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The Earth is surrounded by a thin gas cover, the atmosphere, and fast charged particles, plasma, are moving in space above it. Auroras arise when some of those particles enter the Earth's atmosphere and collide with atoms and molecules. When the particles collide the energy used to give them their velocity changes into a light, the aurora.
The particles that make auroras come from the ionosphere but have an extremely high velocity due to the energy from the solar wind. The particles are caught by the Earth's magnetic field and are steered towards the poles. When a particle reaches the atmosphere it collides with one of the many present atoms.
When the particle collides with an atom, the atom takes over some of the energy that has given the particle its velocity. The particle keeps on moving but with less velocity, since it has lost some energy to the atom. The particle soon collides with a different atom.
The atom that has taken over some energy from the particle has now got too much energy and lets go of it. The surplus energy becomes light. The next atom that collides with the particle also takes over some of its kinetic energy, resulting in the particle losing even more velocity. The new atom also lets go of the energy. As the particle moves down through the atmosphere the atoms become more and more crowded, resulting in more collisions for the particle. Each time the particle collides it moves a bit slower and more light is emitted.
When the particle has collided a number of times it has lost so much of its kinetic energy that it stops moving. This occurs when the particle is approximately 100 kilometres from the Earth's surface. When a lot of particles collide with atoms, releasing light, an aurora occurs.
For auroras to arise on a planet five things are required.
First of all the planet has to have an atmosphere. The atmosphere is the screen upon which the aurora is shown. If there was no atmosphere the particles from space would find no atoms to collide with and no light would be visible.
Second, there must be charged particles, plasma, that can collide with the atmosphere. If there were no particles the situation would be the same as above.
Third, something that can steer the plasma particles down to the atmosphere is needed. That is a magnetic field. If the magnetic field was not present most of the particles would miss the earth and keep moving through space.
Fourth, an energy source that can give the plasma particles all the energy they need to create auroras is required. On Earth that energy source is the Sun. If the particles were not provided with all the necessary energy auroras would not occur.
Lastly, something to carry the energy from the Sun to the particles is needed. This is the solar wind.