Shapes and Forms

The variety of shapes, colors, and movements of auroras are infinite. Like snowflakes, no two are alike. Faced with such a bewildering variety, early scientists tried to take control of the situation by classifying auroras into types and subtypes. These often unwieldy classifications may be found in the International Auroral Atlas (1963). For example, "a3p2mR2B3e" would describe a bright red and green multiple-rayed band, with medium-length rays showing horizontal movement and flaming activity!

In 25 years of auroral research, I do not recall ever having seen such classifications used in any productive scientific way. Indeed, the days of useful amateur visual auroral observations are probably past. Auroral research is much more sophisticated now and relies on low-light-level imaging or interferometric systems (often deployed in arrays covering a range of latitudes), together with simultaneous observations carried out by ground-based magnetometers and radars as well as instruments on satellites.

Nevertheless, a simplified vocabulary of auroral types is good to have. Here are what I consider useful descriptive terms for auroral forms, structures, motions, and brightnesses.

Forms. Auroras come in five basic shapes.
1. Arcs are simple, slightly curved bands of light with smooth bottom edges.
2. Bands have continuous but irregular lower edges characterized by kinks or folds.
3. Patches are small, isolated regions of glow.
4. Veils are extensive, uniform glows covering an appreciable fraction of the sky.
5. Rays are shafts of light. They follow the Earth's magnetic field lines and tend to extend vertically.

Structure. The five forms above show one of three types of internal structure.
1. Homogeneous, or smooth.
2. Striated, made of irregular fine lines or filaments.
3. Rayed, made of definite rays larger than filaments.

Two especially striking combinations occur frequently and deserve special mention. A curtain (or drapery) is a rayed band looped in dramatic folds. A corona is a group of rays that appears to converge on a point high in the sky. This is a perspective effect caused when we look directly up along a bundle of parallel magnetic field lines.

Temporal Behavior and Brightness

Temporal Behavior. An aurora may be motionless, or it may change or move.
1. Quiet means steady for a fairly long time, with only gradual, nonrepeating changes.
2. Pulsating denotes brightness increasing and decreasing more or less periodically.
3. Flickering refers to fast pulsating (say 5 or 10 times per second, up to a speed the human eye can no longer resolve).
4. Flaming describes bursts of luminosity appearing at the base of a form and rushing upward.

Brightness. Auroras have a wide range of brightness, usually rated from 0 to 4 on a logarithmic scale (the International Brightness Coefficient, or IBC) in which each number is 10 times brighter than the previous one.
Zero means subvisual, detected only with instruments.
One is comparable to the Milky Way; whitish with no discernible color.
Two is comparable to moonlit cirrus clouds; color is barely identifiable (usually yellow-green).
Three is like brightly lit cirrus or moonlit cumulus clouds; colors are evident.
Four is much brighter than three; may cast discernible shadows; good colors, many of them fleeting.

If you can see any color, the aurora is at least IBC 2. When it's hard to see bright stars through an aurora, it's IBC 3 or brighter.

These terms will help you recognize, describe, and understand what you see. Although the maximum of the current sunspot cycle has passed, there will still be numerous opportunities to see fine displays of the northern (and southern) lights for some time to come. Although still an inexact science, aurora predictions are available on a number of Web sites. So make the most of the opportunity, and good luck!