Markus Roth, Wolfgang Zima, Aaron C. Birch, Laurent Gizon

We extend an existing Born approximation model for calculating the linear sensitivity of helioseismic travel-times to flows from Cartesian to spherical geometry. This development is necessary to use the Born approximation for inferring large-scale flows in the deep solar interior. Two consistency tests show that results for our sensitivity kernels agree with reference values to within a few percent. Consequently, we evaluate the impact of different data analysis filters on the kernels for a meridional travel-distance of 42 degrees. When mainly low-degree modes are used (roughly l < 70), the sensitivity is concentrated in deeper regions and it visually best resembles a ray-path like structure, otherwise the sensitivity is concentrated near the surface. Among the different low-degree filters used, we find the phase-speed filtered kernel to be best localized at depth.

Markus Roth, Jason Jackiewicz

We present the current status of an undergoing validation of a recently developed model for computing spherical Born approximation sensitivity functions for flows. In a first step, power spectra and reference cross-correlations from the model and a simulation of Hartlep et al. (2013) are matched. Some difficulties in obtaining such a match are discussed. In a second step, travel times from the forward model and from the simulation, which includes a standard meridional flow profile, are to be compared. The analysis procedure including the use of phase-speed filters is identical to the one employed in Jackiewicz et al. (2015). Furthermore, we present a novel approach for a fast computation of integrated sensitivity functions which can be used for interpreting rotationally symmetric flows such as differential rotation and meridional flow.

Emmanuel Hecht, Markus Roth

In current approaches to time-distance helioseismology, the line-of-sight projection effect on the traveltimes is not fully taken into account. Furthermore, filtering of full-disc data induces leakage due to the projection onto the CCD, which has so far not been accounted for. We develop a theoretical approach to consider these effects when computing sensitivity functions. As the formulas obtained do not seem to give results for spherical Born approximation sensitivity functions in a reasonable computation time, we develop tests to estimate the strength of the effects.

Markus Roth, Jason Jackiewicz, Shukur Kholikov

Accurate meridional flow measurements are important for understanding the solar dynamo. Recent in- versions for meridional flows have not yet reached a consensus on the nature of the meridional flow in depths greater than about 0.9 solar radii. In time-distance helioseismology, current modelling of the solar interior for meridional flow inversions is performed using ray kernels, which assume that waves propagate along infinitely thin ray paths. The Born approximation may constitute a more accurate approach as it models the first order perturbation to the wave field in the whole solar interior. We present the current status of an undergoing validation of a recently developed model for computing spherical Born approxi- mation sensitivity functions suitable for inferring meridional flows. In addition, we compare Born and ray approximations using flow models.