In a recent Forum article—“What is the Aurora?” (Eos Trans. AGU, 85(52), 567, 28 December 2004)—John Clarke has raised an important issue: how we define the aurora. I, too, was confronted by the same question when I was writing a review entitled “Auroral emissions of the giant planets” with Randy Gladstone [Bhardwaj and Gladstone, 2000]. I widely searched the literature then, to find how the aurora should be defined.
Finally we defined the aurora as “electromagnetic radiation emanating from the high latitudes of a planet (also called auroral or polar latitudes).” We also defined auroras as “(generally) high-latitude atmospheric emissions that result from the precipitation of energetic charged particles from a planet's magnetosphere.”