Understanding the Kp index and G-scale
The short version
Every geomagnetic storm this dashboard reports on is expressed two ways: a Kp index value (0-9, the raw measurement) and a G-scale rating (G1-G5, NOAA’s plain-language severity band derived from it). This glossary defines both, term by term, so a number on the chart always maps to something concrete.
Kp index
The planetary K-index (Kp) is a 0-9 scale describing how disturbed Earth’s magnetic field is over a rolling window, derived from ground-based magnetometer stations distributed around the globe. NOAA SWPC publishes an estimated Kp value on a continuous cadence, plus a 3-day forecast.
- Kp 0-4 — quiet to unsettled. Background geomagnetic activity; no G-scale storm level applies.
- Kp 5 — minor storm, G1.
- Kp 6 — moderate storm, G2.
- Kp 7 — strong storm, G3.
- Kp 8 — severe storm, G4.
- Kp 9 — extreme storm, G5.
Kp is an index, not a physical unit — it is a standardized composite of magnetometer deflections, not a direct measurement of a single physical quantity like wind speed or magnetic field strength.
G-scale
NOAA’s G-scale (Geomagnetic Storm Scale) translates Kp into five plain-language severity levels, each with documented, source-cited real-world effects:
- G1 (Minor) — weak power-grid fluctuations possible; aurora visible at high latitudes.
- G2 (Moderate) — high-latitude power systems may experience voltage alarms; aurora visible further from the poles than usual.
- G3 (Strong) — voltage corrections may be required in some grids; aurora sometimes visible at mid-latitudes.
- G4 (Severe) — widespread voltage control problems possible; protective systems may mistakenly trip parts of the grid; aurora visible well into mid-latitudes.
- G5 (Extreme) — grid systems may experience voltage control problems and protective-system trips; some grids may experience complete collapse or blackouts; aurora visible at unusually low latitudes.
This dashboard surfaces the current and forecast Kp/G-scale reading directly from NOAA SWPC data — the same source these definitions are drawn from.
Related terms used elsewhere on this dashboard
- Solar wind — the continuous stream of charged particles from the Sun; its speed and density (measured near the L1 point by ACE and DSCOVR/IMAP) are leading indicators for geomagnetic activity.
- Bz — the north-south component of the interplanetary magnetic field carried by the solar wind. A sustained negative (southward) Bz is the condition most associated with strong geomagnetic coupling.
- X-ray flux / R-scale — GOES-observed solar X-ray flux, classified A through X and mapped to NOAA’s R-scale (Radio Blackout) — a separate scale from G, tracking a different physical effect (ionospheric radio disruption, not geomagnetic disturbance).
- CME (Coronal Mass Ejection) — a large eruption of solar material; NASA DONKI tracks detected CMEs and their forecast Earth-arrival windows, shown separately on this dashboard.