1OSCILLATION SPECTRA OF LINE DEPTH, INTENSITY AND VELOCITY FROM RADIATIVE TRANSFER CALCULATIONS

Theoretical spectra for solar oscillations, as they are ob-served by the MDI instrument, have been calculated by modifying a model of the solar interior according to the oscillations of temperature, pressure, density, and veloc-ity, and calculating the detailed radiative transfer for the Ni I absorption line at 6768 A ̊ with the radiative trans-fer code COSI. Using the filter functions from the MDI instrument, MDI velocity, intensity, and line depth spec-tra are modelled. We investigate the intrinsic mode peak asymmetry of velocity, continuum intensity, and line depth spectra. We compare this to the recent results of Georgobiani et al. (2003) who found positive asymmetry for resonance peaks of high-l modes in intensity. Fur-thermore, we look at the mutual phases of the different quantities, and we show that the decay in power with in-creasing frequency is most pronounced in velocity and least pronounced in line depth spectra. This is a possible explanation for the high pseudo-mode visibility in line depth spectra. Key words: Sun: oscillations, radiative transfer. 1. MDI OBSERVATIONS The MDI (see Scherrer et al. (1995) for a detailed de-scription) consists of two tunable Michelson interferom-eters and a Lyot filter, which realize successively five narrow-banded filters F0: : : F4 with a separation of 75 mÅ. These filters cover the observed Doppler shift of the Ni I ¸ 6768 absorption line. The filter profiles and the ab-sorption line are displayed in Figure 1. From the function ® ®> = ((F1 ¡ F3) + (F2 ¡ F4)) =(F1 ¡ F3); ®< = ((F1 ¡ F3) + (F2 ¡ F4)) =(F4 ¡ F2); (1) ( ® = ®> for a positive numerator, ® = ®< for a nega-tive numerator), the line-of-sight velocity is obtained b