I think it's time to show you the 2nd card showing aurora borealis that I've received some time ago :) This one comes from Finland and actually I was almost sure that if I ever receive a card showing aurora borealis, it'll certainly come from Finland (and then I received 2 on the same day - from Finland and from Russia) :)
There's some information on the back side of this card. It says: "When the particles of the solar wind hit the atmosphere of the earth the oxygen of the air flares in green light". I understand the words, but still don't understand how it works. And I don't really think I want to understand it. I think that aurora borealis is beautiful even for people, who understand physics and astronomy very well, but I like the feeling that it's something mysterious, even if I know that in fact there is some scientific explaination. I read that auroras occur occasionally also in temperate latitudes, but I've never been lucky enough to see it :(
Aurorae are produced by the collision of charged particles from Earth's magnetosphere, mostly electrons but also protons and heavier particles, with atoms and molecules of Earth's upper atmosphere (at altitudes above 80 km (50 miles)). The particles have energies of 1 to 100 keV. They originate from the Sun and arrive at the vicinity of Earth in the relatively low-energy solar wind. When the trapped magnetic field of the solar wind is favourably oriented (principally southwards) it connects with Earth's magnetic field, and solar particles enter the magnetosphere and are swept to the magnetotail. Further magnetic reconnection accelerates the particles towards Earth.
The collisions in the atmosphere electrically excite electrons to take quantum leaps (a mechanism in which the electron's kinetic energy is converted to visible light); and molecules in the upper atmosphere. The excitation energy can be lost by light emission or collisions. Most aurorae are green and red emissions from atomic oxygen. Molecular nitrogen and nitrogen ions produce some low level red (pink) and very high blue/violet aurorae. The light blue and green colors are produced by ionic nitrogen and the neutral helium gives off the purple colour whereas Neon is responsible for the rare orange flares with the rippled edges. Different gasses interacting with the upper atmosphere will produce different colors, caused by the different compounds of oxygen and nitrogen. The level of solar wind activity from the Sun can also influence the color of the aurorae.