In this work , we investigate the formation of a magnetic flux rope ( MFR ) above the central polarity inversion line ( PIL ) of NOAA Active Region 12673 during its early emergence phase .
Through analyzing the photospheric vector magnetic field , extreme ultraviolet ( EUV ) and ultraviolet ( UV ) images , extrapolated three-dimensional ( 3D ) non-linear force-free fields ( NLFFFs ) , as well as the photospheric motions , we find that with the successive emergence of different bipoles in the central region , the conjugate polarities separate , resulting in collision between the non-conjugated opposite polarities .
Nearly-potential loops appear above the PIL at first , then get sheared and merge at the collision locations as evidenced by the appearance of a continuous EUV sigmoid on 2017 September 4 , which also indicates the formation of an MFR .
The 3D NLFFFs further reveal the gradual buildup of the MFR , accompanied by the appearance of two elongated bald patches ( BPs ) at the collision locations and a very low-lying hyperbolic flux tube configuration between the BPs .
The final MFR has relatively steady axial flux and average twist number of around 2.1 \times 10 ^ { 20 } Mx and -1.5 , respective .
Shearing motions are found developing near the BPs when the collision occurs , with flux cancellation and UV brightenings being observed simultaneously , indicating the development of a process named as “ collisional shearing ” ( firstly identified by ) .
The results clearly show that the MFR is formed by “ collisional shearing ” , i.e. , through shearing and flux cancellation driven by the collision between non-conjugated opposite polarities during their emergence .