The Inner Magnetosphere Explorer (IMEX), will study the response of Earth's Van Allen radiation belts to variations in the solar wind. The energetic charged particles (mainly protons and electrons) that comprise Earth's trapped radiation zones are potentially hazardous to both astronauts and satellite systems. IMEX will be launched into a 217-mile by 21,748-mile (350-kilometer by 35,000-kilometer) elliptical orbit around Earth with instruments to measure on a regular basis the populations of energetic particles and related magnetic and electric fields throughout the radiation belts.
(taken from http://lasp.colorado.edu/stp/imex/imex_main.html)