The Sun sporadically expels trillions of tons of million-degree hydrogen gas in explosions called coronal mass ejections (CMEs). Such clouds are enormous in size (spanning millions of miles) and are made up of magnetized plasma gases, so hot that hydrogen atoms are ionized. CMEs are rapidly accelerated by magnetic forces to speeds of hundreds of kilometers per second to upwards of 2,000 kilometers per second in several tens of minutes. CMEs are closely related to solar flares and, when they impinge on Earth, can trigger spectacular auroral displays. They also induce strong electric currents in Earth's plasma atmosphere (i.e., the magnetosphere and ionosphere), leading to outages in telecommunications and GPS systems and even the collapse of electric power grids if the disturbances are very severe.

Since the first observation of a solar flare in 1859, solar eruptions ("explosions") have attracted much attention from scientists around the world and have been studied with a succession of increasingly sophisticated international satellite missions in the past three decades. A major challenge has been that enormous and complicated plasma structures accelerating away from the Sun can only be observed remotely. As a result, it has been difficult to test theoretical models to establish a correct understanding of the mechanisms that cause such eruptions. But in 2006, an international twin-satellite mission called STEREO was launched to continuously observe the erupting plasma structures from the Sun to Earth. [ SCIENCE DAILY ]