A two-stage CME
posted: October 6, 2015

A movie of this event can be found here.

On 30 September, a prominence near the southwest solar limb got ejected into space. Prominences are clouds of plasma (charged particles) which are suspended in the corona, squeezed between large magnetic fields of opposite polarity, but denser and cooler than the surrounding coronal plasma. The structure had been visible during the last 2 weeks, before the surrounding magnetic field became unstable and ejected it into space starting around 07:30UT.

As can be seen in the widefield PROBA2/SWAP image above, the escaping material seems to have triggered more reconnections in the corona, with material raining down near the northwest solar limb. Hence, the first coronal mass ejection (CME) is quickly followed by another CME this time directed to the northwest. CACTus combined the two CMEs seen in SOHO's coronagraphs (image below) into one 150-degrees-wide partial halo CME, but they can clearly be distinguished from each other, both in direction and timing. The first CME became visible at 09:24UT and was headed in the southwest direction, the second CME became visible around 10:36UT and was headed to the northwest. Both CMEs were slow, with a speed of around 330 km/s, and they were not directed to Earth.

The most impressive features were observed in the aftermath of the eruption. Indeed, over the northwestern footpoint one can see supra-arcade downflows, i.e. sunward moving voids often seen above developing post-eruption coronal loops. The downflows became visible in SDO's AIA 094 filter starting around 12:00UT. This filter shows the corona at temperatures of several million degrees, and the features are not visible in lower temperature filters such as SDO/AIA 171 (700.000 degrees) or PROBA2/SWAP (about 1 million degrees). The appearance of these structures add to the complexity of the eruption as a whole.

The movie first shows the eruption in extreme ultraviolet (resp. first a combo from AIA193/304 and SWAP; then purely SWAP), followed by two clips combining the PROBA2/SWAP images with coronagraphic imagery from SOHO/LASCO, finally ending with AIA094 (full disk) and AIA094/171 combo (zoom) of the supra-arcade downflows.

Credits - Data and imagery for the movie clips were taken from SDO/AIA, SOHO/LASCO, PROBA2/SWAP and (J)Helioviewer.