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HomeEducationalSpace WeatherThe AuroraCharacterising the Aurora Tuesday, Dec 12 2017 12:17 UT
Space Weather

Characterising the Aurora

The aurora are the result of light generated by ionisation in the upper atmosphere caused by electrons from the Sun injected into the Earth's magnetosphere. The aurora can occur at an altitude of 70 to 600 km, but most aurora occur from 100 to 300 km. They may be observed from the ground or from space.

Sun
Photo: John Hicks - New Forest Solar Observatory

The aurora is an extremely variable phenomenon and occurs in a wide variety of forms. Ground observers use single letters to first specify shape and structure. These descriptors are fairly self-explanatory.

Shape Structure
A - arc D - diffuse
B - band F - glowing
C - corona H - homogeneous
D - drapery P - pulsating
G - glow R - rayed
R - rays
S - surface

Both the shape and the form descriptors are then combined to describe a specific auroral form. An atlas of auroral forms is a great help in serious auroral observing.

Forms
HA - homogenous arc
RB - rayed bands
PA - pulsating arcs
FC - glowing corona

A ground observer will also specify the brightness of an aurora either by measurement with a photometer (the unit used is the "Rayleigh"), or more frequently by estimation according to the table below.

IBC kiloRayleigh Description
I 1 Faint, brightness of the Milky Way. No colour apparent.
II 10 Brightness of thin moonlit cirrus cloud.
III 100 Brightness of moonlit cumulus cloud.
IV 1000 Bright as the full moon. Casts shadows. Very rare.

IBC = International Brightness Coefficient

A satellite can also be used to measure the integrated brightness or activity of the aurora by measuring the electron power input to the polar region. The brightness of an aurora is proportional to the precipitating electron flux (with energy < 10 keV), which may be up to 1017 electrons per square metre per second for a very bright aurora. The power associated with this flux is measured in gigawatts (GW, 109 W = 1 GW) according to the table below.

Activity Index Power (GW)
1 < 2.5
2 2.5 - 4
3 4-6
4 6-10
5 10-16
6 16-24
7 24-39
8 39-61
9 61-96
10 >96

Material prepared by Richard Thompson

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