Sondakylä magnetogram

Real-time measurement of the Earth's magnetic field, updated every 5 min, issued by the Sodankylä Geophysical Observatory, Sodankylä, Finland. Please note the information called "Fullscale" in the titles of the diagrams, if this number grows, it is getting interesting (normal quiet situation: 100 nT or so). Check the curves for rapid variations, meaning changes of a few hundreds of units (nT) within minutes, then you are sure that you can see aurora in the vicinity of Sodankylä and very likely also in Oulu. If the drop is 400 nT or so, then you can almost certainly see something in Oulu, but at 700 nT it'll be more beautiful. Beyond 1000 nT the aurora might even be too far south to be seen in Sodankylä, but probably there'll be beautiful lights above Southern Finland, which might be visible even in Tallinn. Remember, however, that clear skies are a must!