A Taxonomy of Auroras

I have been socked in under a cloud cover for about a week, admiring
the many photographs being posted on the web of auroras that resulting
from the recent geomagnetic storm. Their beauty, color and diversity
got me thinking about the classification of auroras by color, form and
brightness. I did some reading and this post is a summary of what I
learned, principally from two sources:

Davis, Neil. 1992. The Aurora Watcher's Handbook. Univ. of Alaska
Press. Sections 4.6 through 5.6 and 7.3

RASC. 2005. Observer's Handbook. pp. 108.

To illustrate the various types of aurora by color and form, web links
to some of the recent aurora photographs are included. I hope the
following vocabulary of the types of auroras will facilitate
descriptions used in the future postings by other members of this
Usenet group.

(I hope this vocabulary is right. My observing point is only 41N and I
do not see auroras often. If it is not, corrections are welcomed and
appreciated.)

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A Taxonomy of Auroras
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1.0 A color taxonomy of auroras

1.1 What causes the main auroral colors - greenish-yellow, red,
blue-purple and grey?

Auroral colors, typically a greenish-yellow and/or red, are caused by
high energy particles colliding with various layers of atmosphere that
contain differing percentages of oxygen and nitrogen:

0km to 100km: a mixture of 78% N_2 and 21% O_2 dominates
100km to 200km: Nitrogen dominates
200km to 500km: Oxygen dominates
500km+: H and He dominate, but oxygen is present

When exposed to a weak beam of high energy particles, electrons
around oxygen and nitrogen atoms are excited one energy level and they
emit red light. When hit by higher energy beams, oxygen is excited
two energy levels and emits green light (at 558nm). Weakly excited
nitrogen also emits green light (at 391nm to 470nm). When strongly
excited, the nitrogen_2 molecule emits a strong red light (at 630nm
and between 650-680nm). Secondary resonance excitation of nitrogen_2
weakly emits blue-purple light (at 391nm and 428nm).

Some photographs recently posted on the web (11/2004) from a
geomagnetic storm show all three of these colors in one frame:

Anthony Arrigo, Park City, Utah, Nov. 7, from www.utahskies.org
Glow - red and purple on top, green on bottom
http://www.utahskies.org/image_libra...041107-2sm.jpg

George Nov. 7 Montreal
Rayed arc - green arc, purple at the very top, green in the middle,
red on bottom, seen directly from underneath the sheet, sheet at
zenith
http://www.cam.org/~georgea/7_11_2004_16mod.jpg
http://www.cam.org/~georgea/7_11_2004_11mod.jpg
http://www.cam.org/~georgea/aurora2004.htm

Grey aurora are also aurora colored green, red and/or blue-purple.
They are either so far away or so faint that they do not trigger the
color receptors in your eye. So you so them in black and white.

1.2 Color Type D - All red aurora and red glow

A homogeneous but relatively weak beam of high energy particles
penetrates the upper 500km+ layer dominated by H, He and O. Oxygen is
weakly excited. A red sky glow results.

Some photographs posted on the web (11/2004) from the recent
geomagnetic storm that illustrate a Type D color aurora include:

Brian A. Klimowski,Flagstaff, Arizona Nov. 09 from Spaceweather.com
Glow - red
http://spaceweather.com/aurora/image.../Klimowski.jpg

Larry Koehn, Nashville, Tennessee, USA Nov. 07 from Spaceweather.com
Glow - red (with some raying)
http://spaceweather.com/aurora/image...04d/Koehn3.jpg

1.3 Color Type C - All green aurora

A homogeneous higher energy stream penetrates to the 100-200km layer.
Oxygen becomes highly excited and dominates over nitrogen.

Some photographs posted on the web (11/2004) from the recent
geomagnetic storm that illustrate at Type C color aurora include:

Carlos Moreno Nov 9 Montreal Canada
Rayed arc - all green - side angle view - classic veil sheet
http://www.mochima.com/personal/aurora_2004-11-09/

Daniel Tardif, Beauport, Quebec Canada Nov. 10 from Spaceweather.com
Homogeneous arc (nearly all green, with some rays, one small red ray)
- side-view green drapery
http://spaceweather.com/aurora/image...4g/TARDIF1.jpg

Vic & Jen Winter, Warrensburg, Missouri, USA - November 7, 2004 from
Spaceweather.com
Homogeneous arc overhead - all green (with red at the horizon) - arc
seen going from east-west horizon
http://spaceweather.com/aurora/image...04h/Winter.jpg

1.4 Color Type A - Green on bottom, red shading at top

A mixed stream of particles consisting of some high energy and some
relatively weaker energy particles penetrates the atmosphere. The
higher energy particles reach the lower 100-200km layer. The weaker
beam only penetrates higher layers of the atmosphere. A Type C color
all-green aurora is produced in the lower layer, a Type D all-red
color aurora is produced higher up.

