Formation of a Magnetic Flux Rope in the Early Emergence Phase of NOAA Active Region 12673. (arXiv:1908.06360v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Liu_L/0/1/0/all/0/1">Lijuan Liu</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Cheng_X/0/1/0/all/0/1">Xin Cheng</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Wang_Y/0/1/0/all/0/1">Yuming Wang</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Zhou_Z/0/1/0/all/0/1">Zhenjun Zhou</a>
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
$2.1times 10^{20}$~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 Chintzoglou et
al. 2019). 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.
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
$2.1times 10^{20}$~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 Chintzoglou et
al. 2019). 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.
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