Quantifying Variability of YSOs in the Mid-IR Over Six Years with
  NEOWISE
  Variability in young stellar objects (YSOs) can be caused by various
time-dependent phenomena associated with star formation, including accretion
rates, geometric changes in the circumstellar disks, stochastic hydromagnetic
interactions between stellar surfaces and inner disk edges, reconnections
within the stellar magnetosphere, and hot/cold spots on stellar surfaces. We
uncover and characterize $\sim$1700 variables from a sample of $\sim$5400 YSOs
in nearby low-mass star-forming regions using mid-IR light curves obtained from
the 6.5-years NEOWISE All Sky Survey. The mid-IR variability traces a wide
range of dynamical, physical, and geometrical phenomenon. We classify six types
of YSO mid-IR variability based on their light curves: secular variability
($Linear, Curved, Periodic$) and stochastic variability ($Burst, Drop,
Irregular$). YSOs in earlier evolutionary stages have higher fractions of
variables and higher amplitudes for the variability, with the recurrence
timescale of FUor-type outbursts (defined here as $\Delta$W1 or $\Delta$W2 $>1$
mag followed by inspection of candidates) of $\sim$1000 years in the early
embedded protostellar phase. Known eruptive young stars and subluminous objects
show fractions of variables similar to the fraction ($\sim55\%$) found in
typical protostars, suggesting that these two distinct types are not distinct
in variability over the 6.5-year timescale. Along with brightness variability,
we also find a diverse range of secular color variations, which can be
attributed to a competitive interplay between the variable accretion luminosity
of the central source and the variable extinction by material associated with
the accretion process.
