Collapse of the fan tracked along six spines. Panel a: minimum value of the Hα emission intensity. This shows the fan in the first time frame (11:59:20 UT). The six spines used are shown using black lines, with a white dot denoting the edge of the fan as derived from the process shown in panel b. Panel b: LOS velocity values on the y-axis (normalized to the same maximum value for comparison) against position along the length of the spine in pixels on the x-axis (zero being the base of the spine). This data is shown at six different times (in different colors) for the fourth spine from the top.