What is Kp?
The Kp index summarises how disturbed Earth's magnetic field is on a scale from 0 to 9. It is computed for three-hour blocks from magnetic observatories around the world: 0 is quiet, 9 is an extreme storm. Values also come in thirds (5-, 5, 5+). Definitive values are published by GFZ Potsdam; NOAA SWPC provides real-time estimates and a three-day forecast.
Kp is quasi-logarithmic: Kp 7 is not slightly more than Kp 6, it is a clearly stronger storm.
NOAA's G scale
NOAA classifies Kp 5 and above as geomagnetic storms (NOAA space weather scales):
| Level | Kp | Name | Approx. occurrences per solar cycle (~11 yr) | Aurora typically seen down to (geomagnetic) |
|---|---|---|---|---|
| G1 | 5 | Minor | 1700 | ~60° |
| G2 | 6 | Moderate | 600 | ~55° |
| G3 | 7 | Strong | 200 | ~50° |
| G4 | 8 | Severe | 100 | ~45° |
| G5 | 9 | Extreme | 4 | ~40° |
The latitudes are NOAA's "aurora has been seen as low as" figures: a best-case horizon view.
How far south? SWPC's rule of thumb
From SWPC's viewing tips:
- At Kp 0 the equatorward edge of the oval sits near 66° geomagnetic latitude.
- It moves about 2° equatorward per Kp step.
- At Kp 9 it reaches about 48°.
| Kp | Oval edge (geomagnetic) |
|---|---|
| 0 | ~66° |
| 3 | ~60° |
| 5 | ~56° |
| 7 | ~52° |
| 9 | ~48° |
That is where aurora may be overhead. With a clear view toward the pole, for example from a hill, you can see aurora low on the horizon even when it is about 1,000 km further poleward, which is why the G-scale latitudes are lower. SWPC calls these "approximate averages" that don't always hold.
The rule uses geomagnetic, not geographic, latitude. Because the magnetic pole is offset towards Canada, European cities sit several degrees lower in geomagnetic terms. Istanbul is at 41°N geographically but about 38.6° geomagnetic.
Why Kp alone is not enough
You can miss the aurora at Kp 7 and see a sky full of green at Kp 2 in Tromsø, because other factors decide visibility:
- Clouds: low cloud blocks the aurora completely. This is the most common deal-breaker.
- Darkness: high-latitude summer nights never get dark; twilight swallows faint aurora.
- Moon: a bright moon washes out faint aurora (the aurora itself is not weaker).
- Light pollution: city lights erase faint displays.
- Timing: Kp is a three-hour average. The real show often arrives in 20–40-minute bursts (substorms).
- Forecast bias: SWPC's 0–24 h Kp forecasts have been reported to run high at Kp 3 and above.
That is why our 0–100 score combines activity with clouds, darkness, moon and light pollution. See how the score works.
Bz and solar wind basics
Real-time data come from spacecraft at the L1 point, about 1.5 million km sunward. NOAA's SOLAR-1 has supplied operational data since 10 June 2026 (NOAA). Solar wind measured at L1 reaches Earth in roughly 30–90 minutes, your short-notice warning.
- Bz: the north-south component of the solar wind's magnetic field. A negative (southward) Bz that stays negative lets energy in. Brief flips mean little; tens of minutes of clearly negative Bz matter.
- Speed: quiet solar wind is typically around 300–400 km/s. Fast streams and CMEs push it much higher, and faster wind drives stronger activity.
- Density: a sudden jump in density and speed usually marks a CME shock arriving.
In practice: when Bz turns south and stays there, and you are somewhere dark and clear, go outside.