Northern lights scale — G scale and Kp index explained

Two scales, one phenomenon

People often encounter both "Kp 5" and "G1 storm" in aurora forecasts and wonder what the difference is. Both describe the same thing: the intensity of geomagnetic disturbance caused by the solar wind interacting with Earth's magnetosphere. The Kp index is the scientific measure; the G scale is NOAA's simplified public communication tool introduced in 1999.

The relationship is direct. G1 = Kp 5. G2 = Kp 6. G3 = Kp 7. G4 = Kp 8. G5 = Kp 9. Below Kp 5 there is no geomagnetic storm by NOAA's definition - that is G0, the background state. The G scale exists because "G1 minor storm" communicates severity better to the public than "Kp 5" does on its own.

For aurora watching, understanding both scales removes any ambiguity when reading forecasts, news reports, or space weather alerts. News headlines tend to use the G scale. Forecast tools and scientific sources use Kp. Knowing that they map directly to each other means neither causes confusion.

The Kp index (0-9)

The Kp index - planetarische Kennziffer, or planetary index - measures how disturbed Earth's magnetic field is on a scale of 0 (quiet) to 9 (extreme storm). It is calculated every three hours from magnetometer readings at 13 observatories positioned between 44-60°N magnetic latitude. Each station measures the deviation of its local field from the quiet-day baseline; these are averaged and converted to the global Kp value.

The scale is logarithmic, not linear. A jump from Kp 5 to Kp 7 is not twice as strong - it represents roughly 10 times the field disturbance. This is why aurora expands so dramatically southward during high-Kp events: the relationship between Kp and auroral oval latitude is non-linear, with disproportionately large expansion at the top of the scale.

The full Kp guide covers how the index is calculated and its limitations as a forecasting tool. The forecast page shows the current live Kp reading updated every 30 minutes.

The G scale (G1-G5)

NOAA's Geomagnetic Storm scale runs from G1 (minor) to G5 (extreme). Each level corresponds to a specific Kp threshold and has a defined set of impacts on aurora visibility, power systems, and communications infrastructure.

Which scale should you use?

For personal aurora watching, Kp is more useful because it is more granular. The difference between Kp 4 and Kp 5 matters enormously for viewers in northern England or southern Scotland - both fall within the broad band around G1, but only Kp 5 actually constitutes a storm and reliably reaches those latitudes. The G scale compresses five separate storm levels into a range that starts at Kp 5, so anyone at 55-60°N benefits from tracking the full 0-9 scale rather than waiting for a G-scale label.

Use the G scale when reading news reports about geomagnetic events or when explaining storm severity to someone unfamiliar with space weather. "G2 storm" means something immediately to a general audience in a way that "Kp 6" does not.

Use Kp when deciding whether to go out tonight. Check your location's Kp threshold on the locations page, compare it to the current and forecast Kp on the forecast page, and make the call.

The Bz context - what drives the scale

Both the G scale and Kp measure effect, not cause. A Kp 7 (G3) storm happens because the Bz component of the solar wind turned strongly negative - typically below -15 to -20 nT - for a sustained period. Negative Bz means the interplanetary magnetic field is oriented southward, matching Earth's polar field lines. This alignment allows energetic particles to flow along those lines down into the upper atmosphere at the poles, producing aurora.

The G/Kp reading tells you what happened to Earth's field over the past three hours. Bz tells you why it happened and, crucially, whether it is continuing. A Kp 6 reading from three hours ago combined with a current Bz of +5 nT means the storm is fading. The same Kp 6 combined with a current Bz of -20 nT and falling means it is intensifying.

The Bz guide explains the mechanism in full and shows which thresholds produce which storm levels.

Real examples

The G5 storm of 10-11 May 2024 is the benchmark event of the current solar cycle. Kp reached 9 - the maximum on the scale - for the first time since the Halloween storms of October 2003. Aurora was photographed from Spain, Mexico, Florida, and New Zealand's North Island. Red aurora, caused by oxygen atoms at 200-300 km altitude energised by the extreme particle flux, appeared across the entire UK including London. The event produced naked-eye aurora in locations that had not seen it in two decades.

A G2 event (Kp 6) in early 2026 produced a more typical strong-storm scenario. Aurora was visible across northern England and all of Scotland on a single clear night, with green bands photographed from the Yorkshire Dales and Peak District. These events - strong enough for naked-eye viewing from dark sites in northern England, too weak to reach the south - are the bread-and-butter of UK aurora watching during solar maximum.

The solar maximum guide explains why storm frequency has been unusually high since 2024 and what to expect through the rest of the cycle.

Current Kp and storm status

The forecast page shows the live Kp reading, current storm status on the G scale, and the OVATION aurora oval map. The 7-day outlook shows upcoming elevated-Kp windows from NASA DONKI CME predictions, letting you plan ahead for the most active nights.

For location-specific thresholds and visibility assessments, each of these pages lists all locations that reach aurora visibility at the corresponding Kp level: