  Moving vortices in thin superconducting films are considered within the
time-dependent London description. The dissipation due to out-of-core normal
excitations for two vortices moving together turns out to have a minimum for
the separation vector $\bm a$ parallel to the velocity and equal to $a_m
\approx 2.2\, \Lambda$, where $\Lambda $ is the Pearl length. The minimum
entropy production suggests that moving vortices should have a tendency to form
chains along the velocity with a period of the order $a_m$.
