F-Mode Eikonal Heliotomography

Thursday, 18 December 2014
Sjoerd de Ridder, University of Science and Technology of China, Geophysics and Planetary Sciences, Hefei, China and Junwei Zhao, Stanford University, W. W. Hansen Experimental Physics Laboratory, Stanford, CA, United States
We investigate 5 days of Doppler measurements geo-tracked for a sunspot. The data is cross -correlated to create time-distance impulse responses of wave propagation in the plasma. We focus on imaging the f-mode (surface gravity mode), which is separated from the other modes by k-ω filtering.

Recent developments for surface wave imaging in earth-seismology include the development of an eikonal tomography technique. Eikonal tomography relies on evaluation of the local spatial derivatives of a traveltime surface to directly infer the slowness of wave-propagation.

Whereas earth-seismology relies on increasing high signal-to-noise ratio (SNR) by cross-correlating long-time series, helioseismology relies on stacking over geometrically similar virtual sources. We assume there are two symmetries in the geometry of virtual sources around a sunspot. The first is a radial symmetry centered at the sunspot. The second is a radial symmetry of propagation around each virtual source. Because we are interested in the effect of sunspots on wave-propagation, we keep the averaging along the second symmetry axis to a minimum, and rely heavily on averaging each virtual source with equal distance from the sunspot.

We will investigate the spatial variation of f-mode dispersion in a radial geometry with the sunspot at the center by creating f-mode velocity maps at different frequencies.