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Modeling the Subsurface Structure of Sunspots  

Hamed Moradi   Submitted: 2010-08-23 08:58

While sunspots are easily observed at the solar surface, determining their subsurface structure is not trivial. There are two main hypotheses for the subsurface structure of sunspots: the monolithic model and the cluster model. Local helioseismology is the only means by which we can investigate subphotospheric structure. However, as current linear inversion techniques do not yet allow helioseismology to probe the internal structure with sufficient confidence to distinguish between the monolith and cluster models, the development of physically realistic sunspot models are a priority for helioseismologists. This is because they are not only important indicators of the variety of physical effects that may influence helioseismic inferences in active regions, but they also enable detailed assessments of the validity of helioseismic interpretations through numerical forward modeling. In this paper, we provide a critical review of the existing sunspot models and an overview of numerical methods employed to model wave propagation through model sunspots. We then carry out an helioseismic analysis of the sunspot in Active Region 9787 and address the serious inconsistencies uncovered by Gizon et al. (2009a, 2009b). We find that this sunspot is most probably associated with a shallow, positive wave-speed perturbation (unlike the traditional two-layer model) and that travel-time measurements are consistent with a horizontal outflow in the surrounding moat.

Authors: Moradi, H.; Baldner, C.; Birch, A. C.; Braun, D.; Cameron, R.; Duvall, T. L., Jr.; Gizon, L.; Haber, D.; Hanasoge, S.; Hindman, B. W.; Jackiewicz, J.; Khomenko, E.; Komm, R.; Rajaguru, P.; Rempel, M.; Roth, M.; Schlichenmaier, R.; Schunker, H.; Spruit, H.; Strassmeier, K.; Thompson, M. J.; Zharkov, S.
Projects: SoHO-MDI

Publication Status: Solar Physics (accepted).
Last Modified: 2010-08-23 12:55
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Numerical models of travel-time inhomogeneities in sunspots  

Hamed Moradi   Submitted: 2008-11-09 00:01

We investigate the direct contribution of strong, sunspot-like magnetic fields to helioseismic wave travel-time shifts via two numerical forward models, a 3D ideal MHD solver and MHD ray theory. The simulated data cubes are analyzed using the traditional time-distance center-to-annulus measurement technique. We also isolate and analyze the direct contribution from purely thermal perturbations to the observed travel-time shifts, confirming some existing ideas and bring forth new ones: (i) that the observed travel-time shifts in the vicinity of sunspots are largely governed by MHD physics, (ii) the travel-time shifts are sensitively dependent on frequency and phase-speed filter parameters and the background power below the p1 ridge, and finally, (iii) despite its seeming limitations, ray theory succeeds in capturing the essence of the travel-time variations as derived from the MHD simulations.

Authors: H. Moradi, S. M. Hanasoge, P. S. Cally
Projects: None

Publication Status: Accepted, ApJ Letters.
Last Modified: 2008-11-09 18:21
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Hamed Moradi   Submitted: 2008-04-16 23:39

One of the main uncertainties associated with time-distance measurements is how to interpret observations made in regions of strong magnetic field as isolating the effects of the magnetic field from thermal or sound-speed perturbations has proved to be quite complex. One possible way to decouple these effects is by using a 3D sunspot model, with a surrounding stratified, quiet-Sun atmosphere to model the magneto-acoustic ray propagation, and analyze the resulting ray travel-time perturbations that will directly account for the magnetic field. These artificial travel-time perturbation profiles have provided us with several related but distinct observations: i) that strong surface magnetic fields have a dual effect on helioseismic rays - increasing their skip distance while at the same time speeding them up considerably compared to their quiet-Sun counterparts, ii) there is a clear and significant frequency dependence of both skip-distance and travel-time perturbations across the simulated sunspot radius, iii) the negative sign and magnitude of these perturbations appears to be directly related to the sunspot magnetic-field strength and inclination, iv) by ``switching off'' the magnetic field inside the sunspot, we are able to completely isolate the thermal component of the travel-time perturbations observed, which is seen to be both opposite in sign and much smaller in magnitude than those measured when the magnetic field is present. These results tend to suggest that purely thermal perturbations are unlikely to be the main effect seen in travel times through sunspots and that strong, near-surface magnetic fields may be directly and significantly altering the magnitude and lateral extent of sound-speed inversions of sunspots made by time-distance helioseismology.

Authors: H. Moradi, P.S. Cally
Projects: None

Publication Status: Accepted for publication in Solar Phsyics
Last Modified: 2008-09-23 21:10
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