  Data acquired by the Virgo interferometer during the second part of the O3
scientific run, referred to as O3b, were analysed with the aim of
characterising the onset and time evolution of scattered light noise in
connection with the variability of microseismic noise in the environment
surrounding the detector. The adaptive algorithm used, called pytvfemd, is
suitable for the analysis of time series which are both nonlinear and
nonstationary. It allowed to obtain the first oscillatory mode of the
differential arm motion degree of freedom of the detector during days affected
by scattered light noise. The mode's envelope i.e., its instantaneous
amplitude, is then correlated with the motion of the West end bench, a known
source of scattered light during O3. The relative velocity between the West end
test mass and the West end optical bench is used as a predictor of scattered
light noise. Higher values of correlation are obtained in periods of higher
seismic noise in the microseismic frequency band. This is also confirmed by the
signal-to-noise ratio (SNR) of scattered light glitches from GravitySpy for the
January-March 2020 period. Obtained results suggest that the adopted
methodology is suited for scattered light noise characterisation and monitoring
in gravitational wave interferometers.
