A prominence erupts

Solar prominences are clouds of charged particles ("plasma") above the solar surface squeezed between magnetic regions of opposite polarity. Being cooler and denser than the plasma underneath and their surroundings, they appear as bright blobs when seen near the solar limb and as dark lines when seen on the solar disk (then they are called "filaments"). Special filters are required to observe these features, such as in the Hydrogen-alpha (H-alpha) line in the red part of the solar spectrum, or in some extreme ultraviolet (EUV) passbands.

The imagery above is a solar image in extreme ultraviolet taken by the PROBA2/SWAP instrument, overlaid on a coronagraphic image taken by SOHO/LASCO C2 in white light. The prominence is located near the southeast solar limb ("lower left"). Just prior to the eruption, it towered about 160.000 km above the solar surface, that's about half the Earth-Moon distance. The SWAP image shows the prominence shortly after the eruption started. In the second image, taken 10 hours after the first one, the prominence has disappeared but the coronagraphic image now clearly shows a coronal mass ejection (CME) propagating in an eastward direction ("to the left"), so not directed to Earth.

STEREO-A had a much better view on the eruption, as its current location provides a good view on the east limb. Surprizingly, the eruption was a lot less impressive than the views from Earth. Some faint magnetic reconnection can be seen in the southeast portion of the image, as annotated above.

The associated CME was directed to STEREO-A. It can be seen in the STEREO-A/COR2 coronagraphic imagery, however it is very faint and barely visible. In the annotated image the brighter portions of this slow-moving CME have been indicated in dashed green. At the time of this writing, the CME had not reached the spacecraft yet.



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