The first daytime ground‐based optical image of the aurora

Aurorae, spectacular phenomena in the polar night sky, also provide a convenient projection of effects of complex and energetic plasma processes of the outer magnetosphere. Much has been learned about the ionosphere and magnetosphere from night‐time auroral images. However, similar imaging is extraordinarily difficult by day, due to the overwhelming background of atmospherically‐scattered sunlight. This is unfortunate, since many auroral plasma processes may be unique to the sunlit ionosphere. A visible‐light image of the aurora at λ630‐nm wavelength was obtained from Kiruna, Sweden, at sunset on May 2, 1999, by an imaging spectrometer featuring excellent spectral resolution and out‐of‐band rejection. We believe this to be the first such image obtained from the ground under near‐daytime conditions. These observations were obtained as a test of principle during the development of a prototype instrument. We believe this technique holds great promise for future ground‐based studies of the daylit ionosphere and magnetosphere.