Sensitivity of helioseismic travel-times to the imposition of a Lorentz force limiter in computational helioseismology

Abstract

The rapid exponential increase in the Alfvén wave speed with height above the solar surface presents a serious challenge to physical modelling of the effects of magnetic fields on solar oscillations, as it introduces a significant CFL time-step constraint for explicit numerical codes. A common approach adopted in computational helioseismology, where long simulations in excess of 10 hours (hundreds of wave periods) are often required, is to cap the Alfvén wave speed by artificially modifying the momentum equation when the ratio between Lorentz and hydrodynamic forces becomes too large. However, recent studies have demonstrated that the Alfvén wave speed plays a critical role in the MHD mode conversion process, particularly in determining the reflection height of the upward propagating helioseismic fast wave. Using numerical simulations of helioseismic wave propagation in constant inclined (relative to the vertical) magnetic fields we demonstrate that the imposition of such artificial limiters significantly affects time-distance travel times unless the Alfvén wave-speed cap is chosen comfortably in excess of the horizontal phase speeds under investigation.

8 pages, 5 figures, accepted by ApJL

References in corpus (2)

• Seismic Halos Around Active Regions: An MHD Theory
• Numerical Models of Travel-Time Inhomogeneities in Sunspots

Cited by in corpus (5)

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• MHD wave refraction and the acoustic halo effect around solar active regions - a 3D study
• Directional Time-Distance Probing of Model Sunspot Atmospheres
• Helioseismic holography of simulated sunspots: magnetic and thermal contributions to travel times
• Dependence of sunspot photospheric waves on the depth of the source of solar $p-$modes