Illustrations | Key details | Aurora Code | Sample Report
Typical details of the visible aurora are given in the following illustrations. The code for describing the form and behaviour of the aurora together with methods of measuring and recording the activity on a standard form are detailed below.
Typical auroral forms and structures
 |
Aurora Borealis means 'northern dawn'. This name was given because from middle latitudes it most frequently appears as a dawn-like GLOW (N) along the northern horizon. The word 'glow' is to be used only when the auroral light is down on the horizon. |
 |
Sometimes there are patches of auroral light without distinct boundaries, in clear sky (so that it is not the effect of obscuring clouds) and well up from the horizon (so that it is not a glow). These forms are called PATCHES (HP). |
 |
Aurora often takes the form of an ARC (A) extending east-west across the sky. There is an area of clear below the lower edge, which is usually more clearly defined than the upper edge. When an arc has no vertical ray structure it is called a HOMOGENEOUS ARC (HA). |
 |
Sometimes the auroral light forms a BAND (B) without the regular shape of an arc. If there is no ray-structure in it this is called a HOMOGENEOUS BAND (HB) |
 |
When an arc shows vertical ray-structure it is called a RAYED ARC (RA). This form usually exhibits moderate activity, that is, small movements and irregular brightness variations. |
 |
When a band shows ray-structure it is called a RAYED BAND (RB). If the rays are long it may resemble a curtain or drapery waving in the sky. |
 |
A common form assumed by the auroral light is that of a RAY (R), which is like a searchlight beam, usually nearly vertical. Rays may occur singly or in bundles. |
 |
When rays or other forms pass overhead, perspective causes them to to converge to a point to form a crown or CORONA (c). |
| Any
of these forms may disappear and reappear
rhythmically at the same place with a period of
anything from 10 to 100 seconds. This kind of
activity is called PULSATING (p1). The symbols
p1RA, p1HP, etc., are used. Another form of activity called FLAMING (p2) occurs usually after a vigorous display of bright forms. It consists of waves of light moving rapidly upwards towards the zenith, one after the other, lighting up existing forms as it passes over them. |
|
| Top Of Page |
The beginner need only confine himself or herself to noting its location, the double date of the evening and morning of the night concerned (e.g. 18/19th November 1982), the 24 hour clock time (noting U.T. or B.S.T.) and a description of the aurora in writing with or without sketches. More experienced observers should try to observe each change in form and make notes in aurora code at about 5 minute intervals or at the times of change.
Magnetic midnight takes place at about 22:00 hrs U.T. in the United Kingdom so that it is useful to scan the sky nightly at this time when the probability of seeing an aurora is at its highest. Aurorae are more likely to be seen at the equinoxes but for statistical purposes it is desirable to keep an all year round watch for activity. The frequency of the aurora tends to follow the sunspot cycle but peak activity may appear a year or so after sunspot maximum.
In order to measure the position of the auroral forms an observer may make for himself a simple theodolite, or may obtain the azimuths of horizon features from a map, or may convert from the declination and right ascension of the stars in the background. Angles in the sky can be estimated by using the hand at arm's length as a measure : the distance across the fingers is about 10 degrees. This can be checked by counting hand's-breadths from the horizon to the zenith or round the horizon. (Each observer will have his own best measure - across the finger tips or across the knuckles - depending on the length of his arm and on~how he chooses to hold his hand). There is a tendency for observers to over-estimate angles of altitude if an instrument is not used. A meteorological hand held cloud alidade can be used for height measurement, and these are simple to make at home.
| Top Of Page |
The above Diagram illustrates
the key angular measurements required to define
the position of an auroral structure: h
= the base height in degrees above the horizon; h = 10 degrees.
azimuth = 300-060 |
=
the maximum height in degrees above the horizon; azimuth
= the horizontal extent across the horizon from
west to east. The above example shows a rayed arc
(RA) with:In a large and active display it is generally impractical to attempt a complete description of all the forms and types of activity observed; what can be useful is an indication of the time of greatest activity, greatest extent or greatest brightness.
The intensity of auroral light is often too low to stimulate the colour-sensitive parts of the eye and auroral forms may have the grey-white appearance of cloud lit by weak moonlight But at higher intensities (or to a well dark adapted eye) aurora exhibits a variety of colours, mainly greens and deep reds. Look especially for red rays and patches above or to the side of the main display and for red borders to the lower edges of rays, bands and arcs. Interference filters passing the auroral emission colour green of 5577 angstroms may be used to search for auroral light in cloudy, moonlight, hazy or town lit conditions. Such filters are expensive to buy.
| Top Of Page |
Aurora Code
(abbreviations used to
record and report auroral activity)
Aurora code offers a quick and convenient means of recording the condition, structure, form, brightness and colour of a display:
| Top Of Page |
Aurora code is used in the following format: condition|qualifying|structure|form|brightness|colour
a3|cf|R|B|3|c Is a fragment of white, green or yellow rayed band as bright as moonlit cirrus cloud, forming a corona in the magnetic zenith. Confusion between corona (c) and colour class (c) is defined by the columns in which the code letters are entered in the report forms. |
Auroral activity observed should be recorded using the above code on a report sheet drawn up in accordance with the sample provided below. The observer should enter a new line of data whenever the aurora is seen to change in form or activity. It may be necessary to fill in more than one line if more than one form is active at the same time. If in doubt, draw a sketch on which is written the necessary data:
| Top Of Page |