Some photographs posted on the web (11/2004) from the recent
geomagnetic storm that illustrate at Type A color aurora (green on the
bottom, red on the top) include:

Bob Sandy and Carmen Lang, Roanoke County, VA Nov. 07 from
Spaceweather.com
Glow - red on top, green on bottom (with some raying)
http://spaceweather.com/aurora/image...04b/Sandy3.jpg

Dominic Cantin, Ile d'Orléans , Quebec city Nov. 10 from
Spaceweather.com
Rayed arc - red on top, green on bottom
http://spaceweather.com/aurora/image...04g/Cantin.jpg

Tdcarls ) Nov. 7 Canada
Rayed arc - red on top, green on bottom
http://members.aol.com/tdcarls/nov7-2.jpg

1.5 Color Type B Green on top, red on the bottom

A very high energy beam penetrates the lowest layer of the atmosphere
below 100km, to perhaps as low as 70km. Above 100km a Type C
all-green aurora is created. Below 100km, nitrogen_2 is the dominate
gas. When excited, the nitrogen molecules emit strongly in the red.

Some photographs posted on the web (11/2004) from the recent
geomagnetic storm that possibly might illustrate Type B colored aurora
(green on the top, red on the bottom) include:

Scott Beason, Cedar Falls, IA, USA Nov. 07 from Spaceweather.com
Rayed arc - maybe green on top, red at bottom
http://spaceweather.com/aurora/image...4b/Beason1.jpg

Brian Whittaker, Airborne at 36,0000 feet between London, England and
New York, Nov. 08 from Spaceweather.com
Flames - maybe green on top, red on bottom
http://spaceweather.com/aurora/image...Whittaker1.jpg

1.6 Color Type E Green on top, red on the bottom - but fast moving

Color Type E is simply a Type B aurora where the colors are rapidly
moving along the horizon.

The very same high energy streams that penetrate the lowest level of
the earth's atmosphere and cause Type B aurora, can also generate
"walking flames" of red and green color.

No examples.

1.7 Color Type F - Blue-purple colored sunlit aurora

These colors are typically seen in twilight skies at sunrise and
sunset. They result from an secondary excitation of nitrogen
molecules. The incoming high-energy particles ionize nitrogen
molecules. Then the low-angle sunlight in the evening or morning
twilight further excites the ionized nitrogen molecules and those
molecules emit a blue-purple light.

John Chumack, near Dayton, Ohio Nov. 09 from Spaceweather.com
Glow - purple
http://spaceweather.com/aurora/image...f/Chumack2.jpg

Dr Russell Cockman, near Melbourne, Australia Nov. 10 from
Spaceweather.com
Glow - purple
http://spaceweather.com/aurora/image...g/Cockman2.jpg

1.8 Grey aurora - no color type letter designated.

As noted above, grey aurora have color. They are either so far away or
so faint that they do not trigger the color receptors in your eye.

No examples.

2.0 A physical taxonomy of auroras

The following is a simple physical taxonomy for visual aurora
observers developed during the International Geophysical Year in
1957-1958.

The appearance of homogenous or rayed arcs depends on your angle of
observation. The same arc appears different if the observer sees it
from a side angle, from nearly underneath and from directly
underneath.

Examples from the recent geomagnetic storm are listed without comment.

2.1 Discrete auroras - auroras that have a definite edge or boundary

2.1.1 Homogeneous arcs

2.1.1.1 Seen from a side angle

Daniel Tardif, Beauport, Quebec Canada Nov. 10 from Spaceweather.com
Homogeneous arc (nearly all green, with some rays, one small red ray)
- side-view green veil wall
http://spaceweather.com/aurora/image...4g/TARDIF1.jpg

2.1.1.2 Seen from nearly underneath

No examples.

2.1.1.3 Seen from directly underneath

Vic & Jen Winter, Warrensburg, Missouri Nov. 7, 2004 from
Spaceweather.com
Homogeneous arc overhead - all green (with red at the horizon) - arc
seen going from east-west horizon
http://spaceweather.com/aurora/image...04h/Winter.jpg

2.1.2 Rayed arcs

2.1.2.1 Seen from a side angle

Carlos Moreno Nov 9 Montreal Canada
Rayed arc - all green - side angle view - classic veil sheet
http://www.mochima.com/personal/aurora_2004-11-09/

2.1.2.2 Seen nearly underneath

Tdcarls ) Nov. 7 Canada
Rayed arc - red on top, green on bottom
http://members.aol.com/tdcarls/nov7-2.jpg

