A. Jimenez et al., Phase differences and gains between intensity and velocity in low-degree acoustic modes measured by SOHO, ASTROPHYS J, 525(2), 1999, pp. 1042-1055
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.