Active region effects on solar irradiance at Na I D lines

¹ Dipartimento di Scienze Fisiche dell' Università "Federico II", Mostra d' Oltremare pad. 19, I-80125 Napoli, Italy
² Osservatorio Astronomico di Capodimonte, Via Moiariello 16, I-80131 Naples, Italy

Send offprint request to: G. Severino

Received: 9 July 1996
Accepted: 3 January 1997

Abstract

The possibility to detect solar oscillations in the low frequency domain depends crucially on the power contrast among the oscillation signal and other time dependent 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 major 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 fluctuations produced by different solar active region distributions, based on an analytical description of their action. From a grid of models of active regions and from their spatial distribution over the disk, we calculate the synthetic flux profile in the Na I line and determine the velocity measure of a resonance spectrometer. Our velocity results are compared with the offset velocities 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 dominant. The simulation allows to test calibration procedures and to study the effect on the spurious velocities of different 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 find that the inclusion 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.