Aurora Forecast Tonight

WatchSat shows the live planetary Kp index, NOAA’s G-scale geomagnetic storm level, and a 3-day geomagnetic outlook, then combines the current Kp with your latitude to give an honest visibility verdict — whether the aurora oval reaches overhead, might be visible low on the horizon, or is unlikely from where you are tonight.

What Kp and the G-scale mean

The planetary K index (Kp) is a 0–9 measure of global geomagnetic disturbance over a three-hour interval, derived from a worldwide network of magnetometers. NOAA maps it onto the G-scale of geomagnetic storms: Kp 5 is G1 (minor), 6 is G2 (moderate), 7 is G3 (strong), 8 is G4 (severe) and 9 is G5 (extreme); anything below Kp 5 is unsettled-to-active conditions rather than a storm. WatchSat floors the value rather than rounding it, so Kp 5.67 is still G1 and G2 begins only at Kp 6.0 — the same way NOAA defines it.

Why your latitude decides the answer

Aurora rides the auroral oval, which is centred on the geomagnetic pole, not the geographic one — which is why Scotland sees more aurora than Moscow at the same latitude. WatchSat converts your position to corrected geomagnetic latitude through the centred-dipole approximation (IGRF-13 dipole axis, good to a degree or two) and compares it with the equatorward edge of the oval for the current Kp, a linear fit of 66.5° − 2.04 × Kp to the standard per-Kp oval table: about 66.5° at Kp 0, down to roughly 48° at Kp 9. Reach that line and the aurora is plausibly overhead. Within about six degrees equatorward of it, a bright display can still be glimpsed low on the poleward horizon, because the emission sits 100–300 km up and is visible from beyond the point where it is overhead.

Where the numbers come from

Space weather data comes from the NOAA Space Weather Prediction Center (public domain, US Government): the estimated planetary K index product for the current value and SWPC’s 3-day geomagnetic forecast for the outlook, both fetched through WatchSat’s /api/space-weather edge proxy and reduced to a daily maximum Kp per forecast day.

What a forecast cannot tell you

A high Kp reading is necessary but not sufficient. Seeing the aurora also needs darkness, a clear sky, a horizon that is not washed out by city light or moonlight, and patience — displays surge and fade over minutes. Kp is also a three-hour planetary average, so a short, sharp substorm can put aurora overhead while the index still looks quiet. Treat the verdict here as “is it worth going outside tonight”, not as a guarantee. While you are out there, the same dark sky is the best time to catch a satellite: try tonight’s ISS passes, a Starlink train, or the live tracker.

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