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tokens: feature tracking. 3. Results The polar crown prominence in the northwest limb (N63W72) had been stable for over two weeks before our observation began on 2016 June 4. Figure 1 shows the prominence of interest in the H α , AIA 131, 171, and 304 Å images. The prominence is bright in H α , but dark in the AIA images. Note that the intensity of the H α image is reversed, and that darker features in the image are actually brighter. The dark structure in the AIA images is wrapped in the bright emissive gas, indicating that the cool material is wrapped inside the hot coronal gas. The main body of the polar crown prominence lies vertical to the plane of the sky, and an arch-shape prominence newly emerged in the northern region of the polar crown prominence after 04:00 UT. The arch-shape prominence was brighter than the main prominence in the AIA images, but it was fainter in the H α images. The observed arch-shape prominence looks similar to the hedgerow prominence overlying a cavity observed by Ryutova et al. ( 2010 ) except for the horizontal flow within it. We detected a horizontal flow, which is highlighted in the next paragraph. Figure 1. Polar crown prominence observed on 2016 June 4, with the newly emerged arch-shaped prominence in the region north of it. The SDO/ AIA 131, 171, 304 Å data, and H α images (upper-left, upper-right, lower-left, and lower-right panels, respectively) show the formation of a vortex on the arch-shaped prominence. Note that the intensity of the H α image is reversed. The dashed boxes in the H α image represent the FOV of the Figure 3 (small red), Figure 4 (big red), and Figure 8 (blue). A full sequence of the images, covering from 08:09:43 to 09:31:46 UT, is provided in the animation. In the animation, 08:09:43 UT is taken to be the epoch of time t . (An animation of this figure is available.) Figure 2 shows H α images of the observed region at the moment of the rising of the arch-shape prominence and the formation of a 5′′-size vortex. The arrows plotted on the H α images represent the velocity vectors of the stream on the plane of the sky. Near the left-hand side leg of the arch-shape prominence (690′′, 680′′), the dominant direction of the plasma flow is upward along the arch. It rose up slowly (20 km s −1 ) and reached about a height of 20′′. Then it streamed to the right along the arch toward the polar crown prominence with a speed of 20–60 km s −1 . The stream can be clearly identified in both panels of Figure 2 . The measured collimated flow speed of about 20–60 km s −1 is higher than the typical speed of the prominence plasma (10–20 km s −1 ; Zirker et al. 1998 ; Ahn et al. 2010 ; Vial Engvold 2015 ). The full dynamics of the evolution can be seen better in the animation of Figure 1 . Very luckily, the flow lies parallel to the plane of the sky considering the narrow bandpass width of the filter. Figure 2. Velocity vectors superposed on H α images at the moment of the rising of the arch-shape prominence (left)