Aurora forecasting, explained — formula included

The Kp index, in plain English

Every three hours, magnetometers around the planet report how disturbed Earth's magnetic field is. The consensus becomes the planetary K-index — Kp — a 0–9 scale published by NOAA's Space Weather Prediction Center. Kp 0–2 is quiet; Kp 5 is a minor geomagnetic storm (G1); Kp 7 a strong one (G3); Kp 9 the once-a-decade sky-on-fire event. Higher Kp pushes the auroral oval further from the pole, bringing the display to lower latitudes.

Kp is genuinely useful — and genuinely insufficient. It's a global, three-hourly, backwards-looking-ish number that knows nothing about your cloud, your daylight, or where you're standing. Treating a Kp ping as an aurora alert is the founding error of most aurora apps, and the subject of our favourite rant.

Solar wind: the upstream early warning

The solar wind — a stream of charged particles from the Sun — is what powers the whole show. Spacecraft at the L1 point, a million miles upstream (historically DSCOVR and ACE; NOAA is transitioning to SOLAR-1 and IMAP), sample it 15–60 minutes before it reaches Earth. Typical speed is 300–400 km/s; above ~500 km/s geomagnetic activity climbs; coronal mass ejections can drive it past 700. Speed is a live reading, not a forecast — which is why AurorAlert uses it as a now signal and a modest bonus, never as an hourly prediction. (Serious chasers also watch the interplanetary magnetic field's Bz component — southward Bz is the door swinging open — which is on our roadmap for advanced users.)

Geomagnetic latitude: the number nobody tells you

Here's the trap: the aurora doesn't care about your geographic latitude. It circles the magnetic pole, which currently sits over the Canadian Arctic, offset from the geographic pole. Your geomagnetic latitude — angular distance from the magnetic pole — is what decides the Kp you need. The consequences are striking:

Calgary and London sit level on a map; Calgary sees aurora at Kp 4–5 while London waits for 6+. Any forecast that uses geographic latitude — or worse, one global threshold — is wrong for most of the planet. AurorAlert computes your geomagnetic latitude from your exact coordinates (a standard dipole approximation of the field) and derives your personal Kp requirement from it. The same maths, using absolute distance from the magnetic pole, is why the app works identically for the aurora australis.

The AurorAlert formula — published, in full

• Kp needed here = (66 − your geomagnetic latitude) ÷ 2, clamped between 0.5 and 9 — the standard rule-of-thumb for the visible auroral boundary.
• Kp score: compare that hour's forecast Kp (NOAA's 3-hourly product) against your need. At or above by 3+ → 100; +2 → 90; +1 → 75; level → 55; 1 below → 30; 2 below → 10; further → 2. Yes, it's a step function — a forecast Kp a whisker under your need scores 30, which is honest about how sharply visibility falls off.
• Clear-sky = (100 − forecast cloud %) ÷ 100, per hour, for your exact coordinates (Open-Meteo). Full cloud multiplies everything to zero, as it should.
• Darkness = ×1 when your sky is dark, ×0.1 otherwise — computed from your local sunrise/sunset.
• Wind bonus (live solar wind only): +5 above 500 km/s, +10 above 600, +15 above 700.

Prefer no formula at all? Flip on "Use my own conditions" and the percentage becomes advisory: you set minimum Kp, maximum cloud, minimum solar wind and comfort limits (wind, temperature, rain — each with an Off switch), and the app alerts only when every one is satisfied. Your thresholds. Our data. Nobody's black box.

What forecasting still can't do

Honesty cuts both ways. Kp forecasts are ~3-hourly and imperfect; sub-storm timing within a night is genuinely hard; solar wind gives only tens of minutes' lead; and cloud forecasts are cloud forecasts. A good aurora app narrows your nights from 365 to the handful worth standing outside for — it cannot promise the sky. Anyone who says otherwise is selling something. We're just trying to make sure that when your handful comes, you're not asleep.