What is the aurora?



”‘When I use a word,’ Humpty Dumpty said in a rather scornful tone, ‘it means just what I choose it to mean—neither more nor less.’” From Through the Looking Glass, by Lewis Carroll.

While writing a review chapter on Jupiter's aurora, I have given some thought recently to a general definition of the aurora. I have studied auroral emissions from Jupiter, Saturn, and Uranus for the last 20 years, and I at first blithely assumed that there was general agreement for the better understood terrestrial aurora.