Carl Størmer’s auroral discoveries 1This article is part of a Special Issue that honours the work of Dr. Donald M. Hunten FRSC who passed away in December 2010 after a very illustrious career.

In the first half of the 20th century Professor Carl Størmer took aurora research to new heights, devoting all of his energy to solving the riddle of this fascinating natural phenomenon. He began his pioneering research by calculating the trajectories allowed to energetic charged particles. Because the equations of motion did not have analytic solutions, he was forced to invent new numerical methods to follow each particle’s path, step by step. Through a series of treatises he presented now classic solutions to the trajectory problem. To explain the large scale motion of the auroral zone, he was first to introduce the concept of a ring current in 1911. His theoretical work also provided the basis for understanding later discoveries of cosmic rays and the radiation belts. Størmer contributed many important scientific achievements to space physics. In 1909 he constructed the first useful auroral camera needed to make precise space–time mappings of auroral characteristics. Over the course of four sunspot cycles he took more than 100 000 auroral photographs with his network of stations spread across southern Norway. These parallactic auroral photographs gave not only the heights of individual auroral features, but also their occurrence rates, locations, and orientations. He classified the different auroral forms by publishing the first Auroral Atlas (Størmer. Photographic Atlas of Auroral Forms. Brøggers Boktrykkeri, Oslo. 1930.). Among Størmer’s other fascinating discoveries was his identification of sunlit aurorae and descriptions of their remarkable properties.