Towards Assessing, Understanding, and Correcting the Influence of Surface Magnetism in Local Helioseismology

Computer Science – Sound

Scientific paper

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7522 Helioseismology, 7524 Magnetic Fields

Scientific paper

Efforts to probe subsurface wave-speed variations and mass flows near and under solar active regions are complicated and potentially compromised by strong phase and amplitude perturbations introduced in the photosphere by magnetic fields. Recently it has been shown that the phase distortions correlate with surface magnetic field strength and may be corrected to image wave-speed variations underlying active regions. A strong phase asymmetry between waves arriving into and departing from a magnetic region is also shown to produce spurious signatures of horizontal outflows below active regions. Further evidence of the photospheric origin of these phase distortions, as well as a key to their physical origin, is also presented. Using MDI observations, from the SOHO spacecraft, we perform ingression control correlations in the inclined magnetic fields of sunspot penumbra and demonstrate that incoming acoustic waves produce photospheric motion that describes an ellipse tilted towards the inclination of the magnetic field. A consequence is that the phase of the correlation varies with the viewing angle with respect to the field direction. Observations of the vector components of the photospheric fields could be used to correct these phase perturbations analogous to the procedures already developed using line-of-sight magnetograms. A physical understanding of surface distortions will come through MHD modeling, including simulations of the interaction of acoustic and magneto-acoustic-gravity waves with prescribed magnetic and sound-speed perturbations and flows (artificial data). The development of appropriate tools for assessing and correcting the effects of the surface magnetism will be vital for the interpretation of helioseismic data from the upcoming HMI mission on SDO. DCB and CL gratefully acknowledge support from the NSF (SAA/AST) and NASA (LWS, SR&T).

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