11 November 2025: the strongest solar relativistic particle event since 19 years

On 11 November 2025 the Sun produced a remarkable flare, during which protons were accelerated to unusually high energies. The event was detected by neutron monitors (NMs) on the Earth, because the protons and atomic nuclei accelerated at the Sun, while travelling through the interplanetary space, encounter the atmosphere of the Earth. When the particles have sufficiently high energies, their collision with the nuclei of atoms in the atmosphere at tens of kilometres above the ground produce nuclear reactions, releasing secondary protons, neutrons and other particles that can pursue their path downward and produce even more particles by repeated collisions. Solar energetic particle events with such a signature on the Earth are called Ground-Level Enhancements (GLEs). The energy of these particles impacting the atmosphere must be above the threshold of 430 MeV in order to produce measurable effects on the Earth. This is more than two million times the average energy of protons in the corona of the Sun, where the acceleration occurs (see https://www.nmdb.eu/public_outreach/en/01_scr/ ).

The figure above shows the signal recorded by neutron monitors worldwide during GLE77 on 11 November 2025. The signal was normalized to the permanent background level produced by high-energy particles from our Galaxy. After 10 a.m. Universal Time (10 UT), the count rates of many NMs rose sharply from level 0 to a maximum a few minutes later, with the maximum increase in count rate depending on the location of the neutron monitor. The relative count rate increase was strongest in Antarctica (~130% of the pre-event level at Mawson, Australia), but it was also significant in Oulu (Finland). Other NMs detected only a slow rise over hours, followed by a gradual decay. Still others saw no enhancement at all. These neutron monitors are located at low latitudes, where the magnetic field of the Earth prevents the primary protons from impacting the atmosphere. However, it seems that the neutron monitor in Mexico also observed a significant increase in the count rate during GLE77. Its low latitude would mean that the maximum energy of the protons accelerated at the Sun s at least 7 GeV (7 billions of eV) during the early peak, i.e. about 15 times the minimum energy for a proton needed to trigger a GLE.

In order to better appreciate this energy gain we may compare the situation with a human analogy: the average pedestrian walks at a speed of about 4 km per hour. A sprinter making 100 metres in 10 seconds has an average speed of 36 km/h, nine times more than the pedestrian. Since the energy goes with the square of the speed, the sprinter's energy is 81 times the average pedestrian's. The sprinter is with respect to the average pedestrian what the relativistic nucleon is with respect to the average nucleon in the solar corona. The energy gain of the particles at the Sun is, however, more than two million! The underlying processes are not only characteristic of the Sun and its dynamics, but they are relevant to any process of particle acceleration in the Universe! Their understanding is still to be achieved. The proximity of the Sun offers us especially favourable conditions for the observation of particle acceleration events like the GLE of 11 November 2025.

The particle event was related to a flare in the region outlined by the sunspot group in the upper right sector.

The worldwide network of neutron monitors is well established since the 1960s. It remains the state of the art instrument to detect the most energetic protons accelerated at or near the Sun. The event of 11 November is indeed a rare occurrence of the first of this strength observed since December 2006. Its origin is related to a group of sunspots in the upper right sector of the Sun shown for instance by the snapshot taken at Paris Observatory in Meudon, France on the same day. Sunspots are a sign of complex magnetic fields that tend to be unstable, leading to brightenings, known as flares, and mass ejections. The changing magnetic fields also create electric fields that accelerate charged particles. But we still do not know how they can accelerate by the factors we measure during a GLE, especially within a short time less than a few minutes. The data collected on 11 November by neutron monitors and many other complementary instruments on the Earth and in space will be intensely used by researchers worldwide in the coming weeks to gain more insight into the physical processes at the Sun.

In addition to their scientific interest, GLEs are also a topic of concern for space weather, as the high-energy particles generated in the atmosphere interact with electronics and the human body in aircraft.

The issue for electronics is the failure of devices due to the generation of spurious electric currents or, in extreme cases, to the physical damage produced by the energetic particles (see https://www.nmdb.eu/public_outreach/en/05/ ). Enhanced radiation exposure may pose a health risk for people on board aircraft, either in the long term due to the accumulation of radiation doses (particularly for flight crew and frequent flyers), or in the short term during a highly irradiating event (which occurs only once per century or even per millennium). The 11 November event produced enhanced radiation doses at aircraft altitudes, but of low significance judging from the time profile of the NM records.