PANORAMA OF NATURE - An aurora is a natural light display in the sky (from the Latin word aurora, "sunrise" or the Roman goddess of dawn), predominantly seen in the high latitude (Arctic and Antarctic) regions.[nb 1] Aurorae are caused by cosmic rays, solar wind and magnetospheric plasma interacting with the upper atmosphere (thermosphere/exosphere). Their charged particles, mainly electrons and protons, enter the atmosphere from above causing ionization and excitation of atmospheric constituents, and consequent light emissions. Incident protons can also produce emissions as hydrogen atoms after gaining an electron from the atmosphere.
The altitudes where auroral emissions occur were revealed by Carl Størmer and his colleagues who used cameras to triangulate more than 12,000 auroras. They discovered that most of the light is produced between 90 and 150 km above the ground, while extending at times to more than 1000 km. Images of auroras are significantly more common today than in the past due to the increase in use of digital cameras that have high enough sensitivities.Film and digital exposure to auroral displays is fraught with difficulties, particularly if faithfulness of reproduction is an objective. Due to the different color spectrum present, and the temporal changes occurring during the exposure, the results are somewhat unpredictable. Different layers of the film emulsion respond differently to lower light levels, and choice of film can be very important. Longer exposures superimpose rapidly changing features, and often blanket the dynamic attribute of a display. Higher sensitivity creates issues with graininess. File:Aurora Timelapse.ogvPlay media Aurora timelapse video (40 minutes) Northern lights over Calgary
The aurora frequently appears 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 ("active aurora"), they evolve and change constantly. 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.
David Malin pioneered multiple exposure using multiple filters for astronomical photography, recombining the images in the laboratory to recreate the visual display more accurately.For scientific research, proxies are often used, such as ultra-violet, and color-correction to simulate the appearance to humans. Predictive techniques are also used, to indicate the extent of the display, a highly useful tool for aurora hunters.[14] Terrestrial features often find their way into aurora images, making them more accessible and more likely to be published by major websites.[15] It is possible to take excellent images with standard film (using ISO ratings between 100 and 400) and a single-lens reflex camera with full aperture, a fast lens (f1.4 50 mm, for example), and exposures between 10 and 30 seconds, depending on the aurora's brightness.
Auroras take many different visual forms. The most distinctive and brightest are the curtain-like auroral arcs. They eventually fragment or ‘break-up’ into separate, and 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. 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. Occasionally there is a fast, sub-second, flickering. A typical auroral display consists of these forms appearing in the above order throughout the night.
Visual forms and colors
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 aurorae.
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 its role here as well, 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.
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.
Blue: At yet lower altitudes, atomic oxygen is uncommon, and ionized molecular nitrogen takes over in producing visible light emission; it radiates 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.
Ultraviolet: Ultraviolet light from aurorae (within the optical window but not visible to virtually all humans) has been observed with the requisite equipment, and otherwise invisible aurorae of this type were produced on a very small scale by certain HAARP experiments. Ultraviolet aurorae have also been seen on Mars.
Infrared: Infrared light, in wavelengths that are within the optical window, is also part of many aurorae.
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