NOAA’s Space Weather Prediction Center logged electron flux above 1,000 particles per square centimeter per second per steradian on four consecutive days, Oct. 23 through Oct. 26, 2026, a level at which it issues an electron alert because of the threat to spacecraft electronics. The daily peaks reached 3,169, 3,259 and 2,864 particles in those units, known as pfu, more than twice the alert line each time.
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The unusual part is the timing. The geomagnetic storm that preceded the electron surge was only a minor one, rated G1 on NOAA’s five-step scale, and no later reading in the alerts feed reached that level through Oct. 26. What stayed elevated was not a storm in the usual sense but a population of fast electrons in near-Earth space.
A G1 storm that reached its threshold on Oct. 21
The sequence began on Oct. 20 at 19:12 UTC, when the center posted a watch predicting a G1 (Minor) geomagnetic storm for Oct. 21. The center’s alerts feed then recorded a K-index of 4 at 04:29 UTC on Oct. 21 and again at 20:59 UTC, followed by a K-index of 5 at 23:59 UTC. That K5 reading is what defines a G1 storm, and the center issued its alert for it at 00:03 UTC on Oct. 22.
A storm at this rung is mild by design. NOAA’s explanation of its space weather scales lists weak power-grid fluctuations, a possible minor effect on satellite operations and aurora commonly visible at high latitudes such as northern Michigan and Maine. It also puts the frequency at about 1,700 G1 events per 11-year solar cycle, or roughly 900 days of them, so a G1 storm is routine.
Extended warnings for a K-index of 4 ran through Oct. 22 and into Oct. 23, and a further K4 alert was posted when that level was reached at 19:53 UTC on Oct. 23. None of those readings rose to the K5 mark of Oct. 21. The Oct. 23 alert for K4 noted that weak power-grid fluctuations and high-latitude aurora were possible during that three-hour window, the kind of modest effect the scale describes for the lower rungs.
The 2 MeV electron alert and its 1,000 pfu threshold
The electron alert is a separate product from the storm alerts. According to the center’s page on GOES electron flux, two integral channels on the GOES satellites count electrons with energies above 0.8 MeV and above 2 MeV, and an electron event alert is issued when the flux in the higher channel exceeds 1,000 particles per square centimeter per second per steradian. The alert text on Oct. 23 attributed its detection to GOES-19.
The reason for watching that channel is spacecraft charging. The page says high-energy electron fluxes are associated with deep-dielectric charging: electrons penetrate satellite components and build up charge, and “a discharge or arching can occur,” which can result in temporary or permanent loss of functionality. The page notes that GOES electron measurements began in 1975 with the first GOES satellite, and that observations from GOES-14 and GOES-15 were discontinued after Jan. 31, 2020. It adds that the 2 MeV channel can be contaminated by energetic protons and its data are not valid during significant proton events.
It is in this sense that the electron levels are described as satellite-damaging. The notice does not say any satellite was harmed in this episode, and nothing in the center’s feed reports an anomaly. The alert marks a condition in which operators may want to watch their spacecraft, not a confirmed loss.
Four alerts, three recorded peaks
The first electron alert was issued at 11:47 UTC on Oct. 23, with the threshold crossed at 11:40 UTC. That was about 36 hours after the K5 reading on Oct. 21. Three continuation alerts followed at 09:04 UTC on Oct. 24, 09:27 UTC on Oct. 25 and 05:17 UTC on Oct. 26, each stating that the flux had remained above 1,000 pfu and giving the prior day’s maximum.
The recorded maxima were 3,169 pfu at 15:40 UTC on Oct. 23, 3,259 pfu at 15:30 UTC on Oct. 24 and 2,864 pfu at 18:20 UTC on Oct. 25. The highest of them, on Oct. 24, was roughly three and a quarter times the alert level, and even the lowest was nearly three times it. The figures are daily peaks reported in the alerts; they do not describe how long the flux sat above threshold within each day.
The forecast through Oct. 28
The center’s forecast discussion, issued at 00:30 UTC on Oct. 26, said the electron flux above 2 MeV “is expected to continue reaching high levels through 28 Oct,” following the 2,864 pfu peak on Oct. 25. The same discussion said the geomagnetic field was likely to remain at mostly quiet levels over Oct. 26 through Oct. 28.
The three-day forecast, issued at 12:30 UTC on Oct. 26, put the greatest expected three-hour Kp for Oct. 26 through Oct. 28 at 2.00, below the level of any NOAA scale, with no G1 or greater storm expected. The two products together describe a quiet field and an elevated electron environment, a pairing that separates this episode from a storm-driven one. Whether the flux fell below the alert line after Oct. 28 will be shown by the center’s later alerts.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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