Auroras emit light because of the emission of photons by oxygen and nitrogen atoms in the upper atmosphere. Those atoms get excited, or ionised, by the collision with solar wind particles, which are accelerated by the Earth's magnetic field. As the atoms get excited or return to their normal state, they emit visible energy. When it is an oxygen atom, the light emitted is either green or brownish-red, depending on the energy level absorbed by the molecule. Blue happens when nitrogen gets ionised, and red when it returns to ground state.
This red aurora was caught by the International Space Station on September 26, 2011. [NASA Goddard Flickr]