Solar Orbiter observations of the largest ground level enhancement of Solar Cycle 25 to date (GLE 77)

On 11 November 2025, Solar Orbiter’s STIX instrument observed an X5-class solar flare, which peaked at approximately 10:01 UTC, originating from active region AR 14274 located at N23W23. Associated with this flare, a fast coronal mass ejection (CME) was observed by several coronagraphs, with a reconstructed 3D speed of about 2100 km s⁻¹. Solar Orbiter, near 0.82 au and ~18° west of Earth in heliolongitude, encountered the associated interplanetary CME (ICME) at around 09:00 UTC on 12 November, as observed by the MAG and SWA instruments. The ICME arrived at Earth roughly 9 hours later, around 18:00 UTC on 12 November.Solar Orbiter/EPD detected electrons with energies above 7 MeV, as well as penetrating ions exceeding 400 MeV. At Earth, the neutron monitor network recorded the ground level enhancement 77 (GLE 77), showing increases at least above a rigidity of 8 GV shortly after 10:00 UTC on 11 November. This event represents the largest GLE of Solar Cycle 25 observed to date and displays clear prompt and delayed components, presumably associated with flare-related and CME-driven particle acceleration.In this study, we analyze Solar Orbiter and near-Earth observations of the flare, CME, and associated solar energetic particle (SEP) event. We also include multi-spacecraft SEP measurements, in particular from ESA’s BepiColombo mission, which was nominally well connected to the parent active region.