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No. 127: Jan-Feb 2000

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Black Auroras

July 23/24, 1998. Gulf of St, Lawrence. Aboard the m.v. Appleby, Port Cartier to Immingham.

"The aurora borealis was sighted at 0330 UTC. There was a band measuring 35 long in azimuth, with deepblue almost black vertical bands lying between the altitudes of 15 and 25. The colour changed to brilliant blue at 0335, lasting for about 15 minutes before cloud began to obscure it."

What were the "almost black" vertical bands embedded in the main band? Aurora expert R. Livesey replied as follows.

"There is a phenomenon called the 'black aurora' which consists of small regions of very low luminosity embedded in brighter auroral light; the 'black' rays reported from the Appleby could have been a phenomenon of this type."

(Wilson, J.L.; "Aurora Borealis," Marine Observer, 69:112, 1999.)

Black auroras may actually be more than just the contrast effect suggested by Livesey. Low-light TV systems detect clockwise vorticity in the black bands, and the bands seem to be associated with upward electron beams. In other words, black auroras are apparently a distinct phenomeon in their own right and not just nonluminous parts of the visible aurora.

(Stenbaek-Nielsen, H.C., et al; "Why Do Auroras Look the Way They Do?" Eos, 80:193, 1999.)

From Science Frontiers #127, JAN-FEB 2000. 1997 William R. Corliss