Space Weather & Aurora

How to read space weather

The Kp index is a planetary measure of how disturbed Earth's magnetic field is: a network of ground magnetometers produces one number from 0 to 9 every three hours. Kp 0–2 means a calm magnetosphere, 3–4 an unsettled one, and from Kp 5 upward a geomagnetic storm is under way, which NOAA describes with the G scale from G1 (minor, Kp 5) to G5 (extreme, Kp 9). Storms are driven by the solar wind, and the parameter that matters most here is Bz, the north–south component of the interplanetary magnetic field. While Bz points north, Earth's field deflects the stream; when it turns south (negative values, especially below −5 nT), the field lines reconnect with Earth's own and the wind's energy pours into the magnetosphere. That is why a southward Bz combined with a fast, dense wind is the best one-hour predictor of a bright aurora.

The X-ray flux from the GOES satellites reveals solar flares. A flare's class is simply the order of magnitude of the 0.1–0.8 nm flux: A and B are background, C are minor flares, M medium, X major, and the number after the letter refines the value within the class (an M5 is five times an M1). A flare hits Earth with light, arriving in eight minutes: its ultraviolet and X-rays abruptly ionise the dayside ionosphere and short-wave radio on the sunlit half of the planet fades out. That is the R scale — R1 corresponds to an M1 flare, R3 to X1, R5 to X20. The S scale covers radiation storms, floods of protons hazardous to satellites and to crews on polar flights. A flare alone does not guarantee a storm: the aurora is caused not by the light but by a coronal mass ejection, which takes one to three days to reach Earth.

The map shows the OVATION Prime model: it takes the measured solar wind and estimates where, and with what probability, particle precipitation — the aurora — will occur in the next 30–90 minutes. It is a forecast built on wind that is already on its way, so it is accurate for the coming hour and says nothing about tomorrow; for that, use the three-day Kp forecast. The "chance to see it" estimate computes your place's geomagnetic latitude with a dipole model (pole at 80.7° N, 72.7° W) and compares it with the equatorward edge of the oval at the current Kp: at Kp 3 the edge sits near 60° geomagnetic latitude, at Kp 7 near 52°. Even a perfect forecast is useless without darkness and a clear sky: summer white nights hide the aurora at high latitudes, and city light swallows faint arcs. Data come from the NOAA Space Weather Prediction Center; snapshots refresh every 5–10 minutes and the page reloads them on its own.