The Different Colors of Aurora
Luminous Moving Lights
Auroras are the luminous moving lights seen at the northern (aurora borealis) and southern (aurora australis) latitudes. Aurora is caused by the interaction between electrified charged particles and the molecules of gases in the upper atmosphere (ionosphere) of Earth. The different colors of aurora depend on the altitude, as well as the composition of the Earth´s atmosphere and the different levels of density at low and high altitudes.
Aurora is caused by the collisions between high energy particles (mostly electrons) and neutral atoms in the Earth´s upper atmosphere. The interaction among these particles creates the spectacular aurora displays at altitudes ranging from 80 km to about 500 km which is where aurora is most commonly seen.
When the charged particles from the solar wind reach the upper atmosphere at the Polar Regions, they collide with atmospheric atoms and ions. These collisions produce excitation in the atoms. As the excited atoms return to their original energy levels, they emit a photon of a distinct wavelength which is what produces the variety of aurora colors.
Aurora Borealis and Australis
Magnetic Storms can Increase Aurora Luminosity
High energetic activity can be produced during a magnetic storm, allowing the glow of auroras to be seen at lower latitudes of the globe. Magnetic storms are most commonly produced when the Sun´s activity reaches its solar maximum. During a solar maximum, the Sun´s activity increases, producing solar flares and solar wind which reaches the entire solar system.
The chance of an aurora occurring depends on the slant (Bz) of the interplanetary magnetic field lines. Geomagnetic storms known to produce auroras take place more often during the equinoxes when the interplanetary magnetic field and Earth´s magnetic field coincide, opening a path through which energetic particles from the solar wind can enter the inner magnetosphere surrounding the Earth.
Varied Colors at Different Altitudes
The atmosphere consists principally of oxygen and nitrogen molecules which emit visible light which is characteristic of their respective line spectra. Oxygen emits two colors: green and red, while nitrogen emits blue and deep red.
Oxygen is more abundant at altitudes above 300 km (186 miles). The collisions of oxygen with high energy particles, causes its electrons to become excited. As the electrons return to their original energy levels, they emit visible light. Oxygen can take less than a second to emit a green photon or up to two minutes to emit a red photon.
Collisions among atoms and molecules can absorb the excitation energy preventing the emission of photons; however, since oxygen is abundant and is sparsely distributed at upper altitudes, collisions don´t occur often, allowing adequate time for the emission of red photons.
Intense Aurora Activity
Collisions among atoms and molecules become more frequent descending down the atmosphere where the density of atmospheric particles is increased. Gradually, red emissions don´t occur at lower altitudes and eventually even green light emissions are interrupted.
At altitudes between 180 km (112 miles) to 300 km (186 miles), the strong yellow-green lights are produced by the combinations of green light emissions from oxygen and blue and red emissions from nitrogen. At these denser altitudes, oxygen has lost its capability to emit red photons
Below 120 km (74 miles) emissions of blue light from ionized nitrogen and red from excited nitrogen create the intense purple edge of curtains. At these lower altitudes, oxygen atoms don´t have a chance to emit neither green nor red light; the increased density of the atmosphere does not permit them to emit any light. aurora
Facts about Auroras
Auroras are the real evidence that the interaction between the Sun and the Earth is more than just sunlight.
Auroras are known to exist in other planets which have magnetic fields, including Jupiter, Saturn, Uranus and Neptune, as well as Venus and Mars; Io, Europa and Ganymede have also been observed to have auroras.
Nasa´s polar spacecraft takes images from aurora and studies Earth´s interaction with the solar wind.