Phase Differences and Gains between Intensity and Velocity in Low-Degree Acoustic Modes Measured by SOHO*

摘要

Helioseismic instruments aboard SOHO are making possible a more accurate way of investigating the internal structure of the Sun. Making use of the different techniques and characteristics of these instruments, it is possible to measure solar oscillations as variations of the photospheric velocity (GOLF, MDI) or as irradiance and radiance fluctuations (VIRGO, MDI). Among the other advantages of observing solar oscillations simultaneously with different instruments and techniques, the study of velocity and irradiance measurements provides information on nonadiabatic effects in the radiatively cooled solar atmosphere. The thermodynamical properties of the atmosphere determine a phase shift between intensity and velocity (downward positive) oscillations of -90° in the case of an adiabatic atmosphere. Here we compute the phase differences and gains between intensity and velocity acoustic modes measured by SOHO to quantify the nonadiabatic degree of the solar atmosphere. After correcting the observed phase differences of the solar background influence, we find not exactly an adiabactic behavior, but close to it. Finally, we compare our results with three different theoretical models of the solar atmosphere, finding the best agreement with a model that includes turbulent pressure associated with convection and fluctuations of the superadiabatic temperature gradient.