Evolution of thermodynamical variables along a magnetic field line. a) We show the plane-of-sky coordinates of a magnetic field line, represented with dotted (t = 0 s), dashed (t = 0.23 s), dash-dotted (t = 1.14 s), and solid lines (t = 13.74 s), from light to dark pink. In the background, B_z at z = 0.05 Mm and t = 0 s is shown in greyscale, with negative values in black and positive values in white. b) Temperature. c) Vertical velocity along the field line from the left footpoint to the apex (top panels) and from the right footpoint to the apex (bottom panels). The variables are plotted for the same instants in time as in panel a.