How to See the Northern Lights: Reading the Kp Index
The northern lights are not as rare as many people think. NOAA's Space Weather Prediction Center (SWPC), the US government's space weather forecaster, says aurora "exists nearly all the time" and can be seen somewhere on Earth nearly every night of the year. The hard part is being under it, in the dark, on a night when it is strong enough to reach you.
That is what the Kp index is for. Once you know how to read it, an aurora forecast stops being a guess.
1. What the aurora is
SWPC's aurora tutorial explains that the glow comes from electrons flowing down Earth's magnetic field and colliding with atoms and molecules high in the atmosphere, in a ring or oval centered on each magnetic pole. In the north it is the aurora borealis, or northern lights; in the south, the aurora australis.
The colors come from different gases at different heights:
- Pale green, the most common color, comes from oxygen typically 120 to 400 km (80 to 250 miles) up.
- Deep red comes from oxygen above about 300 km (180 miles).
- Other tints, including purple, come from nitrogen and other molecules.
SWPC adds a useful note: people often report the aurora as pale white. That is the eye's night vision seeing brightness before color. Only fairly bright aurora shows its true colors to the eye, while modern digital cameras can often record color when the aurora is too dim for you to see it.
2. What the Kp index measures
SWPC says the level of geomagnetic activity is indicated by the planetary K index, or Kp, which ranges from 0 to 9. SWPC derives it every three hours from a network of ground-based magnetometers, and its scales rate Kp 5 as a minor (G1) geomagnetic storm. Higher Kp means a brighter, more active aurora that reaches farther from the poles.
3. How far south each Kp level reaches
SWPC's viewing tips give an approximate rule. At Kp 0, the equatorward edge of the auroral oval sits near 66 degrees magnetic latitude, and it moves about 2 degrees toward the equator for each step up in Kp:
| Kp | Edge of the oval (magnetic latitude) |
|---|---|
| 0 | 66 degrees |
| 3 | 60 degrees |
| 5 | 56 degrees |
| 7 | 52 degrees |
| 9 | 48 degrees |
Two cautions come with that table. First, it uses magnetic latitude, not the latitude on a map, because the oval is centered on the magnetic pole, which SWPC says lies about 400 km (250 miles) from the geographic North Pole in northeast Canada. Second, you do not need to be under the oval to see it. SWPC says that from a hilltop with a clear view north, you can see aurora as much as 1,000 km (600 miles) farther north of you.
4. What the storm levels look like on the ground
SWPC also rates geomagnetic storms on a G scale tied to Kp, and lists where aurora has been seen at each level:
| G level | Kp | Aurora has been seen as far south as |
|---|---|---|
| G1 (minor) | 5 | Commonly visible at high latitudes, such as northern Michigan and Maine |
| G2 (moderate) | 6 | New York and Idaho |
| G3 (strong) | 7 | Illinois and Oregon |
| G4 (severe) | 8 | Alabama and northern California |
| G5 (extreme) | 9 | Florida and southern Texas |
The scale also shows how uncommon the big storms are. SWPC lists about 1,700 G1 storms per 11-year solar cycle, but only about 4 at G5.
SWPC's viewing tips translate this into plain terms: at Kp 0 to 2, the aurora stays far north and dim; at 3 to 5, it brightens and moves south; at 6 to 7, it may be visible from the northern edge of the United States; and at 8 to 9, it may appear overhead in the northern states.
5. When to look
Hours: SWPC says the best aurora is usually within an hour or two of midnight, between about 10 PM and 2 AM local time. Evening aurora tends to be a quiet arc, and early-morning aurora often turns patchy and dim.
Seasons: Larger storms tend to happen around the spring and fall equinoxes. There is a catch for travelers: far-north summers barely get dark, so the aurora may be there but invisible.
The solar cycle: Activity rises and falls over roughly 11 years, with more storms in the 4 to 5 years near solar maximum. SWPC notes that the biggest flares and storms often come near the end of that period, as activity starts to wane.
6. How far ahead forecasts work
SWPC's tutorial explains three useful time scales:
- 15 to 45 minutes: satellites about 1.5 million km (1 million miles) toward the Sun measure the incoming solar wind, giving short but accurate warnings.
- A day or so: forecasters track eruptions and fast solar wind leaving the Sun, with less certainty.
- 27 days: the Sun rotates in about that time, so if there was aurora 27 days ago, there is a good chance of aurora again, plus or minus a day or two.
7. A practical aurora checklist
- Check the Kp forecast and SWPC's Aurora Dashboard during the day.
- Head away from city lights to a spot with a clear northern horizon.
- Watch most closely between 10 PM and 2 AM.
- Remember that a full moon dims how bright the aurora looks.
- Try a phone or camera photo if you see a pale glow; it may reveal color your eyes cannot.
8. Key takeaways
- The Kp index runs from 0 to 9; higher values mean brighter aurora reaching farther from the poles.
- Each step in Kp moves the auroral oval about 2 degrees of magnetic latitude toward the equator.
- At Kp 7 (G3), aurora has been seen as far south as Illinois and Oregon.
- The best viewing is usually 10 PM to 2 AM local time, with stronger storms near the equinoxes.
- A pale white glow may be faint aurora; a camera can often record its color.