In a few years, the conditions of propagation were made more precise due to the Fourier spectra in time and space.
It appeared that the oscillations, with a period of close to 5 minutes, were due to acoustic waves modified by gravity, resulting in velocity and pressure (p mode) fluctuations.
Therefore some brilliance fluctuations, as well as velocity fluctuations, appear. In 1975, Deubner highlighted the global nature of the oscillations (after long observation sequences), the separation of "modes," and the comparison with theoretical models of the internal structure of the Sun.
The agreement between observations and theoretical models, which were already strongly convincing in 1975, have been significantly improved since.
The oscillations observed in the spectrum come from irregularities of the surface of the Sun associated with matter movements.