The Rotation of Magnetic Flux Ropes Formed during Solar Eruption

Eruptions of solar filaments often show rotational motion about their rising direction, but the mechanism governing such rotation, and how the rotation is related to the initial morphology of the preeruptive filament (and cospatial sigmoid), filament chirality, and magnetic helicity, remains elusive. The conventional view of rotation as a result of a magnetic flux rope (MFR) undergoing ideal kink instability still has difficulty explaining these relationships. Here we propose an alternative explanation for the rotation during eruptions by analyzing a magnetohydrodynamic simulation in which magnetic reconnection initiates an eruption from a sheared arcade configuration, and an MFR is formed during eruption via reconnection. The simulation reproduces a reverse-S-shaped MFR with dextral chirality, and the axis of this MFR rotates counterclockwise while rising, which compares favorably with a typical filament eruption observed from dual viewing angles. By calculating the twist and writhe numbers of the modeled MFR during its eruption, we found that, accompanied by the rotation, the nonlocal writhe of the MFR's axis decreases while the twist of its surrounding field lines increases, and this is distinct from kink instability, which converts magnetic twist into the writhe of the MFR axis.