Aurora

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Aurora

The sporadic radiant emission from the upper atmosphere over middle and high latitudes. It is believed to be due primarily to the emission from nitrogen - atomic N I and N II, molecular, N₂, and ionic N₂+; atomic oxygen (O I and O II); atomic sodium (Na I); the hydroxyl radical (OH); and hydrogen. Compare

airglow.
According to various theories, auroras seem definitely to be related to magnetic storms and the influx of charged particles from the sun. The exact details of the nature of the mechanisms involved are still being investigated, but release of trapped particles from Van Allen belt apparently plays an important part. The aurora is most intense at times of magnetic storms (when it is also observed farthest equatorward), and shows a periodicity which is related to the sun's 27-day rotation period and the 11-year sunspot cycle. The distribution with height shows a pronounced maximum near 100 kilometers. The lower limit is probably near 80 kilometers.
The aurora can often be clearly seen, and it assumes a variety of shapes and colors which are characteristic patterns of auroral emission.
The following is the general classification and abbreviations of the forms of the auroras adopted by the International Union of Geodesy and Geophysics in 1930 for reporting of visual observations. The classification was modified slightly and expanded in 1963. The new classification is described in the , Aldine Pub. Co., Chicago, 1963.

I. Forms without ray structure:
HA (abbr for homogeneous quiet area). These can appear near the horizon, and between the arc and the horizon a dark segment is often seen. These arcs can be narrow or broad, and are very often diffuse along the upper border but sharp along the lower one.
HB (abbr for homogeneous bands). These forms do not have the regular shape of the arcs; they are more rapidly moving phenomena. The lower border is often irregular and sharp. The breadth can vary from a very narrow band to a band which is so large that it resembles a curtain hanging down. These bands very often turn into bands with ray structure.

PA (abbr for pulsating arcs). Parts of an arc flash up and disappear regularly within a period of about 20 seconds. This form quite often stands isolated in the sky without other auroras.
DS (abbr for diffuse luminous surfaces). These either appear like a diffuse veil or glow over great parts of the heavens without distinct boundaries, often appearing after intense displays of rays and curtains, or as more isolated feeble luminous streaks which sometimes bear a striking resemblance to clouds. Sometimes large areas of the heavens can be discolored by a green, violet, or red diffuse light.
PS (abbr for pulsating surfaces). Diffuse patches appear and disappear rhythmically at the same place, retaining the same irregular shape, When the patches are lying near the magnetic zenith the contours can be more sharp, and form a sort of corona. These forms appear often in connection with flaming auroras.
G (abbr for feeble glow near the horizon resembling the dawn). Of white or redlike color, this form is often the upper part of an arc whose lower border is below the horizon.

II. Forms with ray structure: These forms consist of short or long rays which can be arranged in different ways.
RA (abbr for arcs with ray structure). A homogeneous arc which has remained quiet and unaltered for a rather long time may become sharp and luminous along the lower border and they very rapidly change into an arc of rays. The rays can be short or long.
RBI (abbr for bands with ray structure). These resemble the bands mentioned under HB but are constituted of a series of rays which are arranged close to each other along the band, or they can appear more scattered. Often a series of parallel bands appear. When a band is near the magnetic zenith is may have the form of a corona.
D (abbr for draperies). If the ray become very long the band appears like a curtain or drapery whose lower border is often more luminous. Several parallel curtains frequently appear at the same time. Near the zenith the curtain may have a fanlike form on account of the perspective.
R (abbr for rays). The rays can be isolated, narrow or broad, short or long. They may appear in great segments or like masses or rays, very often resembling curtains.
C (abbr for corona). When the rays approach the magnetic zenith they seem, on account of the perspective, to converge to this point and form a corona. This may be formed by long rays or by short ones, it may be completed or incomplete. A corona can also be formed by bands, draperies, or more diffuse forms near the magnetic zenith.

III. Flaming auroras (abbr F). A characteristic, rapidly moving form, consisting of strong waves of light which move upwards, one after the other, in the direction of the magnetic zenith. The waves have the form of detached arcs which move upwards normally to the direction of the arc; they can be compared to invisible waves illuminating broad rays and patches which appear and disappear rhythmically when the waves pass them. The flaming aurora frequently appears after strong displays of rays and curtains and is often followed by the formation of a corona.
For more information,about auroras, visit Auroras: Paintings in the Sky.

References

This article is based on NASA's Dictionary of Technical Terms for Aerospace Use