Aurora forecast: Kp now and the next 3 days

How to read an aurora forecast

An aurora forecast is really a space weather forecast: it tells you how hard the solar wind is pushing on Earth's magnetic field, and how far toward the equator the ring of aurora will spread. Five numbers do most of the work.

Kp index: the headline number

The planetary K-index runs from 0 to 9 and describes how disturbed Earth's magnetic field is over a 3-hour block. It comes from a network of magnetometers at mid-latitudes, and each step up the scale means roughly twice the disturbance. NOAA publishes estimated Kp every minute and a forecast for each 3-hour block of the next three days, which is the bar chart above.

Kp matters for aurora hunters because it predicts how far the auroral oval moves. On a quiet night (Kp 0 to 1) its edge sits near 65° magnetic latitude, over northern Scandinavia, Iceland, northern Canada and Alaska. Each step of Kp pushes the edge about 2° (220 km) toward the equator. At Kp 5 it reaches about 56°, at Kp 7 about 52°, and at Kp 9 below 48°.

The G-scale: when it becomes a storm

NOAA labels geomagnetic storms from G1 to G5. They map straight onto Kp:

Bz: the switch

The solar wind carries its own magnetic field. Its north-south part, Bz, decides whether energy gets in. When Bz points south (negative), it connects with Earth's northward field at the front of the magnetosphere and the solar wind can drive the field like a generator. When Bz points north, most of that energy slides past. A fast stream with Bz near zero can fizzle; a modest one with Bz steady at -10 nT for hours can give a fine show. Watch the Bz trace above: long stretches in the shaded area are what you want.

Solar wind speed and density

The ordinary solar wind blows at 300 to 450 km/s with 2 to 8 protons per cubic centimetre. Fast streams from coronal holes run at 500 to 800 km/s, and the shock at the front of a coronal mass ejection (CME) can arrive faster still, with a jump in density and field strength. Speed sets how fast energy is delivered; density adds the push. A sudden jump in all three together is the sign that a CME has arrived.

Hemispheric power and the OVATION map

NOAA's OVATION model turns the live solar wind into a map of where aurora is likely in the next 30 to 90 minutes, and into a total, the hemispheric power, in gigawatts. Below about 20 GW the aurora is faint and far north. Above 50 GW there is a good display somewhere in the oval, and during big storms it climbs past 100 GW. The map shows the chance of aurora overhead; you can usually see it on your horizon from several hundred kilometres outside the coloured band.

The 27-day outlook

The Sun turns once every 27 days as seen from Earth, so a coronal hole that sent a fast stream past us today may well do it again in 27 days. NOAA's 27-day outlook is built largely on that repeat. It is good at flagging likely active days from recurring streams and blind to new eruptions, so use it to plan a trip and the 3-day forecast to decide on a night.

The aurora in the sky: where to look and why

The auroral oval and magnetic latitude

The aurora is not a curtain over the North Pole. It forms a ring, the auroral oval, around each magnetic pole, where electrons from Earth's magnetic tail rain down along field lines into the upper atmosphere. The magnetic pole is not the geographic one: the axis of Earth's best-fitting magnetic dipole meets the surface at 80.9°N, 72.8°W in 2026 (IGRF-14), in the Canadian Arctic off northwest Greenland, and the real field is lumpier still. That is why North America sees the aurora far more often than Europe or Asia at the same latitude: New York (40.7°N) sits at about 49° magnetic latitude, while Rome (41.9°N) is near 36°.

Why it looks low in the north from mid-latitudes

The lower edge of the aurora is about 100 km up and the red tops reach 300 to 400 km. When the oval sits a few hundred kilometres to your north, you see it end-on across that distance, so it appears as a glow or arc low over the northern horizon (southern horizon in Australia, New Zealand and South America). From 500 km away the lower edge is only about 9° up, roughly a fist at arm's length, and only the red tops rise higher. As a storm grows, the oval moves toward you and the display climbs the sky. When the oval is overhead you see rays converging toward a point near the zenith, the corona. Find a spot with a clear, dark view of the northern horizon; a hill, a lake shore or a beach facing north is ideal.

The colours: green, red, purple and blue

Each colour is the fingerprint of a particular atom or molecule. The common green is atomic oxygen emitting at 557.7 nm, between about 100 and 250 km up. Higher, where the air is so thin that excited oxygen has time to glow before bumping into anything, it emits deep red at 630.0 nm; this is the red seen in great storms and the colour that reaches far south, because the tops of the aurora are visible from furthest away. Molecular nitrogen adds blue (427.8 nm) and a purple to pink fringe at the lower edge, usually when fast electrons dig deep during an active spell. Your eye sees faint aurora as grey-white because night vision has little colour sensitivity; a phone camera on a few seconds of exposure shows the colours plainly.

Substorms and magnetic midnight

Aurora tends to come in bursts. Energy builds in Earth's magnetic tail for an hour or two, then is released in a substorm: a quiet arc suddenly brightens, splits into moving rays and spreads across the sky for 10 to 30 minutes before fading to patchy, pulsing glows. Substorms are most likely around magnetic midnight, when your location faces straight down the magnetic tail. That is roughly 11:45 PM in New York on standard time (12:45 AM in summer), about 11 PM GMT in Britain and about 10 PM CET in northern Norway. The two or three hours around it are the best bet, but keep watching: several substorms can happen in one night.

Solar cycle 25 and the years ahead

The Sun's activity rises and falls on a roughly 11-year cycle. Solar cycle 25 began in December 2019 and peaked in 2024 to 2025, higher than forecasters first expected. In May 2024 it produced the first G5 storm since 2003, with aurora seen from Florida, Mexico and southern Europe, and another severe storm followed in October 2024. Past the peak, the declining phase still brings large eruptions and frequent coronal hole streams, and storms are statistically more common near the equinoxes in March and September to October, when Earth's field is best angled to couple with the solar wind. The next two years remain a good time to look.

When the sky is dark enough

The aurora needs a dark sky: the Sun at least 12° below the horizon, away from city lights and ideally without a bright Moon. Close to midsummer, places north of about 55° never get properly dark, which is why the aurora season in Scandinavia, Iceland, Scotland and Canada runs from late August to April even though the aurora itself is there all year.

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