alone. This accords with the near-symmetry of $\delta\tau$. The smaller total losses around $\phi=90^\circ$ for fixed $\theta\ga30^\circ$ are clearly reflected in $\delta\tau$. On the other hand, the small losses for $\phi\approx0^\circ$ at high $\theta$ run counter to this nexus, presumably because acoustic and magnetic losses affect phase in different ways.

In passing, we mention that the BVP travel time perturbation map for the case $B_0=0.5$ kG, $\nu=3$ mHz, $k_h=0.75$ $\rm Mm^{-1}$ (not shown) does not display the strong positive feature seen in the bottom left panel of Figure \[fig:tau\] at mid-values of $\theta$ and $\phi$ (there is a positive peak, but it is less than 2 s). This indicates that we must be careful with very noisy TD phase fits. Longer time series or broader wave vector filters may be required.

Discussion and Conclusions
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‘Travel time’ perturbations, as measured by the standard techniques of Time-Distance helioseismology, actually determine phase shifts rather than true (group) travel times. Shifts in phase trivially come about through changes in path and in propagation speed. If Fermat’s principle is valid, then travel time is stationary with respect to variations of path, and so wave speed will be the main culprit. This could consist of 