The Kp index is the aurora chaser's number-one tool. But it's often misunderstood - used as a simple "higher = better" scale, without grasping what it actually measures or where it stops. This guide explains what the numbers mean, how they're computed, and how to turn them into concrete decisions based on your position in France.
What Kp measures (and what it doesn't)
The Kp index is a planetary index measuring average geomagnetic field disturbance over a 3-hour window. It's computed from readings of a thirteen-observatory network of magnetic observatories spread across mid-latitudes (Germany, France, USA, Russia, Australia, etc.). Each measures the amplitude of field variations against the expected quiet state for the hour and season. Final Kp is the logarithmic mean of these thirteen, scaled to integer 0-9.
What it measures: global intensity of a geomagnetic storm at planetary scale. The higher, the more shaken the magnetosphere, and the further south the auroral oval descends. What it does not measure: local visibility of an aurora. Two Kp 7 storms will not look the same depending on solar magnetic field orientation (Bz), solar wind density, and the auroral oval's exact position at time T.
From Kp to geomagnetic latitude: the real rule
For an aurora to be visible at your location, the auroral oval must descend to your geomagnetic latitude - which is not the same as your geographic latitude. France is further from magnetic north than a classical map suggests, because that pole sits near the Canadian Arctic, not at the geographic pole.
Approximate geomagnetic latitudes for a few French cities (in 2026, the magnetic pole drifting slowly):
- Calais: ~52° geomag (50.9° geo) - Kp 5 threshold
- Paris: ~49° geomag (48.9° geo) - Kp 6 threshold
- Nantes: ~47° geomag (47.2° geo) - Kp 6-7 threshold
- Lyon: ~46° geomag (45.7° geo) - Kp 7 threshold
- Bordeaux: ~45° geomag (44.8° geo) - Kp 7-8 threshold
- Toulouse: ~44° geomag (43.6° geo) - Kp 8 threshold
- Marseille: ~43° geomag (43.3° geo) - Kp 8-9 threshold
Rule of thumb: aurora visible if Kp ≥ (90 - your geomagnetic latitude) / 5. For Paris: (90 - 49) / 5 ≈ 8.2 - so Kp 6 for low structures, Kp 7+ for clear colours.
Reading a Kp forecast in practice
Pulsar exposes two Kp variants: observed Kp (retrospective, over the past 3 hours) and forecast Kp (3 days ahead, in 3-hour bins). A Kp 7 forecast for the 21:00-00:00 UTC window means the model expects an average Kp of 7 over that window - not necessarily a punctual peak at 7. The instantaneous peak (1-minute measure) can be higher for 15-30 minutes within a 3-hour bin.
For aurora chasing in France, the most useful forecast is the 3 to 6 hours ahead, based on DSCOVR measurements at L1. By that point, solar wind speed, density, and - crucially - Bz polarity are known. When Bz is strongly negative (< -20 nT), magnetic reconnection with the magnetosphere is efficient and observed Kp often comes out higher than the 24-hour forecast.
Classic pitfalls
- Confusing Kp and G. The G scale (G1 to G5) translates that into operational impact on infrastructure (power grids, GPS, satellites). G1 ≈ Kp 5, G5 ≈ Kp 9. For visual observation, Kp is what counts.
- Assuming Kp 9 guarantees aurora everywhere. A Kp 9 storm has a limited time window - if you look at the wrong moment or under cloud, you'll miss it.
- Underestimating the Moon. A full Moon above the horizon cuts 1-2 magnitudes of visual sensitivity. A Kp 6 aurora is markedly harder under full Moon than new Moon.
- Watching a daily average Kp. The useful Kp is the 3-hour window, not the 24 h average.
Pulsar's value-add: Kp contextualized per city
Beyond the raw index, our key contribution is the translation of forecast Kp into per-city observation probability. For every French city, we cross-reference: your actual geomagnetic latitude, average sector light pollution, forecast cloud cover, and lunar phase. The output is a personalized recommendation - "tonight in Lyon, go out if Kp ≥ 7 and northern sky clear". That synthesis is what turns a scientific number into a concrete decision.
Read more
- Understanding the Kp index - full illustrated explainer.
- Pulsar method to observe an aurora - practical protocol.
- Carrington 1859 vs May 2024 - when Kp 9 really matters.
- Tonight's forecast by city - live Kp adapted to your latitude.