the observed downturn in the magnitude of the frequency shifts at high frequencies [@1993ApJ...414..898J]. However, as we now describe, it is not only the mode frequencies that vary throughout the solar cycle.

Solar cycle variations in mode powers and lifetimes
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Solar p modes are excited and damped by turbulent convection beneath the solar surface. The process of excitation and damping varies throughout the solar cycle and these variations are observed as changes in the heights and widths of the mode profiles in a frequency-power spectrum. For typical low-$l$ modes, as the surface activity increases the mode frequencies and widths increase but the mode heights decrease. An increase in the linewidths implies that the modes experience more damping at times of high activity and so the lifetimes decrease.

Numerous authors have observed that lifetimes decrease with solar activity and those observations have been made using data from different instrumental regimes and for a range of $l$ . The size of the variation appears to be dependent on $l$ with lower-$l$ modes showing a larger percentage change than high-$l$ modes [e.g. @2009SoPh..258....1B and references therein]. @2009SoPh..258....1B looked at the lifetimes of modes with $300\le l\le600$ and found that in active regions 