Aurora

Auroras are indicators of the connection between the Earth and the sun. The frequency of auroras correlates to the frequency of solar activity and the sun's 11-year cycle of activity.As the process of fusion occurs inside the sun, it spews high-energy particles (ions, electrons, protons, neutrinos) and radiation in the solar wind. When the sun's activity is high, you'll also see large eruptions called solar flares and coronal mass ejections. These high-energy particles and radiations get released into space and travel throughout the solar system. When they hit the Earth, they encounter its magnetic field.As the charged particles of solar winds and flares hit the Earth's magnetic field, they travel along the field lines. Some particles get deflected around the Earth, while others interact with the magnetic field lines, causing currents of charged particles within the magnetic fields to travel toward both poles -- this is why there are simultaneous auroras in both hemispheres. (These currents are called Birkeland currents after Kristian Birkeland, the Norwegian physicist who discovered them.) When an electric charge cuts across a magnetic field it generates an electric current . As these currents descend into the atmosphere along the field lines, they pick up more energy. When they hit the ionosphere region of the Earth's upper atmosphere, they collide with ions of oxygen and nitrogen. The particles impact the oxygen and nitrogen ions and transfer their energy to these ions. The absorption of energy by oxygen and nitrogen ions causes electrons within them to become "excited" and move from low-energy to high-energy orbitals (see How Atoms Work). When the excited ions relax, the electrons in the oxygen and nitrogen atoms return to their original orbitals. In the process, they re-radiate the energy in the form of light. This light makes up the aurora, and the different colors come from light radiated from different ions. Note: The particles that interact with the oxygen and nitrogen ions in the atmosphere don't come from the sun, but rather were already trapped by the Earth's magnetic field. The solar winds and flares perturb the magnetic field and set these particles within the magnetosphere in motion.

The short answer to how the aurora happens is that energetic electrically charged particles (mostly electrons) accelerate along the magnetic field lines into the upper atmosphere, where they collide with gas atoms, causing the atoms to give off light.