The SDO follows a geosynchronous orbit, so it allows continuous monitoring of the face of the Sun also visible from the Earth. The STEREO satellites follow the orbit of the Earth around the Sun but are faster than the Earth, so that they are now facing the Sun from directions which allow to see parts of the solar surface which are not visible from the Earth at the same time. In three occasions, the Fermi telescope recorded strong gamma ray emission from the “visible” solar surface, each one occurring a few minutes after a flare was recorded by STEREO A or STEREO B “beyond the limb”, on the “back”, invisible side of the Sun. The researchers explain that, during the flare, particles are accelerated to nearly the speed of light, are ejected in the associated Coronal Mass Ejection, and, travelling at high speed along huge magnetic loops, ”rain” back to the solar surface, where they lose their energy and produce the gamma ray emission. The NASA STEREO pages provide all the information needed to understand this beautiful mission, and this video (Credit: NASA) explains the observations.
Supernovae, and the “invisible side” of the Sun – Huffington Post