Observational Analysis on the Early Evolution of a CME Flux Rope: Preflare Reconnection and Flux Rope’s Footpoint Drift

Abstract

We study the early evolution of a hot-channel-like magnetic flux rope (MFR) toward eruption. Combining with imaging observation and magnetic field extrapolation, we find that the hot channel possibly originated from a pre-existing seed MFR with a hyperbolic flux tube (HFT). In the precursor phase, three-dimensional tether-cutting reconnection at the HFT is most likely resulting in the heating and buildup of the hot channel. In this process, the forming hot channel was rapidly enlarged at its spatial size and slipped its feet to two remote positions. Afterward, it instantly erupted outwards with an exponential acceleration, leaving two core dimmings near its feet. We suggest that pre-flare reconnection at the HFT played a crucial role in enlarging the seed MFR and facilitating the onset of its final solar eruption. Moreover, a recently predicted drifting of MFR's footpoints was detected at both core dimmings. In particular, we find that MFR's west footpoint drift was induced by a new reconnection geometry among the erupting MFR's leg and thereby inclined arcades. As MFR's west footpoints gradually 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 and also generating a secondary flare ribbon at their remote feet. This reveals that core dimmings may suffer a pronounced diminishment due to the eruptive MFR's footpoint drift, implying that mapping the real footpoints of the erupting MFR down to the Sun's surface is more difficult than previously thought.

Evolution of the Toroidal Flux of CME Flux Ropes during Eruption

“…The evolutions of the identified footpoint regions are demonstrated in Figure 1 , which present obvious drifting of the flux rope footpoints as mentioned in previous papers. 18 , 39 , 40 , 41 Figure 2 shows the evolution of the toroidal flux during the eruption. It should be mentioned that our method sometimes derives some small and irregular dimming regions that extend from the main dimming region.…”

“…Aiming at the drifting of the footpoints of the CME flux rope, Chen et al. 39 used a trial-and-error method to detect the core-dimming region and found that the magnetic flux there (i.e., the toroidal flux) shows a decreasing trend, if the core-dimming region is considered the footpoint region of the CME flux rope. In addition, Wang et al.…”

Formation of Magnetic Flux Rope During Solar Eruption. I. Evolution of Toroidal Flux and Reconnection Flux

“…It is predicted by theoretical models [35] that if the MFR grows to sufficiently twisted, the hooks of the double-J shaped footprints of the QSL can indeed close onto themselves, becoming two closed rings, although this is not reproduced by the numerical model of [20], possibly because their simulation run is stopped before the MFR grows to such a high degree of twist. Very recently, a few papers reported that there is a systematic decrease of the toroidal flux of erupting MFR after its fast increase (e.g., [40,42,43]). In particular, Xing et al [40] developed a practical method for estimation of the toroidal flux of MFR during eruption by combining twin coronal dimmings and the hooks of flare ribbons.…”

Three-dimensional magnetic reconnection in astrophysical plasmas

“…During eruptive flares, 3D reconnection geometries are more complex than 2D. The photospheric footpoints, for example, are observed to brighten sequentially along the polarity inversion line during the rise phase [158][159][160], as modelled by Priest & Longcope [138] in terms of zipper reconnection ( §3b). Also, Li et al [158] found that one end of an eruptive flux rope is fixed and the other end exhibits apparent slippage along a hook-shaped flare ribbon.…”