Investigation of surface effects of simple flux tubes using numerical simulations

We use the SPARC code for MHD simulations with monolithic flux tubes of varying subsurface topology. Our studies involve the interactions of waves caused by a single source with subsurface magnetic fields. Mode conversion causing acoustic power to trickle downwards along the flux tube has been described before and can be visualized in our simulations. We show that this downward propagation causes the flux tube to act as an isolated source, creating a characteristic surface wavefield. Measuring this wavefield at the surface reveals subsurface properties of the magnetic field topology. Using time distance helioseismology, we demonstrate how to detect such a flux tube signal based on a group travel-time delay of $\varDelta t = 282.6$ sec due to the wave packet spending time subsurface as a slow mode wave. Although the amplitude is small and generally superimposed by the full wave field, it can be detected if assumptions about $\varDelta t$ are made. We demonstrate this for a simulation with solar like sources. This kind of study has the potential to reveal subsurface information of sunspots based on the analysis of a surface signal.

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