Aurora
Kp index and northern lights: what the number tells you
Kp describes global geomagnetic disturbance on a scale from 0 to 9. It helps put aurora activity in context, but it does not tell you the probability of seeing northern lights from your exact location.
Read the number with its time label
NOAA’s estimated planetary Kp uses ground-based magnetic measurements and describes three-hour intervals. An observed estimate describes activity that has happened; a forecast describes what is expected. They answer different questions even when the number is the same.
Before comparing two charts, check the interval, whether the value is observed or forecast, and the time zone. A headline about the highest expected value for a whole day is not a timetable for your local evening.
How NOAA’s storm scale relates to Kp
| Kp level | NOAA category |
|---|---|
| Below 5 | Below G1 storm threshold |
| 5 | G1, minor |
| 6 | G2, moderate |
| 7 | G3, strong |
| 8 and 9− | G4, severe |
| 9o | G5, extreme |
These are space-weather severity categories. Do not read “below storm threshold” as “no aurora,” or a severe category as a promise of a visible display.
Why there is no universal local threshold
NOAA’s relationship between Kp and auroral latitude is approximate and uses geomagnetic latitude, which differs from geographic latitude. Local darkness, clouds, and the horizon are separate questions. Two people looking at the same Kp number can have very different observing conditions.
Instead of saving a rule such as “leave whenever Kp reaches a certain number,” save a viewing routine. Check the current map, the relevant time, and your local sky together. This also makes it easier to understand an unsuccessful outing without assuming that the forecast was a guarantee.
A useful way to compare forecast products
- Write down the period you can actually go outside.
- Check whether the Kp forecast refers to that period or a different part of the day.
- Look at a current aurora map to see the modeled geographic pattern.
- Check local cloud and darkness separately.
- Make a nearby, flexible plan and reassess before departing.
For example, if an elevated daily maximum is forecast while it is daylight at your destination, it may not support the evening plan you had in mind. Keep the time labels attached to the numbers when discussing a forecast with someone else.
Kp and the Next Light map
The Next Light website shows the short-term OVATION aurora model, not a dedicated live Kp chart. A colored area on the globe is modeled activity, not your individual viewing odds. Use NOAA’s Kp product for the index itself and our viewing guide for the local checklist.