of atomic oxygen and higher eye sensitivity in green make green auroras the most common. The excited molecular nitrogen (atomic nitrogen being rare due An aurora (pl. aurorae or auroras) is a natural light display in Earth's upper atmosphere caused by charged particles from the Sun colliding with atoms in the atmosphere. These collisions excite oxygen and nitrogen, which then emit light of different colors such as green, red, and purple. When observed in high-latitude regions they are called polar lights and aurora polaris. In the Arctic they are Oxygen emissions green or orange-red, depending on the amount of energy absorbed. Nitrogen emissions blue, purple, or red; blue and purple if the molecule regains an electron after it has been ionized, red if returning to ground state from an excited state. Oxygen is unusual in terms of its return to ground state: it can take 0.7 seconds to emit the 557.7 nm green light and up to two minutes for the red 630.0 nm emission. Collisions with other atoms or molecules absorb the excitation energy and prevent emission; this process is called collisional quenching. Because the highest parts of the atmosphere contain a higher percentage of oxygen and lower particle densities, such collisions are rare enough to allow time for oxygen to emit red light. Collisions become more frequent progressing down into the atmosphere due to increasing density, so red emissions do not have time to happen, and eventually, even green light emissions are prevented. The change in auroral colour with altitude is therefore explained—oxygen red is predominant at high altitudes, followed by oxygen green and nitrogen blue/purple/red, then finally other hues of nitrogen blue/purple/red where particle collisions prevent oxygen from emissions. Green is the most common colour. Then comes pink, a mixture of light green and red, followed by pure red, then yellow (a mixture of red and green), and finally, pure blue. Precipitating protons generally produce optical An aurora (pl. aurorae or auroras) is a natural light display in Earth's upper atmosphere caused by charged particles from the Sun colliding with atoms in the atmosphere. These collisions excite oxygen and nitrogen, which then emit light of different colors such as green, red, and purple. When observed in high-latitude regions they are called polar lights and aurora polaris. In the Arctic they are called the northern lights or aurora borealis; in the Antarctic, the term southern lights or aurora australis is used. Auroras display dynamic patterns of radiant light that appear as curtains, rays, spirals or dynamic flickers covering the entire sky. These are also known as discrete auroras, which are at times bright enough to read a newspaper at night. These forms are consistent with auroras being shaped by Earth's magnetic field. The appearances of arcs, rays, curtains, and coronas are determined by the shapes of the luminous parts of the atmosphere and a viewer's position. === Colours and wavelengths of auroral light === Red: At its highest altitudes, excited atomic oxygen emits at 630 nm (red); low concentration of atoms and lower sensitivity of eyes at this wavelength make this colour visible only under more intense solar activity. The low number of oxygen atoms and their gradually diminishing concentration is responsible for the faint appearance of the top parts of the "curtains". Scarlet, crimson, and carmine are the most often seen hues of red for the auroras. Green: At lower altitudes, the more frequent collisions suppress the 630 nm (red) mode: rather the 557.7 nm emission (green) dominates. A fairly high concentration of atomic oxygen and higher eye sensitivity in green make green auroras the most common. The excited molecular nitrogen (atomic nitrogen being rare due to the high stability of the N2 molecule) plays a role here, as it can transfer energy by collision with an oxygen atom, which then radiates it away at the green wavelength. (Red and green can also mix together to produce pink or yellow hues.) The rapid decrease in concentration of atomic oxygen below about 100 km is responsible for the abrupt-looking end of the lower edges of the curtains. Both the 557.7 and 630.0 nm wavelengths correspond to forbidden transitions of atomic oxygen, a slow mechanism responsible for the graduality (0.7 s and 107 s respectively) of flaring and fading. Blue: At yet lower altitudes, atomic oxygen is uncommon, and molecular nitrogen and ionized molecular nitrogen take over in producing visible light emission, radiating at a large number of wavelengths in both red and blue parts of the spectrum, with 428 nm (blue) being dominant. Blue and purple emissions, typically at the lower edges of the "curtains", show up at the highest levels of solar activity. The molecular nitrogen transitions are much faster than the atomic oxygen ones. White/continuum: White auroral emission (often appearing mauve) has been observed in the colour spectrum of STEVE, and within the aurora in the oval and on the poleward edge . Nitrogen emissions blue, purple, or red; blue and purple if the molecule regains an electron after it has been ionized, red if returning to ground state from an excited state. Oxygen is unusual in terms of its return to ground state: it can take 0.7 seconds to emit the 557.7 nm green light Collisions become more frequent progressing down into the atmosphere due to increasing density, so red emissions do not have time to happen, and eventually, even green light emissions are prevented. The change in auroral colour with altitude is therefore explained—oxygen red is predominant at high altitudes, followed by oxygen green and nitrogen blue/purple/red, then finally other hues of nitrogen blue/purple/red where particle collisions prevent oxygen from emissions. Green is the most common colour. Then comes pink, a mixture of light green and red, followed by pure red, then yellow (a mixture of red and green), and finally, pure blue.