  We study the optical light curves - primarily probing the variable emission
from the accretion disk - of ~ 900 extreme variability quasars (EVQs, with
maximum flux variations more than 1 mag) over an observed-frame baseline of ~
16 years using public data from the SDSS Stripe 82, PanSTARRS-1 and the Dark
Energy Survey. We classify the multi-year long-term light curves of EVQs into
three categories roughly in the order of decreasing smoothness: monotonic
decreasing or increasing (3.7%), single broad peak and dip (56.8%), and more
complex patterns (39.5%). The rareness of monotonic cases suggests that the
major mechanisms driving the extreme optical variability do not operate over
timescales much longer than a few years. Simulated light curves with a damped
random walk model generally under-predict the first two categories with
smoother long-term trends. Despite the different long-term behaviors of these
EVQs, there is little dependence of the long-term trend on the physical
properties of quasars, such as their luminosity, BH mass, and Eddington ratio.
The large dynamic range of optical flux variability over multi-year timescales
of these EVQs allows us to explore the ensemble correlation between the
short-term (< 6 months) variability and the seasonal-average flux across the
decade-long baseline (the rms-mean flux relation). We find that unlike the
results for X-ray variability studies, the linear short-term flux variations do
not scale with the seasonal-average flux, indicating different mechanisms that
drive the short-term flickering and long-term extreme variability of accretion
disk emission. Finally, we present a sample of 16 EVQs, where the approximately
bell-shaped large amplitude variation in the light curve can be reasonably well
fit by a simple microlensing model.
