Picture of the Day: Green sky at night, northern delight...

The aurora borealis, or northern lights, hangs like a fluorescent curtain over the village of Ersfjordbotn, near Tromso in northern Norway.

The atmospheric phenomenon is caused by streams of electrically charged particles from the Sun, which are funnelled to the polar regions by the Earth's magnetic field. As the particles hit atoms in the upper atmosphere they excite them to higher energy levels, and this energy is then released in the form of light. This aurora is caused by the excitation of oxygen atoms, which emit a characteristic green coloration.