subsequent analysis. Travel times are estimated by measuring the location of the wave packet, and a number of different methods are used to do that. Flows, magnetic fields and inhomogeneities experienced by the waves during their propagation from A to B shift the location of the wave packet (and this in turn is measured as a travel-time shift) as well as potentially modifying the width and amplitude of the wave packet. Also, flows can cause a difference in travel time depending on whether the waves are traveling from A to B or from B to A. By measuring travel times between many different pairs of surface points, 3-D subsurface maps of flows and wave-speed inhomogeneities can be made using inverse techniques [@2004ApJ...603..776Z].

For further discussion of the signatures of flows on the ring analysis and time-distance helioseismology measurements, see Section \[sec:2.1\]. For now, however, we return to concentrate on the global modes.

An introduction to global helioseismology and the solar cycle {#section[global intro]}
=============================================================

It has been known since the mid 1980s that p-mode frequencies vary throughout the solar cycle with the frequencies being at their largest when the solar activity is at its maximum .[^1] For a low-$l$ mode 