Different oscillation modes are sensitive to different regions of the solar interior; for example, high- modes sample only the near-surface layers whereas low- modes penetrate much deeper. The oscillation frequencies are weighted integrals over the sampling region (loosely, the ray path) so some inversion procedure is necessary to infer solar interior properties such as the variation of the sound speed with depth (Christensen-Dalsgaard, 2002). The inversions are usually assumed to be linear so weighted summations over different frequencies can be used to derive averaging kernals which are sensitive to localized regions of the solar interior. Parametric representations may also be used, with minimization procedures to determine the best fit to solar data. Global inversions generally become less reliable in the polar regions and in the deep interior which are not well-sampled by observable oscillation modes.
With regard to solar interior dynamics, the most important feature of global acoustic oscillations is their so-called rotational splitting. In a non-rotating star, the frequencies of resonant acoustic oscillations are independent of the spherical harmonic order (neglecting the asphericity caused by flows or magnetic fields). This is no longer the case when the effects of rotation are included. The resulting frequency shifts are small relative to the reference frequency so they can be reliably treated as perturbations. Helioseismic inversions can then be used to infer the internal rotation profile as a function of latitude and radius as shown in Figure 1.
A limitation of global helioseismology is that the inversions used to infer rotation profiles or structural quantities such as sound speed are only sensitive to the component which is symmetric about the equator. Furthermore, they are insensitive to meridional circulations and non-axisymmetric convective motions. In order to probe such dynamics other techniques are necessary, the most promising being local helioseismology.
© Max Planck Society and the author(s)