Reference

Kp Index

What each planetary K index value from 0 to 9 means, with NOAA G-scale storm levels and an estimate of how far equatorward the aurora may reach for a given Kp.

The Kp index summarizes global geomagnetic activity for each three-hour interval on a quasi-logarithmic scale from 0 (very quiet) to 9 (extreme storm), reported in thirds such as 4+ and 5-. This page explains the scale and estimates aurora visibility for a Kp value you enter. It does not report current conditions: get the live value from the NOAA or GFZ links below, then look it up here.

Estimate aurora visibility for a Kp value

Choose a Kp value, and optionally tell the tool where you are. It converts a geographic position to an approximate geomagnetic latitude with a simple centered-dipole formula and compares it with the typical equatorward edge of the auroral oval at that Kp. The estimate is rough: real aurora also depends on the interplanetary magnetic field, substorm timing, darkness, clouds, and light pollution.

The estimator needs JavaScript. The full Kp reference table and explanations below work without it.

Kp levels at a glance

The equatorward boundary column gives the approximate geomagnetic latitude down to which aurora can appear overhead at local midnight, following NOAA SWPC's rule of thumb: about 66 degrees at Kp 0, moving equatorward roughly 2 degrees per Kp step. Aurora low on the poleward horizon can often be seen several degrees equatorward of that line. Storm frequencies are NOAA's averages per 11-year solar cycle.

Planetary K index reference, with NOAA G-scale mapping
Kp NOAA scale Activity level Typical frequency Overhead aurora down to What to expect
0 none Very quiet Common throughout the cycle about 66° A thin, often faint auroral oval sits at very high geomagnetic latitudes. Aurora only for far-northern or far-southern sites such as Svalbard or central Alaska.
1 none Quiet Common throughout the cycle about 64° Quiet arcs over the usual auroral zone: northern Scandinavia, Iceland, central Alaska, northern Canada, Antarctica.
2 none Quiet to unsettled Common throughout the cycle about 62° The oval brightens and drifts slightly equatorward. Good displays inside the auroral zone; nothing unusual farther south.
3 none Unsettled Common, especially near solar maximum about 60° Active arcs and bands for the auroral zone; horizon glow possible from places a few degrees farther equatorward, such as central Scandinavia.
4 none Active Regular during active periods about 58° Just below storm level. Bright, moving displays at high latitudes; aurora can appear low on the horizon from around 53 to 58 degrees geomagnetic.
5 G1 Minor storm About 900 days per cycle about 56° Weak power-grid fluctuations possible at high latitudes; minor effects on satellite operations. Aurora commonly reaches the northern United States, Scotland, and southern Scandinavia.
6 G2 Moderate storm About 360 days per cycle about 54° High-latitude grids may see voltage alarms; orbital drag increases on satellites. Aurora can push toward roughly 54 degrees geomagnetic, with horizon views well beyond.
7 G3 Strong storm About 130 days per cycle about 52° Voltage corrections may be required; surface charging on satellites; intermittent satellite navigation problems. Aurora possible on the horizon from around 47 to 52 degrees geomagnetic.
8 G4 Severe storm About 60 days per cycle about 50° Widespread voltage-control problems possible; satellite tracking and orientation issues. Aurora deep into the mid-latitudes; overhead displays for much of northern Europe and the northern half of the United States.
9 G5 Extreme storm About 4 days per cycle about 48° Grid collapse and transformer damage possible in the worst cases; extensive satellite charging. Aurora overhead to about 48 degrees geomagnetic and visible on the horizon far beyond, occasionally into the subtropics.

How Kp is written: the thirds notation

Kp is not a continuous measurement. Each three-hour interval receives one of 28 discrete values: 0o, 0+, 1-, 1o, 1+, 2-, and so on up to 9-, 9o. The letter o marks the whole value, while minus and plus mean one third of a unit below or above it. So 4+ equals 4.33 and 5- equals 4.67, and those two values are one third of a unit apart even though the leading digits differ. There is no 0- and no 9+. Data services often publish the same values as decimals (0.00, 0.33, 0.67, 1.00, and so on).

The scale is quasi-logarithmic: each step upward represents a much larger geomagnetic disturbance than the one below, so Kp 8 is far more than twice as disturbed as Kp 4.

Where to get the live Kp

This page deliberately shows no live data. Two authoritative, free sources publish current values:

About the Kp index and this estimator

The K index was introduced by Julius Bartels and colleagues in 1939 (Bartels, Heck, and Johnston, "The three-hour-range index measuring geomagnetic activity", Terrestrial Magnetism and Atmospheric Electricity, volume 44) as a 0 to 9 quasi-logarithmic measure of the irregular disturbance of the local geomagnetic field over three hours. Bartels derived the planetary version, Kp, in 1949 by standardizing and averaging the K indices of a network of mid-latitude observatories. Today the official Kp is produced by GFZ in Potsdam, Germany from 13 designated geomagnetic observatories, with the modern derivation described by Matzka and colleagues (Space Weather, 2021). NOAA's Space Weather Prediction Center computes a near-real-time estimated Kp from its own station network and maps Kp 5 through 9 to its G1 through G5 geomagnetic storm scale.

The aurora estimate uses two published approximations. First, the equatorward edge of the auroral oval at local midnight sits near 66 degrees geomagnetic latitude at Kp 0 and moves equatorward about 2 degrees per Kp step, reaching roughly 48 degrees at Kp 9; this rule of thumb comes from NOAA SWPC's aurora viewing guidance, and this tool interpolates it linearly for third values. Overhead aurora is reported when your geomagnetic latitude lies within about 10 degrees poleward of that edge, and a possible horizon sighting when you are within about 5 degrees equatorward of it, since auroral rays 100 to 300 kilometers up can be seen from several hundred kilometers away on a clear, dark horizon.

Second, geographic coordinates are converted to geomagnetic latitude with a centered-dipole approximation using a northern geomagnetic pole at about 80.7° N, 72.7° W (the IGRF-13 dipole for the 2020 epoch; the pole drifts slowly, so this is stated as approximate). The formula is sin(mlat) = sin(lat) sin(lat_p) + cos(lat) cos(lat_p) cos(lon - lon_p). This simple model can differ by several degrees from the corrected geomagnetic coordinates that professional aurora products use, and the mismatch is largest over Europe, eastern Asia, and the South Atlantic; over western Europe it tends to run a few degrees optimistic. Treat every result here as a planning-grade estimate, not a forecast.

Be honest with your expectations: Kp is a three-hour, globally averaged, quasi-logarithmic index. A high Kp does not guarantee aurora at your site, and spectacular local substorms can occur at modest Kp. Visibility also requires darkness, clear skies, and a low light-pollution horizon toward your geomagnetic pole, and the southward interplanetary magnetic field component (Bz) largely decides how much solar-wind energy actually couples into the magnetosphere. Nothing you enter on this page is stored or transmitted, and no value shown here is an official record or suitable for operational or safety-critical use.