  We present a statistical analysis of the variability of broad absorption
lines (BALs) in quasars using the large multi-epoch spectroscopic dataset of
the Sloan Digital Sky Survey Data Release 12 (SDSS DR12). We divide the sample
into two groups according to the pattern of the variation of C iv BAL with
respect to that of continuum: the equivalent widths (EW) of the BAL decreases
(increases) when the continuum brightens (dims) as group T1; and the variation
of EW and continuum in the opposite relation as group T2. We find that T2 has
significantly (P_T<10-6 , Students T Test) higher EW ratios (R) of Si iv to C
iv BAL than T1. Our result agrees with the prediction of photoionization models
that C +3 column density increases (decreases) if there is a (or no) C +3
ionization front while R decreases with the incident continuum. We show that
BAL variabilities in at least 80% quasars are driven by the variation of
ionizing continuum while other models that predict uncorrelated BAL and
continuum variability contribute less than 20%. Considering large uncertainty
in the continuum flux calibration, the latter fraction may be much smaller.
When the sample is binned into different time interval between the two
observations, we find significant difference in the distribution of R between
T1 and T2 in all time-bins down to a deltaT < 6 days, suggesting that BAL
outflow in a fraction of quasars has a recombination time scale of only a few
days.
