  During eruptive flares, vector magnetograms show increasing horizontal
magnetic field and downward Lorentz force in the Sun's photosphere around the
polarity-inversion line. Such behavior has often been associated with the
implosion conjecture and interpreted as the result of either momentum
conservation while the eruption moves upward, or of the contraction of flare
loops. We characterize the physical origin of these observed behaviors by
analyzing a generic 3D MHD simulation of an eruptive flare. Even though the
simulation was undesigned to recover the magnetic field and Lorentz force
properties, it is fully consistent with them, and it provides key additional
informations to understand them. The area where the magnetic field increases
gradually develops between current ribbons, which spread away from each other
and are connected to the coronal region. This area is merely the footprint of
the coronal post-flare loops, whose contraction increases their shear field
component and the magnetic energy density in line with the ideal induction
equation. For simulated data, we computed the Lorentz force density map by
applying the method used in observations. We obtained increase of the downward
component of the Lorentz force density around the PIL -consistent with
observations. However, this significantly differs from the Lorentz force
density maps obtained directly from the 3D magnetic field and current. These
results altogether question previous interpretations based on the implosion
conjecture and momentum conservation with the CME, and rather imply that the
observed increases in photospheric horizontal magnetic fields result from the
reconnection-driven contraction of sheared flare-loops.
