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A low- or middle-latitude, more or less steady, faint photochemical luminescence visible in the upper atmosphere at night.
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
(Physical Geography) the faint light from the upper atmosphere in the night sky, esp in low latitudes
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014
a dim light, usu. visible at night, that results from ionic radiation in the upper atmosphere.
Random House Kernerman Webster's College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.