Aurora conditions in Rovaniemi

What you are actually looking at

Most aurora pages show a forecast. They read the solar wind at a satellite roughly a million and a half kilometres away and estimate what will happen here in the next half hour or so. That estimate is useful, but it is still an estimate, and it is made a long way from Lapland.

The figure on this page is different. It comes from the IMAGE magnetometer network — instruments buried in the ground across northern Finland that record how much the Earth's magnetic field is being pushed around. When an aurora is overhead, electric currents flow in the upper atmosphere, and those currents tug on the field in a way the magnetometers pick up immediately. The Finnish Meteorological Institute turns that into an R-index: a single number describing how disturbed each station is right now.

So a high reading here does not mean an aurora might arrive. It means something is happening over Lapland at this moment. Whether you can see it from where you are standing is a separate question, which is what the cloud figure is for.

The map at the top of the page is the other kind of thing: NOAA's OVATION model, which takes the solar wind measured out at the satellite and draws where it expects the auroral oval to sit about half an hour from now. It is useful for seeing the shape of the night — whether the oval is sitting north of us, directly overhead, or pushed down far enough that the whole of Lapland is underneath it. Treat it as the weather map and the numbers below as the thermometer.

The stations we read

Four stations cover our operating area, and the page reports whichever one is showing the strongest disturbance relative to its own normal range — not simply the nearest. A substorm breaking out over Ivalo is worth knowing about even when Rovaniemi itself is quiet, and on a clear night it is perfectly reasonable to drive towards it.

• Ranua — just south of Rovaniemi, the closest station to town.
• Pello — north-west, towards the Swedish border.
• Muonio — further north-west, well inside the usual auroral oval.
• Ivalo — north-east, the furthest north of the four.

Every reading is labelled with the station it came from and how far away that is. We would rather tell you a number was measured 180 km north of here than quietly pass it off as local.

What the activity number means

The figure labelled Activity (R-index) is the one worth explaining, because on its own it looks like a score out of nothing.

It is built from the magnetometer's own readings. Every ten seconds the instrument records the magnetic field in three directions. FMI add up how much those readings move between one sample and the next across a ten-minute window — weighting the most recent five minutes twice as heavily as the five before — and subtract a constant for that station's own quiet-time noise. A still field gives a number near zero. A field being pushed around gives a large one.

The result is a plain number rather than a measurement in any unit, and it does not mean the same thing at every station. What makes it readable is that each site has two thresholds, which FMI established by comparing years of readings against all-sky camera footage of what was actually overhead at the time:

• Past the lower threshold, there is roughly a 50% chance of weak aurora — an arc that may be faint to the naked eye, or sitting low on the northern horizon, and greener to a camera than to you.
• Past the upper threshold, there is roughly a 50% chance of strong aurora, and weak aurora becomes considerably more likely than not.

That is why the display shows the station's own threshold next to the number, and why the wording above it — quiet, faint, active, strong — is worked out against those limits rather than one universal scale. A reading of 40 can be an ordinary night at one station and a notable one at another.

Beneath our wording you will also see FMI's own description of the conditions, in their words. If the two ever read differently, theirs is the authoritative one.

What else has to go right

Auroral activity is only one of four things, and it is the only one we cannot do anything about.

Cloud is the one that decides most nights. The figure shown is total cloud cover for Rovaniemi, but Lapland is large and the weather is rarely uniform across it — a 90% reading in town frequently sits next to a clear gap an hour's drive away. Finding that gap is most of what a guide is for.

Darkness sets the season. From late August to early April there is enough true night in Rovaniemi to see anything at all; in midsummer the sky never gets dark, whatever the magnetometers say. The "dark tonight" figure is the gap between sunset and sunrise, so treat it as the outer edge of the window rather than the whole of it.

Moonlight matters less than people expect. A full moon washes out a faint arc, but it lights the landscape, which usually makes for better photographs than a pitch-black foreground under a weak display.

Why we publish this

We run aurora tours out of Rovaniemi, and the honest answer to "will I see them tonight" is that nobody can promise it. What we can do is show the same instrument readings we use ourselves when deciding where to drive, and let you judge. The same data runs on the screen in the van.

Data sources

Auroral activity from the IMAGE magnetometer network, operated by the Finnish Meteorological Institute and partner institutes, via FMI's R-index service. Cloud cover, temperature and sun times from Open-Meteo. Both are published under CC BY 4.0. The green layer on the map is the OVATION model from NOAA's Space Weather Prediction Center, public domain; the coastlines beneath it are drawn from Natural Earth, also public domain. Readings are shown as supplied; any interpretation on this page is ours.