What is a solar radiation storm and why it matters
Solar radiation storms are extreme space weather phenomena in which high-energy particles from the Sun reach near-Earth space, posing operational risks to satellites and aviation.
X1.2 solar flare on May 13, 2025. Credit: NASA/SDO AIA 304, Helioviewer, The Watchers
Solar radiation storms occur when large numbers of high-energy particles, primarily protons, are accelerated away from the Sun and reach near-Earth space. These particles travel much faster than coronal mass ejections (CMEs) and can arrive within minutes to hours after a major solar eruption, posing immediate risks to spacecraft, aviation, and human activity in space.
A solar radiation storm does not mean that harmful radiation reaches Earth’s surface in dangerous amounts. Earth’s magnetic field and dense atmosphere absorb and deflect most incoming solar energetic particles well above ground level.
Even during extreme events, radiation increases at the surface are small, short-lived, and detectable mainly by specialized scientific instruments. Studies show solar radiation storms are not associated with increased health risk for the general population at ground level and do not cause radiation sickness, cancer spikes, or environmental contamination.
Their impacts are confined primarily to space-based systems, high-altitude aviation, and human activity beyond Earth’s atmospheric shielding.
The latest solar radiation storm, S4 – Severe on January 19, 2026, is the strongest solar radiation storm observed since 2003, according to the Space Weather Prediction Center. It followed an X1.9 solar flare and a fast, Earth-directed coronal mass ejection that erupted on January 18, making it a clear example of how extreme solar activity can rapidly translate into hazardous space weather conditions at Earth.
How solar radiation storms form
Solar radiation storms are driven by solar energetic particles (SEPs). These particles are accelerated in two main ways: directly during powerful solar flares, and more efficiently by shock waves driven ahead of fast coronal mass ejections as they propagate through the solar wind.
In strong events, CME-driven shocks act as continuous particle accelerators, injecting high-energy protons into interplanetary space over extended periods. When Earth lies along the path of this particle stream, detectors near Earth register sharp increases in proton flux, particularly in the ≥10 MeV energy range used to define NOAA’s radiation storm scale.
The January 2026 event shows this process clearly. Proton flux rose steadily from minor levels late on January 18, rose through S2 – Moderate and S3 – Strong during January 19, and ultimately exceeded 10 000 pfu, reaching S4 – Severe levels. This progression indicates sustained shock acceleration rather than a short-lived impulsive burst.
What the S-scale means
NOAA classifies solar radiation storms using the S-scale, based on ≥10 MeV proton flux, from S1 to S5:
S1 – Minor and S2 – Moderate storms are relatively common during periods of elevated solar activity and usually cause limited operational effects.
S3 – Strong storms are uncommon and can disrupt aviation and satellite operations.
S4 – Severe storms are rare and represent a high-impact space weather scenario.
S5 – Extreme storms are exceptionally rare and associated with historic events.
Reaching S4 requires not only a strong solar flare, but also efficient and sustained particle acceleration, usually provided by a fast, Earth-directed CME with a well-developed shock. Many X-class flares do not produce severe radiation storms, which is why S4 events stand out.
Operational impacts of severe radiation storms
At S4 intensity, impacts extend across multiple sectors.
For aviation, radiation exposure increases for passengers and crew on high-latitude, high-altitude flights. Airlines may reroute polar flights or adjust altitudes to reduce accumulated dose during severe events.
For spaceflight, astronauts conducting extravehicular activities are exposed to unavoidable radiation hazards, and EVA operations are typically restricted or postponed during severe radiation storms.
For satellites, high-energy protons can penetrate spacecraft shielding, causing memory device upsets, increased noise in imaging systems, and disruptions to star trackers used for orientation. Prolonged exposure may also temporarily degrade solar panel efficiency.
During radio communications, severe radiation storms can cause widespread blackouts of polar high-frequency radio links, affecting aviation, maritime operations, and emergency communications at high latitudes.
Where and when solar radiation storms occur
Solar radiation storms occur in near-Earth space, primarily within Earth’s magnetosphere and along interplanetary magnetic field lines connecting Earth to the Sun. They are detected by satellites in Earth orbit and, in rare cases, indirectly at Earth’s surface through atmospheric particle cascades.
These storms are episodic, not continuous. They are most likely during periods of elevated solar activity, particularly near the peak of the approximately 11-year solar cycle, when powerful solar flares and fast coronal mass ejections (CMEs) are more frequent.
Individual radiation storms can begin within minutes to hours after a solar eruption and may persist for hours to several days, depending on how long particle acceleration continues at the CME-driven shock.
Ground level enhancements (GLEs)
A ground level enhancement (GLE) is a rare subset of solar energetic particle events in which protons are accelerated to extremely high energies, typically hundreds of MeV to several GeV. These particles generate secondary radiation cascades in Earth’s atmosphere that can be detected by ground-based neutron monitors.
Since systematic monitoring began in the 1940s, only a few dozen GLE events have been recorded worldwide. The most recent GLE event, associated with an X5.1 solar flare in November 2025, produced the strongest GLE in about two decades. While significant in scientific and operational terms, it remained far weaker than the most extreme GLE on record, which occurred in February 1956.
Not all severe radiation storms produce GLEs. Their occurrence depends on the particle energy spectrum and magnetic connectivity between the Sun and Earth, not solely on proton flux levels measured in space.
Radiation risk to people at ground level
According to studies, solar radiation storms, including severe S4-level events and those that produce GLEs, do not pose a significant radiation risk to people at ground level. Earth’s magnetic field and atmosphere provide effective shielding, absorbing the vast majority of incoming solar particles well above the surface.
Even during the strongest known events, increases in ground-level radiation are small and typically detectable only by sensitive scientific instruments. Research comparing radiation doses during extreme solar particle events consistently shows that surface-level exposure remains far below thresholds associated with adverse health effects.
Meaningful radiation risk from solar storms is confined primarily to space environments and aviation altitudes, where shielding is reduced or absent. For the general population on the ground, solar radiation storms are not associated with increased cancer risk or other direct health impacts.
Radiation storms vs. geomagnetic storms
Solar radiation storms and geomagnetic storms are related but distinct phenomena. Radiation storms are driven by fast-moving particles and can begin before a CME reaches Earth. Geomagnetic storms occur later, when the CME itself interacts with Earth’s magnetosphere.
In the current event, the severe radiation storm developed first, while the associated CME impacted Earth a day after, sparking G4 – Severe geomagnetic storming.
Interestingly, when radiation and geomagnetic storms occur together, operational impacts can compound, affecting satellites, navigation systems, power grids, and communications at the same time.
I'm a dedicated researcher, journalist, and editor at The Watchers. With over 20 years of experience in the media industry, I specialize in hard science news, focusing on extreme weather, seismic and volcanic activity, space weather, and astronomy, including near-Earth objects and planetary defense strategies. You can reach me at teo /at/ watchers.news.
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