Hydrodynamics of the solar interior

We have studied the MHD waves for homogeneous plasmas in Chapter 5. This theory was transformed in Chapter 6 to the higher level of spectral theory in order to facilitate the much more complicated analysis of inhomogeneous plasmas, which we want to undertake in the present chapter. Plasma inhomogeneity is not just a complication in the analysis, but also provides qualitatively new physical phenomena like wave damping, wave transformation, and, most important of all, a very wide class of global MHD instabilities of magnetically confined plasmas.

Explicit examples of inhomogeneous plasma dynamics abound in the solar system, as we will see in Chapter 8. For the Sun, a number of important phenomena may be described neglecting the magnetic field. Therefore, before we turn to magnetized plasmas in Section 7.3, we will first simplify the model to a purely hydrodynamic one and study the effects of sound and gravity separate from the three MHD waves. Since the hydrodynamic waves are clearly identified in solar observations, we will be able to clarify the potential of observing MHD wave propagation for the investigation of astrophysical objects in general (Section 7.2.4).

We summarize some basic facts of the standard solar model (see Priest [190], Stix [217] or Foukal [69]). The Sun is a sphere of hot material, mainly plasma, of radius R⊙ = 7.0 × 108 m and mass M⊙ = 2.0 × 1030 kg.