In brief
The Kp index is a global measure of geomagnetic activity, published every 3 hours by GFZ Potsdam (Germany). It ranges from 0 (calm sky) to 9 (severe storm). For auroras to be visible from central France, typically Kp ≥ 7 is required.
Definition
The Kp index (from Kennziffer planetarisch, German for 'planetary index') is a synthetic measure of Earth's magnetic field variability over a three-hour window. It is computed by aggregating variations observed by a network of thirteen magnetometers distributed across both hemispheres at 44° to 60° geomagnetic latitude. The scale runs from 0 to 9 in thirds (0, 0+, 1-, 1, 1+, 2-, ... 9).
The index is published by the GFZ Potsdam (Helmholtz Centre, Germany), the international reference. NOAA broadcasts a derived version, the estimated Kp, updated in quasi-real-time from a subset of stations.
How to read the scale
Each Kp level matches a NOAA geomagnetic storm class (G0-G5) and a geomagnetic latitude limit where auroras become visible.
| Kp | NOAA class | Aurora visible down to | Days per cycle |
|---|---|---|---|
| 0-3 | G0 (calm) | Northern Scandinavia | daily |
| 4 | G0 (unsettled) | Northern Scotland | ~100 |
| 5 | G1 (minor) | Southern Scotland, Denmark | ~80 |
| 6 | G2 (moderate) | Northern France, Belgium | ~20 |
| 7 | G3 (strong) | Central France | ~4 |
| 8 | G4 (severe) | Southern France | ~1 |
| 9 | G5 (extreme) | North Africa | ~0.1 |
Frequencies are for a full 11-year solar cycle. During solar maximum (currently, 2024-2026), these numbers roughly double.
Why 3-hour windows?
Geomagnetic activity has an inertia of a few hours. On too short a window, we would capture non-significant fluctuations; on too long, we would smooth over important events. Three hours was chosen historically (Bartels, 1949) and has proven robust. Eight Kp values are produced per day, at fixed UTC times (00h, 03h, 06h, ..., 21h).
What Kp do I need to see auroras from France?
The rule of thumb used by most forecasters:
- North of a Brest-Strasbourg line: aurora sometimes visible from Kp 6.
- Central France (Loire, Burgundy, Massif Central): Kp 7 minimum.
- Southern France (Mediterranean, Pyrenees, southwest): Kp 8 or more.
Two practical conditions matter too: a clear northern sky, and low light pollution. A faint Kp 6 aurora in a large city remains invisible for lack of contrast. From rural or Bortle 3-4 sites, the same aurora becomes perfectly observable.
On each city page, Pulsar computes the Kp threshold specific to your position. See for example aurora in Paris, Lyon or Toulouse.
Limits of the Kp index
Kp is useful but imperfect. Its limits:
- Coarse time resolution (3h): an intense aurora can appear and fade within the hour, drowned in the Kp average.
- Global measure: the index says nothing about north/south asymmetry, nor the exact position of the oval.
- No Bz: identical Kp values can produce very different auroras depending on interplanetary magnetic field Bz orientation.
To complement, forecasters use other indices: Dst (for storm main phase), AE (for substorms, 1-minute resolution), and hemispheric power (GW) derived from the NOAA Ovation model. Pulsar shows all three live.