The Sun is constantly roiling with nuclear heat and intense magnetism that make sunspots, flares, coronal mass ejections, and all sorts of space weather. When directed toward Earth, those solar blasts can disrupt satellite and radio communications, damage our electric-powered tools and toys, and create auroras.
But it is not always easy to know when the Sun is spitting plasma and energy in our direction. This is why NASA launched the Solar Terrestrial Relations Observatory, or STEREO, in October 2006. The twin satellites were sent out along roughly the same orbit around the Sun as the Earth itself. The orbit of STEREO-A (ahead) is slightly closer to the Sun and moving faster than Earth; the STEREO-B (behind) orbit is slightly farther from the Sun and moving a little slower than our planet. The difference in speed creates separation between the satellites and a stereoscopic view of our nearest star.
The images above show the surface of the Sun from two different angles on October 14, 2012. The top image is from STEREO-B and shows a dark vertical stripe on the upper middle face of the Sun. The lower image comes from STEREO-A, which was more than 90 degrees ahead of STEREO-B; that is, somewhat beyond a right angle from that vertical stripe. In the lower image, the vertical stripe instead shows up as a large loop stretching into the solar atmosphere, or corona.
The stripe and the loop are differing views of the same dense mass of electrified gas (plasma) held in place by a magnetic field. When viewed straight on, as in the STEREO-B image, the line of plasma appears darker because it is relatively cooler than the solar surface below. (More of its energy is magnetic than radiant.) Solar scientists call these dark lines “filaments.” When viewed from the side, however, the line of plasma looks like a bright loop stretched out against the blackness of space; solar physicists call this a prominence. Essentially, filaments and prominences are the same phenomenon, just viewed from a different perspective.
As of September 1, 2012, STEREO-A and STEREO-B formed an equal-sided triangle together with NASA’s Solar Dynamics Observatory (SDO), which views the Sun from orbit around Earth. This geometry allowed the three craft to provide overlapping views of the entire Sun. It also allows solar physicists to study solar events in three dimensions.
NASA image courtesy of the STEREO science team. Caption by Mike Carlowicz, based on information from Karen Fox and Steele Hill.