Key takeaways
- The aurora is caused by the solar wind. Charged particles follow the earth's magnetic field to the polar regions and make gases in the upper atmosphere glow.
- Check the cloud cover for your own location first, and the Kp index second. In southern Finland you generally need Kp 5 or more, while in Lapland the lights can appear at much lower values.
- The best months are around the equinoxes, in September and October and in March and April, and the best place has a low, open view to the north.
- Start at 5 to 8 seconds and ISO 1600 to 3200 with the lens wide open, and shorten the exposure to 1 to 3 seconds when the lights are bright and fast.
You can plan an aurora evening even though nobody can promise one. Check the cloud cover for your own location, then the Kp index and the aurora forecast, and choose a site with a low, open view to the north.
The northern lights have a reputation for being capricious, and it is true that nobody can promise them. Yet they are far more predictable than most people think, and the difference between a lucky guess and a good evening is mostly a matter of knowing three things: what the forecast is saying, where the clouds are, and how dark your horizon is.
What you are actually looking at
The aurora begins at the sun. Streams of charged particles, the solar wind, reach the earth and are guided by its magnetic field towards the polar regions, where they collide with gases high in the atmosphere. Each gas glows in its own colour, and the colour depends on the gas and on the height at which the collision happens.
| Colour | Cause | Typical height | Notes |
|---|---|---|---|
| Green | Oxygen atoms | About 100 to 150 km | The familiar pale green, and by far the most common colour |
| Red | Oxygen atoms | Above about 200 km | Rarer, and often faint to the eye, though a camera records it well |
| Blue and purple | Nitrogen molecules | Often at the lower edge of a bright display | Fringes that appear in strong displays |
When the solar wind is strong or its magnetic field points in the right direction, the display grows brighter and spreads further south.
Reading the forecast without being fooled by it
Two numbers are worth learning. The first is the Kp index, a scale from 0 to 9 that describes how disturbed the earth's magnetic field is. The second is the aurora probability map, which shows the chance of seeing the lights at your position. AstroNight shows both in its plan and in its calendar, using the forecasts published by the NOAA Space Weather Prediction Center.
| Kp | Storm level | What to expect |
|---|---|---|
| 0 to 2 | None | In northern Lapland the lights can appear on many clear nights; in the south they are not usually visible |
| 3 to 4 | None | Good in Lapland, and often visible low in the north from central Finland; a faint glow is sometimes possible in the south |
| 5 | G1, minor | Southern Finland can see arcs over the northern horizon, and sometimes higher |
| 6 to 7 | G2 to G3 | Strong displays can reach high over southern Finland |
| 8 to 9 | G4 to G5 | Rare, spectacular displays that can fill the sky and turn red |
The figures are a guide rather than a promise, because the same Kp can look very different on different nights. A third layer of information comes from the solar wind itself. A satellite about a million and a half kilometres towards the sun measures its speed and the direction of its magnetic field, and a fast wind with a magnetic field pointing south gives the best chance of a display. It also gives you something like half an hour of warning.
Remember that the forecast describes the sky above the atmosphere, not the sky above your garden. The best forecast in the world will not help under a lid of cloud, so look at the cloud cover for your own location first, and only then at the Kp number.
Look at the clouds first and the Kp number second.
When is the best time of year?
Aurora seasons favour the weeks around the equinoxes, in March and September to October, when the earth's magnetic field is more easily connected to the solar wind. This effect has a name, the Russell-McPherron effect. In the south of Finland the summer is too bright for the aurora to show at all, as explained in a sky that never gets dark, so the season really begins in late August and runs until April. Keep an evening free in the equinox months.
Choose a place with a low northern horizon
Lights in the north are the usual story, so a place with a clear view in that direction matters more than a perfectly dark zenith. A frozen lake, a field or a shoreline gives you a long open horizon. Keep the lights of the nearest town behind you if you can, and arrive early, because a bright display can start and finish within twenty minutes. The effect of town light on the sky is the subject of the Bortle scale article.
Camera settings to start with
An aurora moves, which makes it different from the Milky Way. If you use the same long exposure you would use for a static star field, you will smear its delicate structure into a green fog.
| What the aurora is doing | Exposure | What to expect |
|---|---|---|
| Faint, slow arc or glow | 8 to 15 seconds | A little softness in the structure, and more colour |
| An ordinary moving display | 5 to 8 seconds | A good balance of brightness and shape |
| Bright, fast, with rays | 1 to 3 seconds | Sharp, well-defined rays |
Here is the routine in order.
- Check the sky above you. Look at the cloud forecast for your own location first, and then at the Kp index and the aurora probability map.
- Choose a site. Pick a place with a long, open northern horizon, such as a frozen lake, a field or a shoreline, and keep the lights of any town behind you.
- Dress and prepare. Dress for temperatures well below the forecast, keep a spare battery against your body and arrive early.
- Set up and focus. Use the widest and fastest lens you own, focus on a bright star in live view, and do not touch the focus ring afterwards.
- Choose an exposure. Begin with 5 to 8 seconds at ISO 1600 to 3200, shorten it to 1 to 3 seconds if the lights are bright and fast, and lengthen it gradually if they are faint.
- Watch and adjust. Check the sky every ten minutes, take a test frame, and change one setting at a time.
Cameras cannot see the colour as your eyes do, and often a faint display looks greener and brighter on the screen than it did in person. That is not cheating, only the benefit of collecting light for a few seconds. A recent iPhone in its night mode can also record a good display from a small tripod, and the same advice about the horizon applies.
Be warm, be patient
Most of an aurora evening is waiting, and the cold makes it feel longer. Dress for temperatures well below what the forecast says, keep a spare battery against your body, and bring something hot to drink. Use a red light when you read your settings, so that your eyes stay adapted, as explained in what red light does for your eyes. Check the sky every ten minutes or so, since an aurora often begins as a faint grey arc that is easy to take for a cloud. If the arc slowly changes shape and passes in front of the stars without hiding them, you are looking at the real thing.
Frequently asked questions
What Kp index do I need to see the northern lights in southern Finland?
As a rough guide, Kp 5 or more. At that level arcs appear over the northern horizon and sometimes climb higher. With a dark, open northern horizon you may glimpse a faint glow at a lower value, but a clear sky matters more than the exact number.
What is the best time of year to see the aurora in Finland?
The weeks around the equinoxes, in September and October and in March and April, give the best odds because the earth's magnetic field connects more easily to the solar wind. In the south the sky is too bright in summer, and the dark season runs from late August to April.
What causes the colours of the northern lights?
Oxygen about 100 to 150 kilometres up glows green, oxygen above about 200 kilometres glows red, and nitrogen can add blue and purple fringes at the lower edge of a bright display. The colour depends on the gas and on the height of the collision.
What camera settings should I use for the aurora?
Use your widest, fastest lens with the aperture fully open, an exposure of 5 to 8 seconds and ISO 1600 to 3200. Shorten it to 1 to 3 seconds when the lights are bright and fast, and lengthen it gradually when they are faint. Focus on a star beforehand.
Can I photograph the northern lights with a phone?
Yes. A recent smartphone in its night mode can capture a good display, especially from a small tripod. The same rules apply: a clear sky, an open northern horizon and a screen turned down low so that your eyes stay adapted.
Sources and further reading
AstroNight is now on the App Store. The free planner and coach for astrophotography works on iPhone and iPad with iOS 17 or later, offline, with no account. Get it on the App Store.
