Guide
Reading an aurora forecast
Three things decide whether the northern or southern lights show: how fast the solar wind is, which way its magnetic field points, and how dark and clear your sky is. Here is how to read the numbers, using NOAA's own wording.
What makes an aurora brighter
NOAA's Space Weather Prediction Center puts it this way: as the solar wind increases in speed and the interplanetary magnetic field embedded in it turns southward, geomagnetic activity increases, and the aurora becomes brighter, more active, and moves further from the poles. When the Kp index is high, between 7 and 9, NOAA says the aurora will be bright and the auroral oval will move to lower latitudes.
So there are two live numbers to watch. Solar wind speed is in kilometres per second. The north-south part of the magnetic field is called Bz: a negative value means the field points south, which is the case NOAA describes. Both are measured by spacecraft, which is why they give a short warning: NOAA's page gives 15 to 45 minutes as the lead time of forecasts based on satellite data.
The G1 to G5 scale
NOAA rates geomagnetic storms from G1 (minor) to G5 (extreme) from the Kp index. The table is NOAA's. The wording about where aurora has been seen is NOAA's, not ours, and "has been seen" describes past storms, not a promise for a given night.
| Level | Name | Kp | Where aurora has been seen, as NOAA words it | Typical geomagnetic latitude |
|---|---|---|---|---|
| G1 | Minor | 5 | Aurora is commonly visible at high latitudes (northern Michigan and Maine). | not given |
| G2 | Moderate | 6 | Aurora has been seen as low as New York and Idaho. | 55° |
| G3 | Strong | 7 | Aurora has been seen as low as Illinois and Oregon. | 50° |
| G4 | Severe | 8 (including 9-) | Aurora has been seen as low as Alabama and northern California. | 45° |
| G5 | Extreme | 9 | Aurora has been seen as low as Florida and southern Texas. | 40° |
Geomagnetic latitude is measured from the magnetic pole, not the geographic pole, so it is not the same as a place's ordinary latitude.
Using this on the night
- A high Kp or a G level tells you the oval has moved toward the equator. It does not know about your cloud or your light pollution.
- Look for a stretch where Bz stays negative and the wind is fast. That is when the aurora brightens, by NOAA's description.
- The app shows the current Kp, the last hours of wind speed and Bz, and NOAA's active alerts for your place, each with its source and time.
The app reads NOAA's own feeds and does not predict aurora itself. One thing we could not confirm: which magnetic-field coordinate system NOAA's tutorial means by southward. We use the GSM Bz value and say only that a negative number points south.
Sources
- NOAA Space Weather Scales. The G1 to G5 table
- NOAA SWPC aurora tutorial. How the solar wind and the Kp index relate to aurora
- NOAA Space Weather Prediction Center data feeds. Solar wind, magnetic field and alert files
- NWS disclaimer and public domain statement. The terms for using NOAA and NWS information