From the Behind perspective on the left, the long filament, darker than the Sun's surface, can be seen rising up and then breaking away, spreading out above most of the Sun's surface.

 

As seen from the Ahead spacecraft on the right, the filament is seen in profile and is called a prominence, which NASA describes as clouds of cooler gas suspended above the Sun's surface by magnetic forces.

 

The eruption was so large that both spacecrafts were able to observe the sensation for hours on end. It was one of the most spectacular events observed by the mission.

 

NASA says the cloud lifted up, broke away and headed out into space.

The Stereo mission launched 3 years ago, and its two satellites have been sending back 3D images of the sun, providing data for more accurate alerts for solar storms.