15 August 2008
Vol 321, Issue 5891

About The Cover

Cover image expansion

COVER A dynamic aurora borealis during a storm over Canada. Energy from the Sun's extended atmosphere is stored at Earth's magnetic field and is released explosively, powering the aurorae. Previously stable aurorae brighten, filament, expand poleward, and cover the sky within 1 to 2 minutes. The energy release starts at an altitude of 130,000 kilometers, at the magnetic equator, near local midnight. See page 931. Image: Norbert Rosing/National Geographic/Getty Images