  Examining flare data observed by TRACE satellite from May 1998 to December
2006, we choose 190 (151 M-class and 39 X-class) flare events which display
post-flare loops (PFLs), observed by 171 \AA and 195 \AA wavelengths. 124 of
the 190 events exhibit flare ribbons (FRs), observed by 1600 \AA images. We
investigate the propagation of the brightening of these PFLs along the neutral
lines and the separation of the FRs perpendicular to the neutral lines. In most
of the cases, the length of the FRs ranges from 20 Mm to 170 Mm. The
propagating duration of the brightening is from 10 to 60 minutes, and from 10
minutes to 70 minutes for the separating duration of the FRs. The velocities of
the propagation and the separation range from 3 km/s to 39 km/s and 3 km/s to
15 km/s, respectively. Both of the propagating velocities and the separating
velocities are associated with the flare strength and the length of the FRs. It
appears that the propagation and the separation are dynamically coupled, that
is the greater the propagating velocity is, the faster the separation is.
Furthermore, a greater propagating velocity corresponds to a greater
deceleration (or acceleration). These PFLs display three types of propagating
patterns. Type I propagation, which possesses about half of all the events, is
that the brightening begins at the middle part of a set of PFLs, and propagates
bi-directionally towards its both ends. Type II, possessing 30%, is that the
brightening firstly appears at one end of a set of PFLs, then propagates to the
other end. The remnant belongs to Type III propagation which displays that the
initial brightening takes place at two (or more than two) positions on two (or
more than two) sets of PFLs, and each brightening propagates bi-directionally
along the neutral line.
