We presented a simulation of the spurious velocities that active regions can produce in the measurement of the global Sun velocity with a resonance scattering spectrometer. The simulation is based on the spot and plage areas inferred from the BBSO CaII K line filtergrams, and on the sodium line profiles computed numerically in active regions model atmospheres.
In the simulation, plages play the major role in determining the
spurious velocities. The relative velocity affects the spurious velocities
at 
level through the 
effect.
The simulation allows to test calibration procedures and to
study the effect of different parameters
on the spurious velocities.
However, its ability to fit the offset velocities
observed by the IRIS project, even if rather good, is
not yet enough, and the use of the present approach
as a standard correction procedure to remove the active regions noise
deserves a refinement of the simulation as well as more
than one observing dataset to compare with.
The simulated velocities show a general agreement
with the observations, but cannot reproduce with the
same precision different observation days: the error is random.
We think that the simulation could be improved notably by using a more detailed representation of the plage contrast, to take into account that there is a distribution of plages on the disk with different contrasts or, also, there are large plages with a contrast spatially variable.
Furthermore, it will be useful to include in the model convective effects associated with plage areas, that can alter the plage contrast by shifting the plage profile with respect to the quiet Sun. A confirm might be considered the fact that the mean of all the simulation we ran with different parameters (V0 variations, smoothing time, cell passbands, plage contrast and shift) give the best correlation (0.76) and the best residual relative amplitude (0.68, i.e. variance 0.46).
In perspective, the methods presented in this paper will be used for an improved simulation based on the Doppler and Magnetic images collected by a MOF which is in course of implementation at the Osservatorio Astronomico di Capodimonte, in collaboration with A. Cacciani e P.F. Moretti (Dip. Fisica, Univ. La Sapienza, Roma).
Acknowledgements
It is a pleasure to thank Eric Fossat and Roger Ulrich who made available to us their data. Also, we thank Thomas Straus for reading the manuscript, the anonymous referee, and Jean Maurice Robillot, as second referee, for their helpful comments. This work was partially supported by the Italian Space Agency (ASI).