In a recent study published in The Astrophysical Journal, CHEN Hechao from Yunnan Observatories of the Chinese Academy of Sciences and the collaborators reported a new phenomenon. They found that the line-tied footpoints of erupting magnetic flux ropes (MFRs) can drift to new location during its eruption.

This new finding implies that mapping the real footpoints of the erupting MFR down to the solar surface is more difficult than solar physics scientists previously thought.

Coronal mass ejections (CMEs) are well-known drivers for most extreme energetic space weather in our solar system. These large-scale solar eruptive activities originate from the loss-of-equilibrium of solar magnetic flux ropes, and may easily make our near-Earth space a hazardous place. Mapping the real footpoints of erupting CME flux ropes down to the solar surface is the key to build the physical links between solar eruption and their resulting enegertic space weather.

Core dimmings, the transient dark regions that appeared during the early eruption phase of CMEs, often conjugately present at the extremes of flare ribbons. Thus, they have been tentatively explained as the line-tied footpoints of the evacuated CME flux ropes. At present, this tentative assumption has been used as a basic condition to estimate the mass supply and poloidal flux of CME flux ropes. However, whether this tentatively assumption is reliable remains unclear.

Through the joint observations from New Vacuum Solar Telescope (NVST) and Solar Dynamic Observatory (SDO), the researchers conducted a detailed observational analysis on the early evolution of a CME flux rope in NOAA active region 12010.

The results showed that MFR’s footpoints underwent a dynamical drift near both core dimmings. In particular, MFR’s west footpoint drift was proved to be induced by a new reconnection geometry among the erupting MFR’s leg and thereby inclined arcades. As MFR’s west footpoints drifted to a new position, a set of newborn atypical flare loops connected into the west core dimming, causing a rapid decrease of dimmed area inside this core dimming.

"This new-found footpoint drift phenomenon in our research now clearly reveals that core dimmings indeed cannot accurately map the real footpoints of the erupting CME flux rope in some real eruption events. To better solve this problem, a more rigorous and reliable approach is required in the future,” CHEN said.