Abstract
Magnetic flux ropes (MFRs) constitute the core structure of coronal mass ejections (CMEs), but hot debates remain on whether the MFR forms before or during solar eruptions. Furthermore, how flare reconnection shapes the erupting MFR is still elusive in three dimensions. Here we studied a new MHD simulation of CME initiation by tether-cutting magnetic reconnection in a single magnetic arcade. The simulation follows the whole life, including the birth and subsequent evolution, of an MFR during eruption. In the early phase, the MFR is partially separated from its ambient field by a magnetic quasi-separatrix layer (QSL) that has a double-J shaped footprint on the bottom surface. With the ongoing of the reconnection, the arms of the two J-shaped footprints continually separate from each other, and the hooks of the J shaped footprints expand and eventually become closed almost at the eruption peak time, and thereafter the MFR is fully separated from the un-reconnected field by the QSL. We further studied the evolution of the toroidal flux in the MFR and compared it with that of the reconnected flux. Our simulation reproduced an evolution pattern of increase-to-decrease of the toroidal flux, which is reported recently in observations of variations in flare ribbons and transient coronal dimming. The increase of toroidal flux is owing to the flare reconnection in the early phase that transforms the sheared arcade to twisted field lines, while its decrease is a result of reconnection between field lines in the interior of the MFR in the later phase.
Highlights
Solar eruptions are spectacular manifestation of explosive release of magnetic energy in the Sun’s atmosphere, i.e., the solar corona, and unveiling the relevant magnetic field structures and their evolution holds a central position in the study of solar eruptions
We have studied the magnetic evolution of an Magnetic flux rope (MFR) formed during the eruption in an MHD simulation
The MFR is partially separated from its ambient field by a QSL that has a double-J shaped footprint on the bottom surface
Summary
Solar eruptions are spectacular manifestation of explosive release of magnetic energy in the Sun’s atmosphere, i.e., the solar corona, and unveiling the relevant magnetic field structures and their evolution holds a central position in the study of solar eruptions. With the ongoing of reconnection, the arms of the double-J ribbons separate, and their hooks gradually extends outwards In such process, the flare reconnection, which occurs between the pre-reconnection sheared arcade (as shown in the classic cartoon of tethercutting model, i.e., Figure 1 of [10]), should increase the toroidal (axial) flux by increasing the number of field lines within the MFR. A well-known, unexplained fact is that the feet of the erupting flux rope, as manifested by twin coronal dimmings and by the hook ends of double-J flare ribbons, are found to be drifting progressively away from the main PIL during eruption [37], even though the photosphere can be regarded as motionless during the short time scale of eruption To this end, Aulanier and Dudík [41] analyzed in more details the reconnection process in their simulation of flux rope eruption and showed that the flare reconnection occurs in three different types of events according to their different effect in building up the flux rope. In the second article of this series, we will illustrate the 3D configuration of the different types of magnetic reconnection in building up the MFR and disclose why the QSLs evolve in such a complex way, following the pioneering work by [41]
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