upward or downward propagating Alfvén waves depends in the relative orientation of the magnetic field and the wave propagation direction. Both panels should be regarded as projections, with the fast wave raypath having a component in the $y$-direction; otherwise, there is no fast-Alfvén coupling. (Adapted from @KhoCal12aa) \[fig:Alfschem\]](\figdir/Alfschem){width="\hsize"}

The most widely used local helioseismic technique is Time-Distance Helioseismology [TD; @DuvJefHar93aa; @GizBir05aa]. By correlating observations of perturbations at different times and positions on the solar surface, a causative link is inferred and a travel time between pairs of points determined. Comparing these travel times with those calculated for a standard model, one infers the presence of wave-speed anomalies beneath the surface that may be due to such features as temperature variations or flows. Utilizing the full gamut of wave paths sampled by one’s data set, inversions may be performed for subsurface structure. However, ‘travel time’ is not necessarily as straightforward a concept as we might hope. Proper group travel time is difficult to measure with any precision, and in practice phase variations are used as a travel time proxy.

This approach seems well-founded in quiet Sun, where magnetic effects may be ignored, but is subject to several uncertainties and complications in 