What Causes the Enigmatic Black Aurora Phenomenon?

[Ivan Seeking]

This article is provided as a public service by the Geophysical Institute, University of Alaska Fairbanks, in cooperation with the UAF research community. T. Neil Davis is a seismologist with the Institute.

The name "black aurora" was coined mostly in fun by graduate students and others at the Geophysical Institute, somewhat more than 20 years ago.

When making visual observations of the northern lights, these observers noticed that there sometimes occurred overhead in the sky what appeared to be close-spaced auroral forms with unusually black dark spaces between them. This situation did not occur often, but when it did, the auroral display seemed distinctive enough that people recognized it as special and commented about the apparent blackness of the sky between the auroras.

Being aware that the human eye and brain combination has a remarkable ability to distinguish between different shades of grey, the young observers thought that the apparent blackness between the auroras was not real, instead that it was an optical illusion. So as a joke aimed at themselves and their own limitations, and because the whole concept was obviously ridiculous, the students applied the name black aurora to this type of display. The word black, of course, referred to dark spaces between the real auroras.

It turned out, however, that these visual observers were seeing a real effect. In later years, new types of observations using various instruments revealed that the black auroras were more than an optical illusion. Most bright auroras are embedded in regions of less bright, perhaps barely visible aurora. Between these auroral forms the sky is brighter than the sky some distance away from the aurora, a fact that the careful observer can verify the next time he or she sees aurora overhead.

But, for reasons not yet known, there sometimes appear truly black voids between close-spaced auroras. In these voids there is an abnormal absence of auroral light, so the sky there looks and truly is abnormally black. So what started out as a joke has become an unexplained mystery.

When the mystery is explained, the new knowledge acquired by solving it may be comparatively trivial, or it might be quite important. That uncertainty adds spice to every unsolved problem, be it one in science or in everyday life

http://www.gi.alaska.edu/ScienceForum/ASF4/422.html

 

[Mk]

That was very interesting, but I was let down—there were no pictures.

 

[Ivan Seeking]

http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=29100

...According to the scientists, black aurorae are caused by holes in the ionosphere, where electrons are sped up and out of it by positively charged magnetic structures. This is exactly opposite of the process that creates visible aurorae.

"Now, with the aid of the four Cluster spacecraft , we have been able to study for the first time the complex physical processes that create these auroral holes," said Goran Marklund, a researcer at the Alfven Laboratory in Sweden.

Observations of the "anti-aurora" took place Jan. 14 in the Northern Hemisphere and Feb. 14 in its southern counterpart. The crafts were at a height of more than 13,422 miles (21,600 km), strung above the Earth like a string of pearls. [continued]

http://www.space.com/scienceastronomy/planetearth/black_aurora_011218.html

Note that the page linked may require a re-load. It takes two tries for me, for some reason.