Panel (a): Kinematics of the eruptive structures. Blue and red dots and lines represent the apex height and the derived velocity, respectively. Panel (b): Distribution of the decay index computed from the B_y component (poloidal field). The cyan-shaded regions indicate the typical threshold for TI (n = 0.8–1.5). The green and red lines show the results derived from the potential field at 04:32 and 05:00 UT, respectively. The orange and pink lines mark the critical height and corresponding decay index at the onset (04:46 UT) and peak (04:56 UT) of the second acceleration phase. Panel (c): Decay-index distribution computed from the B_x component (toroidal field), with the vertical dashed green and black lines indicating the heights where the decay index equals zero and reaches its minimum, respectively. Panel (d): Evolution of the maximum J/B along the horizontal line in the inset J/B images. The vertical dashed red line indicates the time when the maximum J/B begins to increase.