GEOMAGNETIC ACTIVITY · 2 min read

Kp index: what the numbers mean

Understand the 0–9 scale, geomagnetic storms and why your location matters.

One index for the whole planet

Kp summarises disturbances in Earth’s magnetic field from ground-based observatories over three-hour intervals. The scale runs from 0 to 9. NOAA’s estimated Kp can update during an interval, but it is not a minute-by-minute reading of brightness above your location.

Reading the scale

Kp 0–2 indicates relatively quiet conditions; 3–4 indicates more disturbance, below geomagnetic storm level. Kp 5 marks the start of NOAA’s G1 geomagnetic storm category, with higher values corresponding to stronger storm levels. These categories describe magnetic disturbance, not a percentage chance of seeing the northern lights.

Your magnetic latitude matters

The northern lights commonly form in a band around the magnetic pole. Stronger activity can expand this band towards lower magnetic latitudes. People closer to the band may see a display at low Kp. Geographic latitude alone, or a single universal Kp threshold, cannot tell everyone when to look.

Observed and forecast Kp are different

The dashboard shows a recent estimate from measurements. The calendar uses predicted Kp from NOAA’s three-day outlook. A forecast value may change as new information arrives. Check its time period, then compare the local clouds and darkness for those same hours.

What to look for

Use Kp as a broad activity indicator. Low Kp can still bring northern lights at high latitudes; high Kp does not remove clouds or daylight.