5.9 Gravity modes

One possible way to improve the constraints on the core rotation would be to use g modes, or gravity waves, instead of p modes. Because these modes have their greatest amplitude in the solar interior, they should be much more sensitive to the core properties. Unfortunately, they also have very small amplitudes at the surface. The history of helioseismology is littered with unconfirmed reports of g-mode identification; see, for example, Delache and Scherrer (1983), van der Raay (1988),Thomson et al. (1995), and the review by Hill et al. (1991b). The most promising recent work has been carried out using long time series from the GOLF instrument aboard SOHO. Appourchaux et al. (2000b) placed an upper limit of 10 mm/s on g-mode amplitudes based on two years of observations, and Gabriel et al. (2002) reduced this limit further, to 6 mm/s, using 5 years of data. Most recently, García et al. (2007) report finding a pattern of peaks with constant spacing in period corresponding to the model-predicted spacing for l = 2 g modes with δl = 0,δn = 1, and with a splitting that they interpret as corresponding to a core rotation rate of 3 – 5 times the surface rate; however, this is still a preliminary result in need of confirmation.

In a related paper, Mathur et al. (2007) point out that the current predictions for low-order g-mode frequencies are much more consistent than was the case a decade earlier, resulting in a period for the fundamental g-mode between 34 – 35 minutes. This finding does make one wonder about the usefulness of the g-mode observations for discriminating among models; on the other hand, it lends somewhat more credence to the current identification.

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