Aurora Morphology (Structure)

Anyone who has witnessed one of nature's greatest shows, the northern lights, one is forever captivated by its beauty and mystique. It's rarely seen in latitudes south of 50 degrees North, but when it does appearwhen it does appear, it usually appears a deep red or with diffuse structure.  However, over much of Canada and Alaska the aurora is quite distinct and never looks or behaves the same.

Aurora's brightness can vary from the faintest hints of the Milky Way to an illumination bright enough to cast shadows and change snow cover to a vivid green! Its structure can go from a diffuse homogeneous glow to one with discrete sharp boundaries and complex internal structure known as rays that zip across the sky in seconds. Its color, which is mostly green, can have red bottoms or tops or be entirely pink or red. Purple aurora results from the tops of the displays that are lit by the sun (which is well below the horizon). Unlike clouds, most stars will be visible through all but the brightest aurora.

glow
Diffuse Glow

This form usually occurs after subatorm breakup (~1:30AM)

weak This form usually occurs before local midnight but can flair up almost any time

shadow
Discrete Strong

This form is most frequently seen within an hour of midnight


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Homogeneous Bands
bandQuiet bands are typical early forms

Red Bottom
red_bottom
Red on bottom of band is the result of low altitude nitrogen

Purple Top
purple_top
The purple-ish color is due to direct sunlight on aurora

While aurora is most frequent around local midnight, it can be viewed when the sun is 12 degrees below the horizon or lower. Generally, aurora starts in the East as a uniform smooth diffuse arc from horizon to horizon and as the night progresses, it rises and forms a discrete band that is irregular and filled with slightly converging brighter areas known as rays. As the display brightens, these bands will begin to fold onto themselves and form a curtain or drapery formation. If the display continues to intensify, the rays will converge to a vanishing point at the magnetic zenith (nearly overhead in Alaska) and form a corona. These displays are pretty rare and a definite trophy to the patient photographer. After the display dies down (sub-storm passes), a general glow or patchiness will persist across most of the sky and a strobing or pulsating effect between 0.1 second to 30 seconds in duration will be seen. The display cycle can always repeat itself over and over during the remainder of the night, so don't quit looking. Evening aurora move from east to west and reverse direction after midnight.

twilight
Twilight

arc
Arc

rayed_band
Rayed Band


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curtain
corona
pulse
Aurora in Moonlight

During extremely bright displays, snow will appear completely green or red and some claim to actually hear the lights emit a crackle or hissing sound. Since the lowest aurora is 60 miles high where there is no air for sound to travel, what might explain this sound has more to do with electrical static discharges on the ground induced by the aurora. I have viewed over 1000 hours under the northern lights without hearing anything unusual. However, as the lights dance across the sky, my mind puts these movements into music. It's like watching fireworks from a distance and hearing the boom before the sound reaches you. It's an anticipation of sorts. My eyes (and brain) are telling me that the aurora should be producing more than just a light show.
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