NOAA’s Space Weather Prediction Center issued a high-energy electron alert on September 26, 2026 and renewed it on each of the next three days, with the daily peak climbing from 1,260 particle flux units on September 26 to 3,060 on September 27 and 4,128 on September 28. The alert triggers when electrons above 2 million electron volts exceed 1,000 pfu at geostationary orbit, the altitude where communications and weather satellites sit.
Four consecutive days of alerts, with a peak more than four times the threshold, are the kind of run satellite operators watch closely. Nothing in the alert record points to a failure, and the alerts exist to warn operators rather than to report damage.
Four days in NOAA’s alert feed
The raw record is the SWPC alerts feed. It logs an initial electron flux alert at 15:51 UTC on September 26, then continued alerts at 05:02 UTC on September 27, 05:00 on September 28 and 05:02 on September 29. The three daily maxima it reports are 1,260 pfu on September 26, 3,060 pfu on September 27 and 4,128 pfu on September 28, the last reached at 14:40 UTC. The September 29 message was still in force when it went out, so the flux had not dropped back under 1,000 pfu at that point. The daily maxima rose by 1,800 pfu from the first day to the second and by another 1,068 pfu to the third, which is a steady climb rather than a spike, and the 05:00 UTC issue times show the continued alerts follow a daily cycle in which each message reports the previous day’s maximum.
A geomagnetic K-index of 4 alert on September 27 marked the same stretch, an active-level reading well short of a storm, and the feed shows nothing stronger in the run. The Belgian centre cited below explains that these energetic electrons result from solar wind interactions with Earth’s magnetosphere, so they can build over days rather than arriving as a single burst, which is the shape the 1,260, 3,060 and 4,128 sequence has.
Why 1,000 pfu is the line for satellites
SWPC’s GOES electron flux page states the rule directly: electron event alerts are issued when the greater-than-2 MeV flux exceeds 1,000 particles per square centimetre per second per steradian. The concern is deep-dielectric charging, in which energetic electrons penetrate a spacecraft’s skin, accumulate inside insulating parts and can eventually discharge, disrupting systems temporarily or permanently.
The alert product is aimed at a narrow audience. In SWPC’s alert catalogue, the electron product, coded ALTEF3, lists its impact as possible differential charging and bulk charging effects on satellites, and it is meant for owners and operators rather than for power-grid or radio users.
The Belgian Solar-Terrestrial Centre of Excellence, in an explainer on enhanced electron flux, points to the worst-known example: the consecutive outage on January 20, 1994 of Telesat Canada’s Anik E1 and E2 geostationary satellites, which interrupted telecommunications and data transmission across Canada for hours. The same page describes a January event it followed, when flux reached the 1,000 pfu threshold on January 27 and peaked at 5,068 pfu on January 31, again without permanent damage to any satellite, an outcome that shows a high reading raises risk without guaranteeing an anomaly.
A quieter fortnight followed a stronger episode
This run is not the strongest of the month. The same feed shows an earlier episode of alerts from August 31 through September 14, fourteen messages in all, with a peak of 7,008 pfu on September 11 at 13:55 UTC, seven times the threshold, and another reading near 3,000 pfu the day before. That episode had its own pauses, including a gap between September 4 and September 6. No alerts appear at all between September 14 and September 26, so the September 26 alert is a fresh enhancement after a twelve-day gap and not a continuation. Across the two episodes the feed carries eighteen electron alert messages in under a month, and the feed gives no baseline for how that compares with earlier years.
The sensor behind these numbers is GOES-19, which NOAA’s satellite service says carries the Space Environment In-Situ Suite, or SEISS, to monitor proton, electron and heavy-ion fluxes in the magnetosphere. The NESDIS release on its first data says the satellite launched on June 25, 2024, began collecting data on August 22, 2024, and that SWPC would use the suite’s measurements to issue radiation belt alerts. SWPC’s electron page adds that GOES electron measurements go back to the first GOES satellite in 1975 and that the product tracks two integral channels, above 0.8 MeV and above 2 MeV, with the higher one driving the alert.
How long the enhancement lasts is the one thing the feed does not say. The most recent alert in the feed, issued September 29 at 05:02 UTC, reported the September 28 peak and did not announce an end.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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