  We show that time variations in the UV ionizing continuum of quasars, on
scales of $\sim$1 year, affect the dynamic structure of the plasmas responsible
for low ionization broad absorption lines. Variations of the ionizing continuum
produce non-equilibrium photoionization conditions over a significant fraction
of the absorbing clouds and supersonically moving ionization fronts. When the
flux drops the contraction of the ionized region drives a supersonic cooling
front towards the radiation source and a rarefaction wave in the opposite
direction. The pressure imbalance is compensated by an increased speed of the
cool gas relative to the front. When the flux recovers the cool gas is
re-ionized and re-heated by a supersonic ionization front traveling away from
the radiation source and a forward shock is created. The reheated clouds
equilibrate to a temperature of $\sim 10^4$ K and are observed to have
different radial velocities than the main cloud. Such fragmentation seems
consistent with the multicomponent structure of troughs seen in some objects.
The velocity differences measured among various components in the quasars QSO
2359--1241 and SDSS J0318--0600 can be reproduced by our model if strong
magnetic fields ($\sim$10 mG) are present within the clouds.
