What Makes the Color of the Aurora Borealis

Aurora borealis is a colored display of lights in the Earth´s magnetic poles. The spectacular display of colored light (ribbons, curtains, rays and spots) is most commonly appreciated in the North (aurora Polaris) and South (aurora australis) poles. Aurora lights are created by the interaction of energetic particles (solar wind) emitted from the Sun with atoms of oxygen and nitrogen found in the high altitude Earth´s atmosphere. The following is an explanation of what makes the color of the aurora borealis.

The origin of aurora begins at the Sun when great amounts of energy are created at the core of the Sun by a process known as fusion. The released energy at the core of the Sun radiates out through convection cells (granules).  The electrical currents of charged gas (plasma) create magnetic fields inside the Sun strong enough to erupt out to the surface of the Sun, producing solar flares and coronal mass storms, and sending millions of tons of plasma out into interplanetary space.

After approximately 1 to 2 days, solar plasma reaches the magnetosphere (boundary where Earth´s magnetic field and the solar wind interact), where the charged particles of the solar wind are deflected into the upper atmosphere (ionosphere) over the Earth´s poles. At altitudes between 100 km (62 miles) and 500 km (310 miles) curtain shaped-like auroras are produced, causing a multicolored display of moving lights.

What makes the color of the aurora borealis is the emissions of photons from atoms returning from an excited state to ground state. The various light emissions depend on the composition and density of the atmosphere, as well as the altitude where the electrically charged particles collide with the atoms. At higher altitudes oxygen predominates, but at lower altitudes, the atmosphere becomes thicker with oxygen and nitrogen molecules, allowing more frequent collisions among molecules.

The color of aurora is determined by which gas (oxygen or nitrogen) is being excited by the collisions generated at different altitudes. Oxygen most commonly emits red or green light, while nitrogen generally emits blue/red light. The combination of these colors creates the other colors which are characteristic of aurora (pink, purple and white).

Oxygen is more common in the upper atmosphere and is sparsely distributed. After oxygen is excited by a charged particle, it takes ¾ of a second to emit green light and up to two minutes to emit red light; however, collisions with other molecules can prevent the emission of these photons by absorption of the energy. Because oxygen is sparsely distributed in the upper atmosphere, collisions are rare; permitting the emission of red photons which is what gives the characteristic color of the upper red curtains of aurora.

At lower denser latitudes, the emission of red, green and blue photons by oxygen and nitrogen respectively are combined to create the green and white curtains of light. At the lowest latitudes, the higher density of the atmosphere prevents the oxygen atoms to emit any kind of photons, allowing nitrogen to emit photons in the red and blue spectrum, leading to the creation the purple curtains of light characteristic of the lower fringes of aurora curtains.

Lighter molecules of hydrogen and helium present in the upper ionosphere produce blue and purple colors, but our eyes cannot perceive them; however, high quality photographic film can be more sensitive to those type of emissions. the human eye is better adapted to perceive in the green-yellow-orange-red part of the spectrum.