SolriseKp index explained: when can you see the aurora?
The Kp index is a 0 to 9 measure of geomagnetic activity: the higher it is, the farther from the poles the aurora can be seen. Kp 5 (a minor storm) is usually when the aurora becomes visible outside the very highest latitudes, and Kp 7 or more can push it to mid-latitudes.
What the Kp index is
The planetary Kp index measures how disturbed Earth's magnetic field is, on a scale from 0 to 9. A quiet field keeps the aurora in rings near the poles. When the field is shaken up, those rings grow and slide toward the equator, so people much farther south get a chance to see the northern lights.
NOAA's Space Weather Prediction Center ties Kp to a geomagnetic storm scale, from G1 (minor) to G5 (extreme), which is the easiest way to read it.
The Kp and geomagnetic storm scale
| Kp | Storm level | Where the aurora may be seen (US) |
|---|---|---|
| 0 to 4 | Quiet | Aurora mostly stays near the poles. |
| 5 | G1 minor | High latitudes, like northern Michigan and Maine. |
| 6 | G2 moderate | As low as New York and Idaho. |
| 7 | G3 strong | As low as Illinois and Oregon. |
| 8 | G4 severe | As low as Alabama and northern California. |
| 9 | G5 extreme | As low as Florida and southern Texas. |
Source: NOAA Space Weather Prediction Center geomagnetic storm (G) scale.
Why Kp changes
Kp rises when a burst of charged particles from the sun, a solar wind gust or a coronal mass ejection, hits Earth's magnetic field. A quiet sun means a low, steady Kp most nights.
The solar wind is worth watching alongside Kp. Even the slow solar wind moves at about 400 km per second, and wind from coronal holes can reach 800 km per second. Aurora chasers watch for speeds climbing above about 500 km/s; combined with a high Kp, that is a good night to head somewhere dark.
Solar flares are a related signal. Big M and X-class flares often come before the kind of solar activity that drives strong aurora displays a day or two later.
What Kp means for you
Everyday
Even when the aurora is visible, dark skies away from city lights make the difference between a clear show and nothing noticeable. A high Kp under thick cloud or bright streetlights will not show you much.
Outdoors
To chase the aurora, head away from city lights to a spot with a clear northern horizon, and give your eyes about 20 minutes to adjust. Phone cameras often pick up color your eyes cannot see, so take a photo even if the sky looks only faintly glowing.
Flying
Strong geomagnetic storms can degrade GPS accuracy, which drones rely on for position hold and return-to-home. On high Kp nights, be ready to fly manually.
Common questions
What Kp index do I need to see the aurora?
Kp 5, a G1 minor storm, is usually when the aurora becomes visible outside the very highest latitudes, such as northern Michigan and Maine. Kp 7 or more can push it to mid-latitudes like Illinois and Oregon.
How high up is the aurora?
The aurora typically forms 50 to 310 miles (80 to 500 km) above Earth's surface, far above the weather, according to NOAA SWPC.
Why can my phone see the aurora when I can't?
Phone cameras often pick up aurora color your eyes cannot see, so take a photo even if the sky only looks faintly glowing. Give your eyes about 20 minutes to adjust to the dark.
Related
See it live in Solrise
Every reading in Solrise Weather comes with its scale, what it feels like, and what it means for your day. Coming soon to the App Store.
See Solrise Weather