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Solar core rotation from asymptotic g modes

Dr. Eric Fossat
Observatoire de la Côte d'Azur

April 11th, 2017


Helioseismology is about 40 years old, still a young science. It has been a tremendous success providing many more results than initially expected, including those coming from Izana of course. Now we really know a lot of the solar internal structure and rotation.

However, one important parameter has still resisted to this investigation, the solar core rotation, which is not accessible to acoustic modes of oscillation, and helioseismlogy successes have all been obtained from acoustic modes. The reason is simple: the second type of seismic oscillations, called g modes (g for gravity, as the waves on the sea) are confined in the deepest layers of the Sun, while the observers are staying outside. These g modes that contain the information on all properties of the solar core have never been convincingly detected despite many efforts and attempts during the last forty years.

We have used a differential parameter of the acoustic modes, carefully selecteded to have a maximum sensitivity to the deepest layers and a minimum sensitivity to the surface layers, to look for its possible modulation produced by periodic motions in the solar core. The frequencies possibly accessible are very low, they correspond to periods between about half a day and two days. The advantage is that in this very low frequency range, if g modes exist, they must follow an asymptotic behaviour that makes possible a collective detection. Using a long data set (16.5 years) from the GOLF instrument onboard  the SOHO space mission, the result is the success of this search, and I will present you these asymptotic parameters, including the measurement of the core rotation within less than 1 percent uncertainty.