The twin STEREO spacecraft (called "Behind" and "Ahead" denoting their relative positions in space), now almost 120 degrees apart, captured this large and dramatic prominence eruption over about a 30-hour period between Sept. 26-27, 2009. Prominences, called filaments when they are viewed against the surface of the Sun, are clouds of cooler gas suspended above the Sun's surface by magnetic forces. This erupting prominence was large enough that both spacecraft were able to observe it for hours on end, one of the first times that has occurred. From the Behind perspective (on 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 (right), the filament is seen in profile and is therefore called a prominence. The very large cloud lifts up, breaks away, and heads out into space. This is one of the most spectacular eruptive prominences either SOHO or STEREO have observed.
We should note that scientists were interested to observe that the side (Ahead) view confirms that the prominence heads from high solar latitudes to near the solar equator, which is what we expect from the usual shape of the extended solar magnetic field at solar minimum. However, it has taken years longer than usual for the field to relax in this cycle