The Kp index, explained
Kp is the number every aurora headline quotes. It's useful — and it's routinely misunderstood. Here's what it actually measures, and why the pros watch something else in the moment.
Kp right now
What Kp measures
Kp is a planetary index of geomagnetic disturbance, from 0 (dead calm) to 9 (extreme storm). It's stitched together from magnetometers at observatories around the world and reported in 3-hour blocks. A higher Kp means the auroral oval has expanded toward the equator — so aurora becomes visible from lower latitudes.
The catch: Kp is a 3-hour average, and it lags
Because each Kp value covers three hours and takes time to compute, it's always a little behind reality. Aurora can flare and fade inside a single Kp block. So Kp is a great backdrop — "is tonight worth watching at all?" — but a poor stopwatch.
Why Bz matters more in real time
The sharper live signal is the solar wind's magnetic field, measured at the L1 point ~1.5 million km sunward of Earth. Its north–south component, Bz, is the key: when Bz turns strongly southward (negative) and stays there for 20+ minutes, energy pours into Earth's magnetosphere and aurora tends to follow. Watch Bz go red on the dashboard and you often have a 30–60 minute head start on the Kp number.
Rule of thumb
Kp tells you if tonight could be good. Bz tells you when to walk outside. Use both. And see the NOAA G-scale for what each Kp level means for storms.
Common questions
What is Bz?
Bz is the north–south component of the interplanetary magnetic field carried by the solar wind. When Bz points south (a negative value), it links up with Earth's field and pours energy into the system — that's what powers a good aurora. A strongly southward Bz sustained for 20+ minutes is a better real-time sign than the Kp number, which is a 3-hour average that lags.
Why does the Kp forecast keep changing?
Kp is a planetary average computed over 3-hour blocks, so it's always a little behind what's happening right now, and the forecast updates as new solar-wind data arrives. Treat Kp as the big-picture backdrop and the live solar wind (Bz, speed) as the minute-to-minute signal.
What Kp do I need to see the aurora in Michigan, Minnesota, or Washington?
For the northern tier of the US you typically need about Kp 5 (a G1 storm) to catch aurora low on the northern horizon, and Kp 6–7 (G2–G3) for it to climb higher or appear overhead. Your geomagnetic latitude matters more than your map latitude — the northern US sits at a higher geomagnetic latitude than its geographic one, which is why it sees aurora more readily than places at the same latitude in Asia. Enter your city on the dashboard for the number at your exact spot.