High speed energetic particles collide with atoms in Earth's atmosphere at a height of anywhere from about 50 to a few hundred miles above Earth's surface to cause the aurora. These high speed particles, which are usually electrons, originate from space, specifically from the solar wind, blowing outward from the Sun.
When the electrons from space strike an atom or molecule in Earth's atmosphere, they give one of the electrons in the atom an energy boost. In scientific jargon, the electron jumps to a higher energy level and the atom is in an excited state. After a while, the electron in the excited atom jumps back down to its original lower energy level. It releases this energy as light causing the auroral glow. This process is the same mechanism that causes emission line spectra and aurora are in fact emission line spectra of the atoms in Earth's upper atmosphere.
The colour of emission line spectra depends on the chemical composition, and each type of atom produces its own unique pattern of colours. Hence, the different colours in auroral displays originate from different elements in Earth's atmosphere.
Oxygen molecules cause the green Aurora, and oxygen atoms cause the red colours. Blue auroral displays result from nitrogen molecules. Molecular nitrogen and oxygen are the most common constituents of Earth's atmosphere, so these are the most common auroral colours. Atomic oxygen occurs at high altitudes, so aurora usually has red above the green. Mixtures of these colours form other colours.
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.
As they 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 with 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 a photon, i.e. by making light.
The high-energy electrons and protons traveling down Earth's magnetic field lines collide with the atmosphere. The collisions can excite the atmospheric atom or molecule or they can strip the atmospheric species of its own electron and create an ion. The result is that the atmospheric atoms and molecules are excited to higher energy states. They relinquish this energy in the form of light upon returning to their initial, lower energy state. The particular colours we see in an auroral display depend on the specific atmospheric gas struck by energetic particles, and the energy level to which it is excited. The two main atmospheric gases involved in the production of auroral lights are oxygen and nitrogen:
· Oxygen is responsible for two primary auroral colours: green-yellow wavelength of 557.7 nanometres (nm) is most common, while the deep red 630.0 nm light is seen less frequently.
· Nitrogen in an ionized state will produce blue light, while neutral nitrogen molecules create purplish-red auroral colours. For example, nitrogen is often responsible for the purplish-red lower borders and rippled edges of the aurora.
The process is similar to the lights that illuminate a neon light or computer and TV screens. In a neon light, neon gas is excited by electrical currents. Likewise, in a picture or computer screen, a beam of electrons controlled by electric and magnetic fields strike the screen, making it glow in different colours, according to the type of chemicals (phosphors) that coat the screen.
Auroras typically occur between 95 and 1,000 km. Auroras stay above 95 km because at that altitude the atmosphere is so dense (and the auroral particles collide so often) that they finally come to rest at this altitude. On the other hand, auroras typically do not reach higher than 500-1,000 km because at that altitude the atmosphere is too thin to cause a significant number of collisions with the incoming particles.
