tokens: estimate the average period of the rotational motion, which turns out to be 569 s (with an error of 91 s). Taking the average width 38 Mm of the jet into account, the average line speed of the rotational motion during the off-limb stage is then estimated to be about 209.8 ± 33.6 km s −1 . As the upward Lorentz force acting on the jet after reconnection resulted from the untwisting motion (Shibata Uchida 1985 ), the nearly invariable rotating period is consistent with the above result that the downward acceleration in the late ascending phase and in the descending phase were almost the same. 3.2. Kinetics of the Twin Jets A similar method reveals almost the same inclination of the twins’ axes as the preceding jet related to the plane of the sky. To probe the axial motions of the twin jets, we place two slices along their axes labeled as “C1” and “C2” in Figure 1 (e) for the left and right twin jet respectively. Their corresponding running-difference time–distance plots are shown in Figures 3 (C1) and (C2) with the y -axis representing the distance along the slices after correcting the projection effects. Figure 3. (C1) and (C2): running-difference time–distance plots for slices “C1” and “C2” in Figure 1 (e) along the axis of the left and right twin jet respectively. The blue stars represent trajectories of materials within the twins, with the blue curves representing their parabolic fitting results. (D1)–(D4): running-difference time–distance plots for slices “D1” to “D4” perpendicular to the twins’ axes as shown in Figure 1 (e). Only very weak rotational motion of the twins could be found at their early off-limb stage from these four time–distance plots. As in Figure 3 (C1), the left twin jet reached a maximum distance along the slice of about 330 Mm at around 01:09 UT. The downward acceleration during its ascending phase and descending phase turned out to be similar. Parabolic fitting (blue dashed curve in Figure 3 (C1)) on one of the trajectories (blue stars in Figure 3 (C1)) gives us an average deceleration of about −228.9 ± 8.1 m s −2 . It is hard to determine the maximum distance of the right twin jet (Figure 3 (C2))—it seemed to reach a distance that was beyond the FOV of SDO observations or faint enough to be invisible. However, the dynamics seem to be similar to the left one with the average downward acceleration of about −234.5 ± 10.7 m s −2 (one of the trajectories is shown as the blue stars with the parabolic fitting result shown by the blue dashed curve in Figure 3 (C2)). All of the above values resemble the average local gravity (−220 m s −2 ), which indicates that there was almost no extra driving force acting on the twins. Meanwhile, we place four parallel slices “D1” to “D4” at different heights perpendicular to the twins’ axes to investigate the rotational motion of the twins if there was any. Corresponding running-difference time–distance plots are shown in Figures 3 (D1) to (D4) with the y -axis representing the distance
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