  We present results from a detailed spectral-timing analysis of a long ~486 ks
XMM-Newton observation of the bare Seyfert 1 galaxy Ark 120 which showed
alternating diminution and increment in the 0.3-10 keV X-ray flux over four
consecutive orbits in 2014. We study the energy-dependent variability of Ark
120 through broad-band X-ray spectroscopy, fractional root-mean-squared (rms)
spectral modelling, hardness-intensity diagram and flux-flux analysis. The
X-ray (0.3-10 keV) spectra are well fitted by a thermally Comptonized primary
continuum with two (blurred and distant) reflection components and an optically
thick, warm Comptonization component for the soft X-ray excess emission below
~2 keV. During the first and third observations, the fractional X-ray
variability amplitude decreases with energy while for second and fourth
observations, X-ray variability spectra are found to be inverted-crescent and
crescent shaped respectively. The rms variability spectra are well modelled by
two constant reflection components, a soft excess component with variable
luminosity and a variable intrinsic continuum with the normalization and
spectral slope being correlated. The spectral softening of the source with both
the soft excess and UV luminosities favour Comptonization models where the soft
excess and primary X-ray emission are produced through Compton up-scattering of
the UV and UV/soft X-ray seed photons in the putative warm and hot coronae,
respectively. Our analyses imply that the observed energy-dependent variability
of Ark 120 is most likely due to variations in the spectral shape and
luminosity of the hot corona and to variations in the luminosity of the warm
corona, both of which are driven by variations in the seed photon flux.
