How Aurora happen?

Auroras are caused by a stream of charged particles (mostly electrons, but some protons and a few other things) from the Sun getting trapped in Earth's magnetic field and interacting with the gasses in our upper atmosphere. The charges electrify the nitrogen and oxygen in the air and make them glow like a neon or fluorescent light bulb.
When photons crash into gas nuclei, electrons can take up that energy. The energized electrons use the extra energy to jump out to higher energy levels around their parent nuclei. The atoms are said to be ionized. The energized electrons then release the extra energy and return to their original orbitals. The released energy is electromagnetic energy - it's light.