The effects of scattering on solar oscillations

Abstract

Acoustic modes are scattered by turbulent velocity fluctuations in the solar convection zone. The strongest scattering occurs near the top of the acoustic cavity where the mode changes character from propagating to evanescent. This layer is located at depth z_1 ~ g/ω^2 below the photosphere. The scattering optical depth τ_s of order M^2_1 where M_1 is the Mach number of the energy-bearing eddies at z_1. The corresponding contribution to the line width is y^s ~ ωM^2_1/π(n + 1), where n is the mode's radial order. At the top of the acoustic cavity the correlation time of energy-bearing eddies is much longer than ω^(-1). Also, the pressure scale height H and the eddy correlation length Λ are comparable to ω/c, where c is the sound speed. Thus scattering couples modes of similar ω and all ℓ and has little effect on the sum of their energies. Observations show that mode energies decline with decreasing n (increasing ℓ) at fixed ω. Consequently, scattering damps p-modes and excites ƒ-modes.

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