“‘An S4 severe solar radiation storm is now in progress this is the largest solar radiation storm in over 20 years,” the Space Weather Prediction Center announced in a public message, pinpointing a moment when the most extreme effects were occurring in a way that was invisible to anyone on the surface.
Such a storm is not characterized by colorful skies but by speed. A solar radiation storm starts when the Sun ejects charged particles, mostly protons, into space and enough of them hit the Earth to register as a significant radiation event. NOAA rates such events from S1 to S5 based on satellite observations, and this event was S4 (severe), which was last recorded in October 2003.
These particles can travel the distance of approximately 93 million miles between the Sun and Earth in tens of minutes, well before the plasma clouds that cause geomagnetic storms. When the most energetic protons finally reach Earth, they do not distribute evenly. They follow the magnetic field lines towards the polar regions, where Earth’s protection is weaker. Notably, Earth’s atmosphere and magnetic field protect the planet from these particles and ensure that they do not pose a direct threat on the surface.
Far above the surface, the needs shift rapidly.
For astronauts, the plans for response are useful, not dramatic. During periods of high radiation risk, the crews on the International Space Station can be instructed to move to areas of the station that are more protected until the flux rate decreases. This is effective because the ISS is always within the magnetosphere, a shielded region that protects it from radiation exposure compared to space. Outside this region, the problem of shielding mass versus launch mass becomes more pressing.
This is what the current lunar planning is centered on. Radiation monitoring and prediction are considered operational requirements rather than research areas, with development underway for real-time sensing concepts for future cislunar habitats and vehicles. “Leaving the magnetosphere is like leaving a safe harbor and venturing out into the open ocean,” ESA has explained, pointing out that radiation exposure at the Moon can be significantly higher than in low Earth orbit and why early warning is important for crews who may have limited options for shelter.
The constraints for aviation are different: planes cannot increase shielding significantly, and route interactions affect overall networks. Solar radiation storms can render high-frequency communications unreliable over the polar regions, and ionospheric disturbances caused by space weather can impact satellite navigation. The space-weather aviation research by the FAA points out that over 2,500 airports in the U.S. are in a environment where GPS operations can be impacted during disturbed space weather, even though most of the major airports have instrument landing systems as a non-GPS option.
However, during the proton storm, another event contributed to the sense of celestial drama that filled the week: a G4 geomagnetic storm that contributed to the energization of aurorae over broad latitudes. Although these two types of storms tend to occur in close proximity to one another, as both can be associated with the eruption of magnetically complex active regions on the Sun, their effects are distinct: radiation storms are concerned with particle exposure and satellite electronics, whereas geomagnetic storms are more directly perturbing power and navigation systems.
The broader context is the Sun’s current cycle. Solar Cycle 25 is now in the solar maximum, where the number of sunspots increases and solar flares occur more frequently, and NASA and NOAA have noted that the solar maximum does not pinpoint a specific “peak month” online. For the engineer or operator, the issue is the uncertainty: space weather is not a singular news story, but a space where data must be taken and accounted for, particularly when the most significant impact is well above the clouds.
