  The VIIRS Day Night Band sensor on the Suomi NPP satellite provides almost a
decade of observations of night light. The daily frequency of sampling, without
the temporal averaging of annual composites, requires the distinction between
apparent changes of imaged night light related to the imaging process and
actual changes in the underlying sources of the light being imaged. This study
characterizes night light variability over a range of spatial and temporal
scales to provide a context for interpretation of changes on both subannual and
interannual time scales. This analysis uses a combination of temporal moments,
spatial correlation and Empirical Orthogonal Function (EOF) analysis. A key
result is the pervasive heteroskedasticity of VIIRS monthly mean night light.
Specifically, the monotonic decrease of temporal variability with increasing
mean brightness. Anthropogenic night light is remarkably stable on subannual
time scales. Overall variance partition derived from the eigenvalues of the
spatiotemporal covariance matrix are 88%, 2% and 2% for spatial, seasonal and
interannual variance in the most diverse geographic region on Earth (Eurasia).
Heteroskedasticity is present in all areas for all months, suggesting that
much, if not most, of observed month-to-month variability may result from
luminance of otherwise stable sources subjected to multiple aspects of the
imaging process varying in time. Given the skewed distribution of all night
light arising from radial peripheral dimming of bright sources, even aggregate
metrics using thresholds must be interpreted in light of the fact that much
larger numbers of more variable low luminance pixels may statistically
overwhelm smaller numbers of stable higher luminance pixels and cause apparent
changes related to the imaging process to be interpreted as actual changes in
the light sources.
