Aurora : The Magnificent Phenomena from Nature

An aurora (Latin word mean “sunrise”) is natural light display in the sky in the poles (north and south) caused by the collision of energetic charged particles with atoms in the high altitude atmosphere. In the northern latitude, the effect is known as aurora borealis (aurora: the goddess dawn, borealis: north wind). While in the Antarctic region, the effect is known as aurora australis. They illuminate the northern and southern horizon as greenish glow or sometimes faint red, occasionally blue but most often in fluorescent green. Auroras can be seen at night because their light is not as strong as the light of the day. 

The aurora phenomenon occurs when the sun produces solar wind. Solar wind is a stream of electrons and protons which are released from the sun due to the high kinetic energy. These particles are charged and contain energy, which means they contribute to electricity. In other way, our planet is surrounded by a super-sized magnetic sheath which is usually called the Magnetic Field of the Earth. Sometimes this solar wind hits the Earth. Some of these charged particles lead to the poles of the earth at a speed that keeps growing. The collision between these particles and atoms present in the earth’s atmosphere, it releases the energy that causes the formation of colorful auroras at the poles of the earth, which looks like a big circle around the pole.

Aurora mostly found on the earth’s poles because the north and South Pole magnetic field is very strong compared to other regions. The poles of this magnetic field pull the protons and electrons from the solar wind. So the phenomenon is more common in the Polar Regions.
However, sometimes the Aurora can also appear at the top of the mountain in a tropical climate, but this phenomenon is extremely rare. Aurora phenomena have been observed on other planets than Earth that have a magnetic field, such as Jupiter, Saturn and more recently Mars. It is believed to be a widespread phenomenon in the Solar System and beyond.

Auroras frequently appear either as a diffuse glow or as "curtains" that extend approximately in the east–west direction. At some times, they form "quiet arcs"; at others they evolve and change constantly. These are called "active aurora".
The most distinctive and brightest are the curtain-like auroral arcs. Each curtain consists of many parallel rays, each lined up with the local direction of the magnetic field, consistent with auroras being shaped by Earth's magnetic field. In-situ particle measurements confirm that auroral electrons are guided by the geomagnetic field, and spiral around them while moving toward Earth. The similarity of an auroral display to curtains is often enhanced by folds within the arcs. Arcs can fragment or 'break-up' into separate, at times rapidly changing, often rayed features that may fill the whole sky. These are the 'discrete' auroras, which are at times bright enough to read a newspaper by at night and can display rapid sub-second variations in intensity. The 'diffuse' aurora, on the other hand, is a relatively featureless glow sometimes close to the limit of visibility. It can be distinguished from moonlit clouds by the fact that stars can be seen undiminished through the glow. Diffuse auroras are often composed of patches whose brightness exhibits regular or near-regular pulsations. The pulsation period can be typically many seconds, so is not always obvious. Often there black aurora i.e. narrow regions in diffuse aurora with reduced luminosity. A typical auroral display consists of these forms appearing in the above order throughout the night.
  • Red: At the highest altitudes, excited atomic oxygen emits at 630.0 nm (red); low concentration of atoms and lower sensitivity of eyes at this wavelength make this color visible only under more intense solar activity. The low amount 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 630.0 nm (red) mode: rather the 557.7 nm emission (green) dominates. 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 high stability of the N2 molecule) plays a role here, as it can transfer energy by collision to 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 of 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, slow mechanism that is 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 takes 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.
  • Ultraviolet: Ultraviolet radiation from auroras (within the optical window but not visible to virtually all humans) has been observed with the requisite equipment. Ultraviolet auroras have also been seen on Mars, Jupiter and Saturn.
  • Infrared: Infrared radiation, in wavelengths that are within the optical window, is also part of many auroras. 
  • Yellow and pink are a mix of red and green or blue. Other shades of red as well as orange may be seen on rare occasions; yellow-green is moderately common. As red, green, and blue are the primary colours of additive synthesis of colours, in theory practically any colour might be possible but the ones mentioned in this article comprise a virtually exhaustive list.
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