How to read an aurora forecast: Kp, Bz and solar wind explained

What the Kp index really means, how far south each level reaches, why the solar wind's Bz matters more than Kp on the night, and a simple checklist for deciding whether to go out.

Green curtains low in the north, with red higher up: what a camera often catches before the eye does.

Where the aurora comes from

The Sun blows a constant stream of charged particles, the solar wind. When it carries a strong magnetic field pointing the right way, it couples into Earth's magnetic field and pours energy into a ring around each magnetic pole, the auroral oval. Particles rain down along the field lines and make the upper atmosphere glow, 100 to 300 km up.

Green, the most common colour, comes from oxygen around 100–250 km. Red comes from oxygen higher still, and is what people at lower latitudes usually see during big storms, low on the horizon. Purple and pink edges come from nitrogen.

The Kp index: how disturbed the field is

Kp is a number from 0 to 9 that measures how disturbed Earth's magnetic field was over a three-hour period, from a network of magnetometers. The higher it is, the wider the auroral oval spreads toward the equator. NOAA's storm scale starts at Kp 5: G1 is Kp 5, G2 is 6, G3 is 7, G4 is 8 and G5 is 9.

As a rough guide for where the aurora can be seen overhead or on the northern horizon (in geomagnetic latitude, which differs from ordinary latitude): around Kp 3, northern Scandinavia, Iceland and Alaska; Kp 5, Scotland, southern Scandinavia and the northern US border; Kp 7, much of northern Europe and the northern half of the US; Kp 9, far lower, as in May 2024 when aurora was photographed from places like southern Europe and Mexico.

On the night, watch Bz and the solar wind

Kp is calculated after the fact, every three hours. For what is about to happen, watch the solar wind measured by spacecraft about 1.5 million km upstream, which gives roughly 15–60 minutes of warning.

The key number is Bz, the north–south direction of the solar wind's magnetic field. When Bz is negative (southward) for a sustained time, energy flows in and the aurora brightens; strongly negative values, below about −10 nT, are what big displays are made of. Speed and density help too: a fast (over 500 km/s), dense wind with a southward Bz is the best combination.

Forecasts days ahead: CMEs and coronal holes

Big storms usually come from coronal mass ejections (CMEs): clouds of solar material that reach Earth one to three days after they leave the Sun. Forecasters announce an expected arrival, but the timing can be off by many hours and the strength is uncertain until the cloud reaches the spacecraft upstream.

Fast streams from coronal holes bring smaller, more regular activity, and because the Sun rotates every 27 days or so, they often return on a 27-day rhythm. Activity also tends to be higher around the equinoxes in March and September.

A checklist for going out

Forecast Kp at or above what your latitude needs, ideally with a CME expected.
Bz negative for the last 30 minutes or more, and a clear, dark sky to the north (south in the Southern Hemisphere).
Go somewhere with a low, open view of the poleward horizon, away from town lights. The Moon is less of a problem than for faint stars, but darkness still helps.
Be patient. Aurora comes in bursts (substorms), often around local midnight, and can go quiet for an hour before flaring up.
Use your phone's night mode or a camera with a few seconds' exposure: sensors see colour in faint aurora that eyes see as grey.