How the northern lights really work.

Why Kp isn't the whole story

Kp is a planet-wide score from 0 to 9 for how disturbed Earth's magnetic field is. It's the number most aurora apps lead with, and it matters most for people far to the south, who need a big storm before the aurora reaches them.

Fairbanks is different. We sit right under the aurora oval, the ring where the lights live, so we can see aurora on quiet nights too. For us, what usually decides the night is what the Sun is sending right now: how fast the solar wind is moving, how dense it is, and above all which way its magnetic field points. When that field, called Bz, turns south, the door opens and energy pours into Earth's magnetic field. A Kp 2 night with a strong southward Bz can outshine a Kp 4 night with the door closed.

That's why ANN shows Kp beside the live solar wind, and why the satellite that measures the wind gives us roughly 30 to 60 minutes of warning before it reaches Earth.