The presence of a fibril magnetic field in the solar envelope not only induces shifts in the p-mode resonant frequencies, but also contributes to the line width of the modes. The augmentation of the line widths results from two related physical processes : the excitation of tube mode oscillations on the individual magnetic fibrils and the attendant mode mixing between p-modes with identical oscillation frequencies. We assay the magnitude of the contribution from the former physical process based upon an idealized model consisting of vertical, slender, magnetic flux tubes embedded in a plane-parallel isentropic polytrope of index m. We restrict our attention to axisymmetric flux tubes that are in mechanical and thermal equilibrium with their immediate nonmagnetic surroundings. For low p-mode oscillation frequencies, ω, this model predicts that the line width, Γ, varies as Γ ∝ fωAl-1/2 ∝ fωm+2 , where Al is the mode mass, and f is the magnetic filling factor reckoned at the surface of the polytrope. This scaling is in better overall agreement with the observations (F ∝ ω4.2) than previous predictions based on the excitation and damping of solar p-modes by turbulent convection (which yields Γ ∝ ω2Al-1 ∝ ω2m+4), or the scattering of p-modes by convective eddies (which yields Γ ∝ ω(4/3)m+3), and it suggests that tube mode excitation on fibril magnetic fields may be a dominant and detectable (through its solar cycle variation) component of the low-frequency p-mode line widths.
- Sun: Magnetic fields
- Sun: Oscillations