  Transiting ultra-hot Jupiters are ideal candidates to study the exoplanet
atmospheres and their dynamics, particularly by means of high-resolution, high
signal-to-noise ratio spectra. One such object is KELT-20b, orbiting the fast
rotating A2-type star KELT-20. Many atomic species have already been found in
its atmosphere, with blueshifted signals that hints at the presence of a
day-to-night side wind. We aimed to observe the atmospheric Rossiter-McLaughlin
effect in the ultra-hot Jupiter KELT-20b, and to study any variation of the
atmospheric signal during the transit. For this purpose, we analysed five
nights of HARPS-N spectra covering five transits of KELT-20b. We computed the
mean line profiles of the spectra with a least-squares deconvolution, and then
we extracted the stellar radial velocities by fitting them with a rotational
broadening profile in order to obtain the radial velocity time-series. We used
the mean line profile residuals tomography to analyse the planetary atmospheric
signal and its variations. We also used the cross-correlation method to study
an already known double-peak feature in the FeI planetary signal. We observed
both the classical and the atmospheric Rossiter-McLaughlin effect in the radial
velocity time-series. The latter gave us an estimate of the radius of the
planetary atmosphere that correlates with the stellar mask used in our work:
R(p+atmo)/Rp = 1.13 +/- 0.02). We isolated the planetary atmospheric trace in
the tomography, and we found radial velocity variations of the planetary
atmospheric signal during transit with an overall blueshift of approximatively
10 km/s, along with small variations in the signal's depth and, less
significant, in the full width at half maximum (FWHM). We also find a possible
variation in the structure and position of FeI signal in different transits.
