Rare X5.1 Solar Super-Flare Sent Atmospheric Radiation to Highest Levels in Nearly 20 Years

Rare Solar Super-Flare Raised Radiation Levels Across the Atmosphere

On Nov. 11 a powerful X5.1 solar flare erupted from sunspot AR4274 and produced a rare Ground Level Event (GLE) that briefly pushed radiation in Earth's atmosphere to levels not seen in nearly 20 years. The flare followed a sequence of smaller flares and coronal mass ejections (CMEs) that created vivid auroras visible as far south as Florida.

What Happened and Why It Matters

Unlike CMEs — which eject clouds of magnetized plasma that take days to reach Earth — energetic protons accelerated by some solar flares can travel nearly at the speed of light and arrive within minutes. When these protons strike molecules at the top of the atmosphere, they trigger cascades of secondary particles (neutrons, muons and electrons) that travel downward and temporarily boost radiation levels at flight altitudes and on the ground.

In this event, stratospheric balloons launched by researchers recorded that radiation at typical commercial cruising altitudes (about 40,000 feet / 12 kilometers) briefly rose to roughly ten times the normal cosmic-ray-related background. The most intense phase lasted about two hours, while the broader event persisted for roughly 15 hours. Benjamin Clewer, a space-weather researcher at the University of Surrey, noted that these increases usually peak quickly and decay: "Typically, these events peak right at the beginning and that might only last about half an hour," he said.

Ground Level Events Are Rare — And Hard To Predict

Ground Level Events are uncommon: only 77 have been recorded since instrumental monitoring began in the 1940s, according to Clewer. Scientists still lack a reliable explanation for why only some solar flares accelerate enough high-speed protons to produce GLEs, which means predicting sudden radiation spikes remains difficult.

"We don't understand the physics of it that well as to why some solar flares eject these really high speed particles and other ones don't,"

Health And Aviation Risks

Although the recent GLE was modest compared with historical extremes — the strongest measured event, in 1956, was about sixty times more intense — paleoclimate records (radioisotope signatures in tree rings) indicate even larger events are possible. Short-lived spikes like Nov. 11 raise concerns for passenger and crew health (short exposures increase radiation dose and long or repeated exposure raises lifetime cancer risk). Researchers point out that a pregnant person continuously exposed to the observed peak levels for more than 12 hours would exceed official fetal safety limits; fortunately, the intense phase here was short.

GLEs can also disrupt aircraft electronics. Two weeks before Nov. 11, a JetBlue Airbus flying over Florida experienced a sudden altitude loss later attributed to possible electronics malfunctions tied to high-energy space particles; that incident caused multiple passenger injuries. Scientists warn that a more intense GLE could trigger simultaneous instrument anomalies across many aircraft, potentially creating hazardous in-flight situations.

Recommendations And Preparedness

Researchers are urging airlines and regulators to consider installing radiation monitors on commercial aircraft so flight crews can detect sudden space-weather-driven radiation increases and take mitigations such as descending to lower altitudes or changing latitude when communication with controllers is possible. During severe events radio communications can be degraded, and because GLEs often arrive with little or no warning, some aircraft may be caught mid-flight without advance notice.

While the Nov. 11 event was brief and relatively mild, it highlights gaps in our ability to forecast extreme particle events and the value of improved monitoring and operational plans to protect passengers, crews and avionics.

Context

Key facts: the flare was classified X5.1 and originated from AR4274; about 20 X-class flares affected Earth this year but only this event delivered a direct high-speed proton stream; GLEs are rare with 77 documented since the 1940s; the 1956 GLE was ~60× stronger and tree-ring records suggest far larger events have occurred historically.