Intensity along the loop plotted as a function of time. The intensity averaged in time has been subtracted from each time slice. a) The linear trend (orange dashed line) gives a projected speed of 212 km s^-1. b) Zoomed-in version of the top figure giving a projected speed of 206 km s^-1. This speed can be approximately corrected for the projection using the 3D estimate of the loop from the STEREO-B/SDO comparison to give a speed of approximately 470 km s^-1, close to the sound speed of 500 km s^-1 at 11 MK.