Phase differences and gains between intensity and velocity in low-degree acoustic modes measured by SOHO

Helioseismic instruments aboard SOHO are making possible a more accurate wa y of investigating the internal structure of the Sun. Making use of the dif ferent techniques and characteristics of these instruments, it is possible to measure solar oscillations as variations of the photospheric velocity (G OLF, MDI) or as irradiance and radiance fluctuations (VIRGO, MDI). Among th e other advantages of observing solar oscillations simultaneously with diff erent instruments and techniques, the study of velocity and irradiance meas urements provides information on nonadiabatic effects in the radiatively co oled solar atmosphere. The thermodynamical properties of the atmosphere det ermine a phase shift between intensity and velocity (downward positive) osc illations of -90 degrees in the case of an adiabatic atmosphere. Here we co mpute the phase differences and gains between intensity and velocity acoust ic modes measured by SOHO to quantify the nonadiabatic degree of the solar atmosphere. After correcting the observed phase differences of the solar ba ckground influence, we find not exactly an adiabactic behavior, but close t o it. Finally, we compare our results with three different theoretical mode ls of the solar atmosphere, finding the best agreement with a model that in cludes turbulent pressure associated with convection and fluctuations of th e superadiabatic temperature gradient.