  We investigate the plasma and magnetic environment of active region NOAA
11261 on 2 August 2011 around a GOES M1.4 flare/CME (SOL2011-08-02T06:19). We
compare coronal emission at (extreme) ultraviolet and X-ray wavelengths, using
SDO AIA and RHESSI images, in order to identify the relative timing and
locations of reconnection-related sources. We trace flare ribbon signatures at
ultraviolet wavelengths, in order to pin down the intersection of previously
reconnected flaring loops at the lower solar atmosphere. These locations are
used to calculate field lines from 3D nonlinear force-free magnetic field
models, established on the basis of SDO HMI photospheric vector magnetic field
maps. With this procedure, we analyze the quasi-static time evolution of the
coronal model magnetic field previously involved in magnetic reconnection. This
allows us, for the first time, to estimate the elevation speed of the current
sheet's lower tip during an on-disk observed flare, as a few kilometers per
second. Comparison to post-flare loops observed later above the limb in STEREO
EUVI images supports this velocity estimate. Furthermore, we provide evidence
for an implosion of parts of the flaring coronal model magnetic field, and
identify the corresponding coronal sub-volumes associated to the loss of
magnetic energy. Finally, we spatially relate the build up of magnetic energy
in the 3D models to highly sheared fields, established due to dynamic relative
motions of polarity patches within the active region.
