results illustrate the dual role of major eruptions in both accelerating relativistic particles and modulating galactic cosmic rays and provide a benchmark event for space-weather forecasting during Solar Cycle 25.