What Is the Kp Index?
The Kp index is the single most important number for predicting whether you'll see the aurora tonight. Here's what it means and how to use it.
The Basics
The Kp index measures geomagnetic activity on a scale from 0 to 9. Higher numbers mean stronger disturbances in Earth's magnetic field, which means brighter, more widespread aurora. It's updated every 3 hours by NOAA's Space Weather Prediction Center based on data from magnetometer stations around the world.
| Kp | Activity Level | What It Means for Aurora |
|---|---|---|
| 0–1 | Quiet | Aurora only visible near the Arctic (Fairbanks, northern Scandinavia) |
| 2–3 | Unsettled | Good displays in Fairbanks/Denali; faint in Anchorage |
| 4 | Active | Visible across most of Alaska; possible in northern Lower 48 |
| 5 | Minor Storm (G1) | Strong displays statewide; visible in northern US states |
| 6–7 | Moderate–Strong Storm | Vivid aurora across Alaska and into the Lower 48 |
| 8–9 | Severe Storm | Rare. Aurora visible from the southern US. Historic display. |
What Kp Do You Need in Alaska?
Each location in Alaska has a different Kp threshold because of its latitude. The further north you are, the less activity you need.
| Location | Kp for Visible Aurora | Kp for Strong Display |
|---|---|---|
| Fairbanks | 1 | 3 |
| Denali Area | 2 | 3 |
| Talkeetna | 2 | 4 |
| Palmer–Wasilla | 3 | 5 |
| Anchorage | 3 | 5 |
| Kenai Peninsula | 4 | 5 |
| Juneau | 5 | 6 |
Kp Isn't the Whole Story
The Kp index tells you about geomagnetic activity, but three other factors matter just as much:
- Cloud cover — A Kp 7 storm is worthless if it's overcast. Always check the weather forecast alongside the Kp.
- Solar wind Bz — When the interplanetary magnetic field (IMF) points southward (negative Bz), it connects with Earth's field more effectively, boosting aurora. A Kp of 3 with strong southward Bz can outperform a Kp of 5 with northward Bz.
- Moon phase — A bright full moon washes out faint aurora. New moon or crescent phases give you the darkest sky.
Alaska Glow combines all four of these factors into a single score for each location, so you don't have to check multiple sources.
Check tonight's aurora conditions across 7 Alaska locations
View Tonight's Forecast →Where Does the Kp Forecast Come From?
NOAA's Space Weather Prediction Center monitors the sun for coronal mass ejections (CMEs) and solar wind streams. They issue Kp forecasts up to 3 days out, though accuracy drops significantly beyond 24 hours. Real-time Kp is measured every minute from ground-based magnetometers worldwide.
For aurora watching, the most useful data is the 1-minute real-time Kp combined with solar wind measurements from the DSCOVR satellite positioned between Earth and the Sun. When a burst of solar wind with strong southward Bz is detected at DSCOVR, aurora follows about 30–60 minutes later.