On the generation, propagation and radiation of magneto-acoustic-gravity waves with application to stars

摘要

The propagation, generation and radiation of magneto-acoustic-gravity (MAG) waves are modelled using the approach of a "wave analogy" (Campos 1987) analogous to the original "acoustic analogy" (Lighthill 1952), viz.: (i) the starting point is the general equations of magnetohydrodynamics (MHD) describing a viscous electrically conducting fluid in the presence of magnetic and gravity fields; (ii) a steady inhomogeneous mean state is considered, i.e. an atmosphere under non-uniform gravity and external magnetic field; (iii) the linear non-dissipative perturbation terms are eliminated for the velocity perturbation, leading to the magneto-acoustic-gravity wave operator, which combines sound, Alfvn and internal waves and their interactions; (iv) the non-linear and dissipative terms are retained to model the sources of MAG waves, which consist of an hydronamic tensor (Lighthill 1952), an analogous hydromagnetic tensor (Campos 1977), and a "new" hydrogravity tensor. For MAG wave propagation within an atmospheric scale height, the dispersion matrix is used to calculate the radiation field, leading to an estimate of the wave energy flux in the far field. The latter is calculated for the solar chromosphere, and is found to be comparable to the solar radiative losses, suggesting that hydromagnetic turbulence and ionized inhomogeneities are the sources of MAG waves which transport a substantial energy flux through the atmosphere of the sun. A second application is made to estimate the magnitude of magnetic non-degenerate stars based on the assumption that their luminosity is due to energy transport by MAG waves, whose sources are predominantly hydromagnetic; this leads to a scaling law between stellar magnitude and magnetic field strength, which may be a reasonable alternative to the Hertzsprung-Russell diagram for magnetic non-degenerate stars. This suggests that hydromagnetic sources of MAG waves dominate the energy transport in stellar atmospheres for r.m.s. magnetic field strengths of the order of 150 G or more.