Observing The Visible Aurora:
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 |


Key Details

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; = 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:

h = 10 degrees.

= 55 degrees

azimuth = 300-060


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:

Code (Condition) Definition
q Quiet. No Movement
a Active
a1 Folding of Bands
a2 Rapid Change of Shape of Lower Form
a3 Rapid Horizontal Movement of Rays
a4 Forms fade quickly to be replaced by others
p1 Pulsing. Rythmic change of form as a whole
p2 Flaming. Variations moving upwards
p3 Flickering. Rapid irregular variations
p4 Streaming. Irregular horizontal variations in homogenious forms.
Code (Qualifying Symbol) Definition
m Multiple. Several groups of forms
f Fragmentary. A part only of an auroral form
c Coronal. Rays converging
Code (Structure) Definition
H Homogeneous. Uniform in shape and intensity
S Striated. Lines of brighter and darker light
R1 Rayed. Short Rays
R2 Rayed. Medium Length Rays
R3 Rayed. Long Rays
Code (Forms) Definition
G Glow. Brightness from horizon upwards.
A ARC. Uniformly curved arch of light.
RA Rayed ARC from which rays appear.
R Ray. A vertical shaft
B Band. Twisted ARC
V Veil. Indefinite
P Patch. Isolated "cloud " of aurora.
N Undefined. Does not identify with other forms.
Code (Brightness) Definition
1 Weak. Barely visible
2 Bright as moonlit cirrus cloud
3 Bright as moonlit cumulus cloud
4 Strong. Bright enough to cast shadow
Code (Colour) Definition
a Red Only in upper portion of form
b Red Lower border only
c White Green or Yellow
d Red
e Red and Green
f Blue or Purple

| Top Of Page |

Aurora code is used in the following format:

condition|qualifying|structure|form|brightness|colour

Example:

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:

British Astronomical Association - Aurora Section

- Report Form -

Report For: Year Month Night Observer Location
2000 April 06-07 J. Smith Paisley, SCOTLAND.
Lat. N55.55' Long. W04.45'
Date Time
(UT)

Cond-
ition

Qual.-
Symbol

Struc-
ture
Form Bright-
ness
Colour Altitude azimuth Notes & Sketches
h

06

2235

a3

cf

R3

B

2

c

15

90

340-010

 
 

2300

q

 

H

N

2

c

 

5

330-040

 
 

2310

q

 

H

A

2

c

5

9

340-020

 
 

2320

p2

 

R2

B

4

c

15

50

320-040

 
 

2325

a4

f

H

P

3

d

20

40

020-040

 
 

2335

q

 

H

A

1

c

6

12

335-025

 
 

2345

q

 

H

N

1

c

 

5

330-040

 

07

0004

q

 

H

B

2

c

6

12

335-025

 
                       
                       
                       
                       

| Top Of Page |