Forecast › Kp index
The Kp index explained
The Kp index measures how disturbed Earth’s magnetic field is, on a scale from 0 to 9. The higher the Kp, the further from the poles the aurora reaches. Kp 5 or more counts as a geomagnetic storm. Near the Arctic Circle a low Kp can be enough; in the northern US you usually need a storm.
What Kp measures
The K-index describes disturbances in the horizontal part of Earth’s magnetic field, from the largest swings a magnetometer records in a three-hour period. The planetary index, Kp, is the average of standardised K-indices from 13 observatories between 44° and 60° geomagnetic latitude. It runs from 0 to 9, where 1 is calm and 5 or more means a geomagnetic storm. The scale was introduced by Julius Bartels in 1938, and the letter K comes from the German word Kennziffer, “characteristic digit” (NOAA SWPC, Planetary K-index).
NOAA publishes a forecast of Kp for each three-hour block over the next three days. Our forecast uses that to find the highest Kp expected while it is dark at your place.
What each Kp level means for the aurora
From NOAA’s viewing tips: at Kp 0 to 2 the aurora stays far north and is dim; at Kp 3 to 5 it moves further from the poles and gets brighter and more active; at Kp 6 to 7 it may be visible from the northern edge of the United States; at Kp 8 to 9 it can be seen overhead from the northern US states (NOAA SWPC tips).
NOAA geomagnetic storm scale
| NOAA scale | Kp | Aurora, according to NOAA | How often (11-year cycle) |
|---|---|---|---|
| G1 Minor | Kp 5 | commonly visible at high latitudes (northern Michigan and Maine) | 1,700 per cycle (900 days per cycle) |
| G2 Moderate | Kp 6 | seen as low as New York and Idaho (typically 55° geomagnetic latitude) | 600 per cycle (360 days per cycle) |
| G3 Strong | Kp 7 | seen as low as Illinois and Oregon (typically 50° geomagnetic latitude) | 200 per cycle (130 days per cycle) |
| G4 Severe | Kp 8, including 9- | seen as low as Alabama and northern California (typically 45° geomagnetic latitude) | 100 per cycle (60 days per cycle) |
| G5 Extreme | Kp 9 | seen as low as Florida and southern Texas (typically 40° geomagnetic latitude) | 4 per cycle (4 days per cycle) |
Source: NOAA Space Weather Scales, checked October 20, 2026. “Per cycle” means per 11-year solar cycle.
Kp needed in different places
NOAA’s rule of thumb is that the equatorward edge of the auroral oval is near 66° geomagnetic latitude at Kp 0 and moves about 2° towards the equator for each Kp step, down to 48° at Kp 9 (NOAA SWPC). Using the geomagnetic pole position from the World Magnetic Model 2025 (80.85°N, 72.76°W, NOAA NCEI), that gives these approximate values:
| Place | Geomagnetic latitude | Kp for aurora overhead (approx.) |
|---|---|---|
| Fairbanks, Alaska | 65.7° | 0.2 |
| Anchorage, Alaska | 61.9° | 2.0 |
| Yellowknife, Canada | 68.5° | 0 (under the usual oval) |
| Reykjavík, Iceland | 68.8° | 0 (under the usual oval) |
| Tromsø, Norway | 67.5° | 0 (under the usual oval) |
| Kiruna, Sweden | 65.7° | 0.2 |
| Rovaniemi, Finland | 63.6° | 1.2 |
| Stockholm, Sweden | 58.0° | 4.0 |
| Edinburgh, UK | 58.1° | 3.9 |
| Minneapolis, USA | 53.4° | 6.3 |
| Seattle, USA | 53.0° | 6.5 |
| Chicago, USA | 50.7° | 7.7 |
| New York, USA | 49.9° | 8.1 |
| Berlin, Germany | 52.2° | 6.9 |
These are averages, and NOAA notes there are times when the relationship does not hold. Aurora can also be seen low on the horizon from hundreds of kilometres away, so you can often see it with a lower Kp than the table shows.
Aurora guides
Questions and answers
What Kp is good for the northern lights?
There is no single number. Under the auroral oval, for example in Fairbanks or Tromsø, aurora can be seen even at Kp 3 or 4, according to NOAA. In the northern US you usually need Kp 6 or more.
Is Kp 5 a storm?
Yes. NOAA calls Kp 5 a G1 (minor) geomagnetic storm, Kp 6 G2, Kp 7 G3, Kp 8 G4 and Kp 9 G5 (NOAA).
How far ahead can Kp be forecast?
NOAA publishes a three-day Kp forecast in three-hour blocks. Forecasts more than a day or so ahead are less certain, so check again on the evening itself.