A Proton Arc Over Lake Superior . ... Credit Copyright: Ken Williams Explanation: The setting had been picked out -- all that was needed was an aurora. And late last August, forecasts predicted that an otherwise beautiful night sky would be lit up with auroral green . ... While setting up for the image, a proton arc -- a rare type of aurora -- appeared. ... APOD Editor to Speak: Saturday, August 8 at Keweenaw Science Engineering Festival . ... NASA Web Site Statements, Warnings, and Disclaimers . ...