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The possibility to detect solar oscillations in the low frequency domain depends crucially on the power contrast among the oscillation signal and other time de- pendent signals in the same frequency range. The signal to noise ratio is increased by our ability to understand and remove solar sources of noise. In measurements of the mean Doppler velocity shift of the integrated solar disk, the solar noise has a line component spectrum with a ma- jor peak at 13.1 days, and a second less prominent peak at 27.2 days. Active region modulation is believed almost completely responsible for this signal. We develop simulations of the flux and velocity fluctu- ations produced by dierent solar active region distribu- tions, based on an analytical description of their action. From a grid of models of active regions and from their spa- tial distribution over the disk, we calculate the synthetic flux prole in the Na I D1 line and determine the velocity measure of a resonance spectrometer. Our velocity results are compared with the oset ve- locities from the IRIS network. There is a rather good agreement between the observed and computed velocities, and the plage contribution to the noise appears to be dom- inant. The simulation allows to test calibration procedures and to study the eect on the spurious velocities of dier- ent parameters, such as the intensity thresholds used to determine the areas of spots and plages, and the contrast of the active regions. In particular, we nd that the inclu- sion of intrinsic line shifts in plages can change strongly both the amplitude and the shape of the simulated signal, and then may be an important source of uncertainty for the simulation.

Active region effects on solar irradiance at Na I D lines