  We discover the significant (significance level of >99%) correlations between
the fractional variation of the ionizing continuum and that of the C IV and/or
Si IV BALs in each of 21 BAL quasars that have at least five-epoch observations
from the Sloan Digital Sky Survey-I/II/III. This result reveals that the
fluctuation of the ionizing continuum is the driver of most of these BAL
variations. Among them, 17 show negative correlations and the other 4 positive
correlations, which agrees with the prediction of photoionization models that
absorption line variability response to ionization changes is not monotonic. 8
quasars out of 21 examples have been observed at least 30 times on rest-frame
timescales as short as a few days, which reveals that changes in the incident
ionizing continuum can cause BAL variability even in such a short period of
time. In addition, we find that most of the 21 quasars show larger variation
amplitude in Si IV than C IV, which reveals the ubiquity of saturation in these
BALs (at least for C IV BALs).