2.1.2.3 Seen from directly underneath

William Biscorner, Memphis, Michigan Nov. 07 from Spaceweather.com
Rayed arc - View standing directly under a rayed arc, veil at zenith
http://spaceweather.com/aurora/image...Biscorner1.jpg

George Nov. 7 Montreal, Canada
Rayed arc - green arc, purple at the very top, green in the middle,
red on bottom, seen directly from underneath the sheet, sheet at
zenith
http://www.cam.org/~georgea/7_11_2004_16mod.jpg
http://www.cam.org/~georgea/7_11_2004_11mod.jpg
http://www.cam.org/~georgea/aurora2004.htm

2.1.3 Flames

Terry Lutz, Plymouth, Ohio Nov. 07 from Spaceweather.com
Flames - Red
http://spaceweather.com/aurora/image...v04c/Lutz1.jpg

Brian Whittaker, Airborne at 36,0000 feet between London, England and
New York, Nov. 08 from Spaceweather.com
Flames - Green on top, red on bottom
http://spaceweather.com/aurora/image...Whittaker1.jpg

Kurt A. Fisher 10/29/2003 Salt Lake City, Utah
Flames - Red on top, green on bottom
http://members.csolutions.net/fisher...CME486_001.JPG

2.1.4 Other adjectives used to describe arcs

Bands or draperies.

2.1.5 Rays alone

No examples.

2.1.6 Spots and patches

A steady spot or patch.

No examples.

3.0 Diffuse aurora

3.1 Glow

Brian A. Klimowski,Flagstaff, Arizona Nov. 09 from Spaceweather.com
Glow - red
http://spaceweather.com/aurora/image.../Klimowski.jpg

John Chumack, near Dayton, Ohio Nov. 09 from Spaceweather.com
Glow - purple
http://spaceweather.com/aurora/image...f/Chumack2.jpg

Dr Russell Cockman, near Melbourne, Australia Nov. 10 from
Spaceweather.com
Glow - purple
http://spaceweather.com/aurora/image...g/Cockman2.jpg

4.0 Pulsating aurora

4.1 Flaming aurora

One amazing animated gif posted on the web during the recent
geomagnetic storm shows these "walking flames":

Christopher Picking, Wairarapa, North Island New Zealand Nov. 08 from
Spaceweather.com
Flames - Animated gif of walking red flames
http://spaceweather.com/aurora/image...4c/Picking.gif

4.2 Fast auroral waves

These waves look like airport light beacons sweeping across the sky.

No examples.

I observed these pulsating waves on 10/29/2003 from Salt Lake City,
Utah. The onset preceeding an hour of rayed arcs was a series of white
light pulses sweeping along the northern horizon.

4.3 Pulsating arc

No examples.

4.4 Pulsating spot or patch

No examples.

5.0 A brightness taxonomy of auroras

Neil Davis' book suggests use of the International Brightness
Coefficient (IBC).

It's hard to tell what the actual brightness is of auroras seen in
recent web posted photographs, considering differing and unknown
photographic exposure times. Some rough illustrative examples are
listed, but the classifications are my own subjective guesses.

5.1 IBC I: Equal to the brightness of the Milky Way.

A recent example might be:

Brian A. Klimowski,Flagstaff, Arizona Nov. 09 from Spaceweather.com
Glow - red
http://spaceweather.com/aurora/image.../Klimowski.jpg

5.2 IBC II: Brightness equal to a thin moonlit cumulus cloud. 100x
times (2.5 mags) brighter than IBC I.

A recent example might be some of Carlos Moreno's pictures in the
middle of the event that he photographed:

Carlos Moreno Nov 9 Montreal Canada
Rayed arc - all green - side angle view - classic veil sheet
http://www.mochima.com/personal/aurora_2004-11-09/

5.3 IBC III: Brightness equal to full moonlit cumulus clouds. 100x
(2.5 mags) brighter than IBC II.

A recent example might be:

George Nov. 7 Montreal, Canada
Rayed arc - green arc, purple at the very top, green in the middle,
red on bottom, seen directly from underneath the sheet, sheet at
zenith
http://www.cam.org/~georgea/7_11_2004_11mod.jpg

5.4 IBC IV: Brightness equal to the full moon. 100x to 1000x brighter
than IBC III.

A recent example might be:

Daniel Tardif, Beauport, Quebec Canada Nov. 10 from Spaceweather.com
Homogeneous arc (nearly all green, with some rays, one small red ray)
- side-view green veil wall
http://spaceweather.com/aurora/image...4g/TARDIF1.jpg

Again, corrections to any of the above are welcomed and appreciated.

I would appreciate hearing from other group members about the "right"
way or other ways to classify and describe differing types of aurora.

Enjoy -

Canopus

P.S. - Copyright stuff. The web links in this post are for the
non-commercial purposes of comment, reporting on a newsworthy event,
and scholarship (self-study).