Linné on line 
Physics and the Cosmos Macrocosmos The sun What are northern lights?
What are northern lights?
During his travels it could happen that Linnaeus saw northern lights. In those days there were many different types of speculations on the origin of this phenomenon. Since northern lights are explained by atomic and particle physics reaction, which in those days were unheard of, these ideas appear as very farfetched today.
| The particles that give rise to northern lights originate from the sun. They come with the solar wind from the sun and are captured by the earth's magnetic field. The motion of the particles is determined by the shape of the magnetic field. At the poles the particles can reach deep into the atmosphere since the magnetic poles are close to the geographic ones. When the particles reach the atmosphere they collide with the molecules there. This will excite the molecules and when they return to their ground state they emit the absorbed energy in the form of photon radiation. These photons will have different energy, producing different colours, depending on which type of molecule that was involved: for example oxygen (O2) gives green light and nitrogen (N2) red. Since oxygen and nitrogen are the dominant gases in the atmosphere it is clear that northern lights will be dominantly red and green. |  Picture of northern lights above Scandinavia taken by a satellite. Cities are seen as bright dots on the picture. |