Alternating Twist Along an Erupting Prominence

Abstract

Triangulation measurements using observations from the two Solar Terrestrial Relations Observatory (STEREO) spacecraft, combined with observations from the Solar Dynamics Observatory (SDO), are used to characterize the behavior of a prominence involved in two successive coronal mass ejections 6 – 7 December 2010. The STEREO separation at the time was 171.6∘, which was functionally equivalent to a separation of 8.4∘, and thus very favorable for feature co-identification above the limb. The first eruption at ≈ 14:16 UT on 6 December of the middle branch of the prominence starts off a series of magnetic reconfigurations in the right branch, which itself erupts at ≈ 2:06 UT the next day, about 12 hours after the first eruption. The cool prominence material seen at 304 A drains back down to the surface, but a flux-rope-like magnetic structure is seen to erupt in both 195 A by the STEREO/Extreme Ultraviolet Imager (EUVI), and in white light by the STEREO/COR1 inner coronagraph. In between the two eruptions, two different signs of helicity are seen in the measured twist of the right branch. This is interpreted to be caused by the overall prominence channel being composed of different segments with alternating helicity signs. The erupting parts on 6 and 7 December both show positive twist, but negative twist is seen in between these positive sections. Negative twist is consistent with the dextral chirality signs seen in the He ii line at 304 A prior to both eruptions. However, during the period between the eruptions, a region of positive twist grows and replaces the region of negative twist, and finally erupts. We interpret these observations in the light of models that predict that helicity cancellation can be an important factor in the triggering of flares and coronal mass ejections.