Genetic magnetohelioseismology with Hankel analysis data

Abstract

Hankel analysis determined that sunspots absorb energy from and shift the phase of f- and p-modes incident upon them. One promising mechanism that can explain the absorption is partial conversion to slow magnetoacoustic-gravity (MAG) waves and Alfvén waves, which guide energy along the magnetic field away from the acoustic cavity. Our recent mode conversion calculations demonstrated that simple sunspot models, which roughly account for the radial variation of the magnetic field strength and inclination, can produce ample absorption to explain the observations, along with phase shifts that agree remarkably well with the Hankel analysis data. In this paper, we follow the same approach, but adopt a more realistic model for the solar convection zone that includes the thermal perturbation associated with a sunspot's magnetic field. Consistent with our earlier findings, we show that a moderately inclined, uniform magnetic field exhibits significantly enhanced absorption (mode conversion) in comparison to a vertical field (depending on the frequency and radial order of the mode). A genetic algorithm is employed to adjust the parameters that control the radial structure of our sunspot models, in order to minimize the discrepancy between the theoretical predictions and the Hankel analysis measurements. For models that best fit the phase shifts, the agreement with the Hankel analysis data is excellent, and the corresponding absorption coefficients are generally in excess of the observed levels. On the other hand, for models that best fit the phase shift and absorption data simultaneously, the overall agreement is very good but the phase shifts agree less well. This is most likely caused by the different sizes of the regions responsible for the absorption and phase shift. Typically, the field strengths required by such models lie in the range 1-3 kG, compatible with observations for sunspots and active regions. While there remain some uncertainties, our results provide further evidence that mode conversion is the predominant mechanism responsible for the observed absorption in sunspots; and that field inclination away from vertical is a necessary ingredient for any model that aims to simultaneously explain the phase shift and absorption data. © 2005 RAS.