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Aurora is a luminous glow of the upper atmosphere caused by energetic particles entering the atmosphere from above.
This definition differentiates aurora from other forms of airglow, and from sky brightness that is due to reflected or scattered sunlight. Airglow features that have "internal" energy sources are more common than aurora. For example, lightening and all associated optical emissions like sprites should not be considered aurora.

On Earth, the energetic particles that make aurora come from the geospace environment, the magnetosphere. These energetic particles are mostly electrons, but protons also make aurora. The electrons travel along magnetic field lines. The Earth's magnetic field looks like that of a dipole magnet where the field lines are coming out and going into the Earth near the poles. The auroral electrons are thus guided to the high latitude atmosphere near each pole. As the electrons penetrate into the upper atmosphere, the chance of colliding with an atom or molecule increases the deeper they go. Once a collision takes place, the atom or molecule takes some of the energy of the energetic particle and stores it as internal energy while the electron goes on at a reduced speed. The process of storing energy in a molecule or atom is called "exciting" the atom. An excited atom or molecule can return to the non-excited state (ground state) by sending off or emitting a photon, i.e. by making light.

Basically, it is very difficult to take night photos with a cellphone. To capture the Northern lights you will need to position your camera on a tripod to make sure you don't have any motion while you take a photo. We recommend to bring a camera which has a manual mode, which most medium-priced cameras have nowadays.

The colours of the aurora are determined by the composition of gases in the Earth's atmosphere, the altitude at which the aurora occurs, the density of the atmosphere, and the level of energy involved.

Green, the most common colour seen from the ground, is produced when charged particles collide with oxygen at lower altitudes (around 100-300 km). Occasionally, the lower edge of an aurora will have a pink or crimson fringe, which is produced by nitrogen molecules (around 100 km).

Higher in the atmosphere (300-400 km), collisions with atomic oxygen produce reds instead of greens. Since the atmosphere is less dense at higher altitudes, it takes more energy and more time to produce red light (up to two minutes), whereas green light can be made quickly at lower altitudes (about one second).

Hydrogen and helium can also produce blue and purple, but those colours tend to be difficult for our eyes to see against the night sky.