Solar Oscillations and Convection. II. Excitation of Radial Oscillations

By Robert F. Stein, Åke Nordlund. 2000

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Solar p-mode oscillations are excited by the work of stochastic, nonadiabatic, pressure fluctuations on the compressive modes. We evaluate the expression for the radial mode excitation rate derived by Nordlund & Stein using numerical simulations of near-surface solar convection. We first apply this expression to the three radial modes of the simulation and obtain good agreement between the predicted excitation rate and the actual mode damping rates as determined from their energies and the widths of their resolved spectral profiles. These radial simulation modes are essentially the same as the solar modes at the resonant frequencies, where the solar modes have a node at the depth of the bottom of the simulation domain. We then apply this expression for the mode excitation rate to the solar modes and obtain excellent agreement with the low ℓ damping rates determined from data obtained by the "global oscillations at low frequencies" (GOLF) instrument on SOHO. Excitation occurs close to the surface, mainly in the intergranular lanes and near the boundaries of granules (where turbulence and radiative cooling are large). The nonadiabatic pressure fluctuations near the surface are produced by small instantaneous local imbalances between the divergence of the radiative and convective fluxes near the solar surface. Below the surface, the nonadiabatic pressure fluctuations are produced primarily by turbulent-pressure fluctuations (Reynolds stresses). The frequency dependence of the mode excitation is due to effects of the mode structure and the pressure fluctuation spectrum. Excitation is small at low frequencies because of mode properties—the mode compression decreases and the mode mass increases at low frequency. Excitation is small at high frequencies because of the pressure fluctuation spectrum—pressure fluctuations become small at high frequencies because they are due to convection, which is a long-timescale phenomenon compared with the dominant p-mode periods.

Subfields: Astronomy and Astrophysics, Atomic and Molecular Physics, and Optics, Computational Mechanics

View Paper: https://www.semanticscholar.org/paper/904eceec37218cae1c968d9b0d64a786e4adb9a0

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Citations in the corpus

Dynamic Hydrogen Ionization (2002, background): “The upper convection zone is the site of acoustic wave generation (Stein & Nordlund 2001; Skartlien et al. 2000; Goldreich et al. 1994), while overshooting convective flows in the photosphere generate gravity waves.”
The Stagger-grid: A grid of 3D stellar atmosphere models - I. Methods and general properties (2013, background): “Also, it was found that with 3D solar models the predicted p-mode excitation rates are much closer to helioseismic observations(Nordlund & Stein 2001; Stein & Nordlund 2001).”
CoRoT sounds the stars: p-mode parameters of Sun-like oscillations on HD 49933 (2008, result): “Stein & Nordlund (2001) showed that the contribution of the entropy fluctuations to the mode driving is rather small in the case of the Sun (of the order of 15%), a result verified by Samadi et al. (2003).”
Improvements to stellar structure models, based on a grid of 3D convection simulations. II. Calibrating the mixing-length formulation (2014, methodology): “They have also been employed in a calculation of p-mode excitation for a range of stars (Stein et al. 2005; Samadi et al. 2007; Stein et al. 2007), based on the formulation by Nordlund & Stein (2001); Stein & Nordlund (2001).”
Line formation in solar granulation IV. (O I), O I and OH lines and the photospheric O abundance (2003, background): “…of observational constraints, such as spectral line shapes, shifts and asymmetries (Paper I; Allende Prieto et al. 2002a), helioseismology (Rosenthal et al. 1999; Stein & Nordlund 2001), ﬂux distribution and limb-darkening(Asplund et al. 1999b) and granulation properties (Stein & Nordlund 1998).”
A publicly available simulation of an enhanced network region of the Sun (2015, unknown): “The relaxation to a given effective temperature is a very slow process and in the en024048 hion simulation the effective temperature is typically lower than that of the Sun, see Section 3.2.”