The NOAA G-scale: G1 to G5
NOAA rates geomagnetic storms on a five-step scale from G1 (minor) to G5 (extreme), keyed to the Kp index. Higher storms push the aurora further from the poles — here's what each level realistically means for seeing it.
| Level | Kp | Name | What it means for viewing |
|---|---|---|---|
| G1 | Kp 5 | Minor | Aurora possible low on the northern horizon from the northern-tier US and similar latitudes. |
| G2 | Kp 6 | Moderate | Aurora fills more of the northern sky at those latitudes and edges a little further south. |
| G3 | Kp 7 | Strong | Widespread lower-latitude visibility — 'drop everything' territory for the northern US and mid-latitudes. |
| G4 | Kp 8 | Severe | Aurora reaches well into the mid-latitudes; overhead displays far south of usual. |
| G5 | Kp 9 | Extreme | The rare, historic event — aurora reported near 40° latitude and, in exceptional storms, lower. |
One honest caveat baked into every level: overhead visibility needs a substantially higher Kp than seeing a glow low on the northern horizon at the same place. Aurorix always shows you both thresholds for your location.
Fractional Kp and the G-scale
You'll see values like "Kp 5-" (4.67) in forecasts. The G-scale is keyed to Kp reaching a whole number: G1 begins at Kp 5 exactly, so "5-" is still below storm level. It's a small thing that trips up a lot of headlines.