Temporal evolution of the counterclockwise single vortex that appeared in the prominence (case 2) at the northern limb of the solar disc on 6 September 2023. Panels (a1)–(a4): NVST Hα images. The yellow arrows in panels (a1)–(a4) indicate the velocity fields of the vortex. The cyan arrows in panels (a2) and (a4) indicate the speeds of features E3 and E4 from the DAVE technique, respectively. Panels (b1)–(b2): Hα images displaying temporal evolutions of E3 in panel (a2). The positions of E3 at two moments are marked with magenta and cyan symbols “×”, respectively. Cyan arrows indicate the evolution orientations. Panels (c1)–(c2): similar to panels (b1)–(b2), but for E4 in panel (a4). Panels (d1)–(d3): AIA 193 Å images. The blue arrows in panels (d1)–(d3) indicate the velocity fields. Panel (e): Average projection speeds with error bars of the vortex from the NVST Hα observations (panels (a1)–(a4)) and the AIA 193 Å data (panels (d1)–(d3)). An animation of NVST Hα observations from 6 September 2023 at 01:27:14 UT to 02:04:27 UT, showing the evolution of the single vortex before the split, is available online. The real-time duration of this animation is 5 s.