Temporal evolution of the three vortices in case 2. Panels (a1)–(a4): NVST Hα images. The yellow arrows in panels (a1)–(a4) indicate the velocity fields of the vortices. In panel (a2), the combination part of the three vortices is pointed by the cyan arrow. Panels (b1)–(b3): AIA 193 Å images. The blue arrows in panels (b1)–(b3) indicate the velocity fields. Panel (c): Average projection speeds with error bars of the vortices from the NVST Hα observations (panels (a1)–(a4)) and the AIA 193 Å data (panels (b1)–(b3)).