The May 2024 Gannon storm: a G5 case study
The short version
This is SolarStormAlert’s first post-event storm write-up — a template as much as a report. Every section below (timeline, magnitude, geomagnetic response, what an alert would have said) is a shape a future write-up fills in for the next storm worth documenting this way; this piece fills it with the 2024-05-10 “Gannon” G5 storm, the first G5-level geomagnetic storm since 2003.
What happened
Active region AR13664 produced a sequence of X-class flares from May 8–11, 2024, launching multiple Earth-directed, partially overlapping CMEs. Their combined arrival at the L1 monitoring point on May 10 drove a sustained, strongly southward interplanetary magnetic field and a G5 (Extreme) geomagnetic storm.
Flare timeline
| Date | Class |
|---|---|
| May 8 | X1.0 |
| May 9 | X2.25 |
| May 9 | X1.12 |
| May 10 | X3.98 |
| May 11 | X5.4–5.7 |
The May 10 X3.98 flare (~06:54 UTC) is the one this write-up’s magnitude figures are anchored to. Every flare in this sequence classified R3 (“Strong radio blackout”) under NOAA’s R-scale — none reached the R4 band, which begins at X10.0.
Geomagnetic and solar-wind response
| Metric | Value |
|---|---|
| Kp index (peak) | 9.0 (G5 Extreme) — reached across three separate 3-hour periods, May 10–11 |
| Solar-wind speed | 750–950 km/s sustained, peak ~1000 km/s |
| IMF magnitude (peak) | 73 nT |
| Bz (southward, minimum) | −50 nT |
Kp 9.0 sits at the G5 (Extreme) boundary — the top rung of NOAA’s scale. The step below, G4 (Severe), covers Kp 8; this storm crossed fully into G5 territory and stayed there across multiple observation windows, not a single brief spike.
What an alert would have said
Had SolarStormAlert been live on May 10, 2024, a subscriber at the G5 floor would have received a source-cited email the moment NOAA’s own G-scale call crossed G5, naming the confirmed Kp reading, not a forecast guess. This is the same alert path SolarStormAlert runs today for any G-scale threshold crossing — no separate “major storm” tier, no different rules for a rarer event. Mobile push notifications, including a planned iPhone Critical Alert option for premium subscribers, are not part of that alert path; per the how it works page, push delivery is planned for a future release, not available in 2024 or today.
Sources
- May 2024 solar storms — Wikipedia (secondary aggregator, cross-referenced against NOAA SWPC and peer-reviewed sources)
- The Gannon Storm — citizen science observations of the geomagnetic superstorm of 10 May 2024, Copernicus GC (peer-reviewed)
- Impacts of the Extreme Gannon Geomagnetic Storm — arXiv (peer-reviewed preprint)
- NOAA SWPC event archives and G-scale definitions