Squeezed between magnetic fields of opposite polarity, a really big blob of plasma (charged particles) rounded the Sun's northeast limb. The prominence offered impressive sights in H-alpha filters, which are suited to observe the inner, "cold" (10.000 degrees) atmosphere of the Sun.
Above pictures show the evolution of the prominence from 22-24 April, as photographed by various astronomers from the Belgian solar section and from public observatories. On 24 April, the prominence towered more than 100.000 km above the solar surface, and had a length of about 300.000 km, i.e. close to the average Earth-Moon distance.
Solar observers are well aware that this kind of huge structures eventually will become unstable and erupt. This case was no different, and the eruption took place on 28 April when the structure was now visible on the solar surface as a dark filament . The magnetic fields seemed to have been rendered unstable by the eruption of a small, nearby filament to the east around 02:30UT. The southern, fuzzy, part of the big filament gradually disappeared between 06:00 and 08:00UT, whereas the northern part got ejected between 12:30 and 14:30UT. Another small filament, located between the previous two, erupted between 21:30 and 22:30UT.
A movie of the prominence rounding the solar limb and the filament eruption can be seen here. It combines imagery in the extreme ultraviolet from SDO showing the dynamics of the structure in cold (80.000 degrees; red; AIA 304) and hot (1.3 million degrees; green; AIA 193) filters. The associated coronal mass ejection seemed mostly directed away from Earth.
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