  Stellar variability strongly impacts the search for low-mass exoplanets with
radial velocity techniques. Two types of planet-free time series can be used to
quantify this impact: models and direct solar observations after a subtraction
of the Solar System planetary contribution. Comparing these approaches is
necessary for simulations. Our objective is to validate the amplitude of the
convective blueshift in plages used in our previous works, particularly in
blind tests, with HARPS-N solar data. We applied our model to the structures
observed at the time of observations and compared the radial velocity time
series. To complete our diagnosis, we studied the observed radial velocities
separately for each diffraction order derived from the individual
cross-correlation functions, as well as our line-by-line radial velocities. We
find that our previous model had been underestimating the amplitude of the
convective blueshift inhibition by a factor of about 2. A direct estimation of
the convective blueshift in the spectra explains the difference with previous
estimations obtained with MDI/SOHO Dopplergrams, based on the properties of the
Ni line. We identified several instrumental systematics: the presence of a 2
m/s peak-to-peak signal with a period of about 200 days in radial velocity and
bisector, which could be due to periodic detector warm-ups, a systematic
dependence of the long-term trend on wavelength possibly related to the
variability of the continuum over time, and/or an offset in radial velocity
after the interruption of several months in Oct. 2017. A large amplitude in the
convective blueshift inhibition of (360 m/s) must be used when building
synthetic times series for blind tests. The presence of instrumental
systematics should also be taken into account when using sophisticated methods
based on line properties to mitigate stellar activity when searching for very
weak signals.
