A wave of activity on the surface of the sun triggered a minor radiation storm and set off a chain of coronal mass ejections that forecasters expect to reach Earth this week. The Space Weather Prediction Center, the federal office responsible for monitoring solar activity that can affect satellites, aviation and power grids, recorded the radiation storm on September 5 and followed it with a geomagnetic storm watch covering both September 8 and 9.
A Minor Radiation Storm on September 5
The radiation event began when the flux of energetic protons crossing the 10 MeV threshold exceeded 10 particle flux units starting at 4:15 p.m. UTC on September 5, eventually peaking at 18 units. That level registers as an S1, or minor, radiation storm on the five-step scale forecasters use to classify solar proton events, the lowest tier on a scale that runs up through S5 for the most extreme events. A separate, higher-energy measurement showed the flux of 100 MeV protons briefly crossing its own alert threshold for roughly half an hour that same afternoon, a sign that the eruption driving the storm produced a genuinely energetic burst of particles rather than a marginal one.
Radiation storms of this kind matter most for specific, exposed systems rather than for people on the ground. Airline crews flying polar routes can accumulate additional radiation dose during an S1 event, and the storm can degrade high-frequency radio communication used on some transoceanic flights, prompting airlines to occasionally adjust routing during stronger events. Ground-based technology is essentially unaffected at this intensity, which is part of why minor radiation storms rarely draw public attention outside the aviation and satellite operations communities that track them closely.
Where the Activity Originated
Space weather observers traced the burst of activity to sunspot region AR4524, an active region on the sun’s surface that produced at least four coronal mass ejections over September 5 and 6. Coronal mass ejections are large expulsions of plasma and magnetic field from the sun’s corona, and when several erupt from the same active region in close succession, their leading edges can catch up to one another and merge into a single, more forceful disturbance by the time they reach interplanetary space.
Forecasters tracking the string of ejections from AR4524 flagged the possibility that the merged material would arrive at Earth in a compressed window spanning September 8 into September 9, which is what prompted the geomagnetic storm watch covering both days. Data feeds published through the agency’s public alerts service, available at SWPC’s alerts feed, logged the sequence of measurements behind each stage of the forecast as it developed.
The Geomagnetic Storm Watch
The Space Weather Prediction Center issued a G1, or minor, geomagnetic storm watch at 4:35 p.m. UTC on September 7, covering the period the merged coronal mass ejections were expected to strike Earth’s magnetic field, according to the agency’s public alerts and warnings page. By early September 8, the agency had already logged a geomagnetic K-index reading of 5, the threshold at which conditions are classified as a minor storm, indicating the disturbance had begun to arrive close to forecast. Some assessments of the incoming material left open the possibility of brief G2, or moderate, storm conditions if the merged ejections arrived with a more favorable magnetic orientation.
Geomagnetic storms at this scale can produce fluctuations in high-latitude power grids, minor disruptions to satellite operations, and interference with GPS accuracy, though effects at the G1-to-G2 level are typically too small to cause the kind of widespread outages associated with the strongest storms on record. Utilities in far-northern regions routinely monitor these watches as a matter of course, since even a minor storm can cause small voltage fluctuations on long transmission lines running through high-latitude terrain.
Radiation Belt Effects Detected in Orbit
The same period of solar activity also showed up in measurements of Earth’s radiation belts. NOAA’s GOES-19 satellite recorded the flux of 2 MeV electrons exceeding 1,000 particle flux units on September 6, a level that indicates the outer radiation belt had become measurably more energized in the days following the initial eruptions. Elevated electron flux in that belt is tracked closely because it can accelerate the degradation of sensitive satellite electronics over time, particularly for spacecraft that spend extended periods in the affected orbital region.
What to Expect at Higher Latitudes
For observers at high latitudes, the practical effect of a G1-to-G2 storm is typically an expanded aurora rather than any disruption to daily life. Forecasters monitoring the merged ejections said the resulting geomagnetic activity could push the aurora further from the poles than usual, with some outlooks suggesting visibility could extend into the northern United States and similar latitudes in Europe if the storm reaches the stronger end of its forecast range. The agency said it would continue monitoring the incoming material and update its alerts as the geomagnetic response through September 9 became clearer, noting that the exact strength of any storm depends heavily on the orientation of the magnetic field carried within the approaching material, a variable that is difficult to measure precisely until the material is close to Earth.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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