  The formation of quasar broad-line region (BLR) clouds via thermal
instability in the presence of Alfven heating has been discussed by Goncalves
et al. (1993a, 1996). In particular, these studies showed the relevance of
Alfven heating in establishing the stability of BLR clouds in the intercloud
medium. The present paper shows the results of time-dependent calculations (we
use a time-dependent hydrodynamic code) following the evolution of BLR clouds,
since their formation from the 10^7 K intercloud medium. We also calculate the
UV and optical line emission associated with the clouds in order to compare
with observations. Our results are compared with those of UV and optical
monitoring of well-studied AGN, which suggest that the BLR is most probably
composed of at least two different regions, each one giving rise to a kind of
line variability, since low- and high-ionization lines present different
patterns of variability. We discuss the alternative scenario in which lines of
different ionization could be formed at the same place but heated/excited by
distinct mechanisms, considering as a non-radiative mechanism the Alfven
heating.
