  Using the United Kingdom Infrared Telescope on Mauna Kea, we have carried out
a new near-infrared J, H, K monitoring survey of almost a square degree of the
star-forming Orion Nebula Cluster with observations on 120 nights over three
observing seasons, spanning a total of 894 days. We monitored ~15,000 stars
down to J=20 using the WFCAM instrument, and have extracted 1203 significantly
variable stars from our data. By studying variability in young stellar objects
(YSOs) in the H-K, K color-magnitude diagram, we are able to distinguish
between physical mechanisms of variability. Many variables show color behavior
indicating either dust-extinction or disk/accretion activity, but we find that
when monitored for longer periods of time, a number of stars shift between
these two variability mechanisms. Further, we show that the intrinsic timescale
of disk/accretion variability in young stars is longer than that of
dust-extinction variability. We confirm that variability amplitude is
statistically correlated with evolutionary class in all bands and colors. Our
investigations of these 1203 variables have revealed 73 periodic AA Tau type
variables, many large-amplitude and long-period (P > 15 day) YSOs, including
three stars showing widely-spaced periodic brightening events consistent with
circumbinary disk activity, and four new eclipsing binaries. These phenomena
and others indicate the activity of long-term disk/accretion variability
processes taking place in young stars. We have made the light curves and
associated data for these 1203 variables available online.
