  Active galactic nuclei (AGNs) have long been observed to "twinkle" (i.e.,
their brightness varies with time) on timescales from days to years in the
UV/optical bands. Such AGN UV/optical variability is essential for probing the
physics of supermassive black holes (SMBHs), the accretion disk, and the
broad-line region. Here we show that the temperature fluctuations of an AGN
accretion disk, which is magnetically coupled with the corona, can account for
observed high-quality AGN optical light curves. We calculate the temperature
fluctuations by considering the gas physics of the accreted matter near the
SMBH. We find that the resulting simulated AGN UV/optical light curves share
the same statistical properties as the observed ones as long as the
dimensionless viscosity parameter $\alpha$, which is widely believed to be
controlled by magnetohydrodynamic (MHD) turbulence in the accretion disk, is
about $0.01$---$0.2$. Moreover, our model can simultaneously explain the
larger-than-expected accretion disk sizes and the dependence of UV/optical
variability upon wavelength for NGC 5548. Our model also has the potential to
explain some other observational facts of AGN UV/optical variability, including
the timescale-dependent bluer-when-brighter color variability and the
dependence of UV/optical variability on AGN luminosity and black hole mass. Our
results also demonstrate a promising way to infer the black-hole mass, the
accretion rate, and the radiative efficiency, thereby facilitating
understanding of the gas physics and MHD turbulence near the SMBH and its
cosmic mass growth history by fitting the AGN UV/optical light curves in the
era of time-domain astronomy.
