K begins with a magnetometer
A local K index describes the maximum disturbance in horizontal magnetic-field components during a three-hour interval compared with a quiet-day reference. It uses a quasi-logarithmic scale from 0 to 9. Conversion thresholds differ between observatories to account for their naturally different magnetic environments.
The planetary Kp index combines standardised K measurements from a network of geomagnetic observatories. Operational centres may also publish estimated or forecast Kp before a final index is available. A chart should therefore label whether a value is observed, estimated or forecast.
What the numbers mean
Low Kp generally describes quieter planetary geomagnetic conditions; larger values describe stronger disturbance. NOAA's G scale begins at Kp 5, but Kp and G are not identical labels for every use. A higher number can support a broader or more active auroral oval, especially during a storm.
The scale is not linear. The physical disturbance represented by successive levels does not increase in equal steps. Reading Kp 8 as ‘twice Kp 4’ is incorrect, and converting Kp to an unqualified percentage adds false precision.
Why Kp is not a local probability
Kp is a global geomagnetic activity index, not an exact local visibility probability. It does not contain your cloud cover, daylight, light pollution, terrain or viewing direction. It also cannot describe every local and short-lived auroral structure.
Latitude matters, but geomagnetic latitude is more relevant than a simple geographic line. During active conditions a bright oval may be visible toward the pole from outside its overhead footprint. Simplified Kp latitude maps are planning aids, not guarantees or hard borders.
Observed, forecast and peak values
A current Kp estimate can help describe the overall state of the magnetosphere. A one-to-three-day Kp forecast is an outlook for activity over broad three-hour periods. A maximum daily Kp hides the lower values and timing within the day, so it should be read with the underlying intervals.
Long-range products are even less suitable for exact local claims. Recurrence patterns and broad geomagnetic outlooks can flag periods worth monitoring, but they cannot responsibly produce a precise future oval or viewing time many days ahead.
A practical reading order
Start with the forecast horizon and whether the Kp is observed, estimated or forecast. Next check the current auroral oval when available. Then look at solar-wind conditions, local darkness and cloud. Finally read the confidence and source timestamps.
GreenHalos uses Kp as one ingredient in an activity category. It displays local sky factors separately and does not present its Viewing Potential as an official probability or observed success rate.
Evidence register
Primary sources
Sources were checked on . Each record identifies which sections it supports.
- The K-index NOAA Space Weather Prediction Center Supports: K begins with a magnetometer; What the numbers mean
- Planetary K-index products NOAA Space Weather Prediction Center Supports: Observed, forecast and peak values
- Aurora NOAA Space Weather Prediction Center Supports: Why Kp is not a local probability