Pixie

Is an electronic camera which images the earth's aurora in x-rays. The x-rays measured by PIXIE are generated when energetic electrons strike the upper atmosphere. By imaging these x-rays and measuring their energies across a broad energy range, PIXIE determines the fluxes and characteristic energies of the parent electrons. This information is used to determine other important characteristics of the ionosphere, and of the interactions between the earth's upper atmosphere, ionosphere, radiation belts, and magnetosphere. 
PIXIE is one of 11 instruments aboard the Polar spacecraft, which is one of two  satellites in the Global Geospace Science (GGS) Program managed by NASA's Goddard Space Flight Center.. GGS represents a major US contribution to the world-wide International Solar Terrestrial Physics Program (ISTP) Program.  Polar/PIXIE is also a mission of the Sun-Earth-Connections NASA Research Program