pressure, and therefore sound speed (since $c_s^2=\frac{\gamma_1P}{\rho}$, where $\gamma_1$ is the first adiabatic exponent). Assuming the direction of travel is not radial the waves follow a curved trajectory which takes them back to the surface where they are reflected by the sharp drop in density. By the time the waves reach the surface they are approximately traveling in a radial direction and so the depth at which the near-surface reflection of the waves occurs depends only on the wave frequency. The p modes are set up by a superposition of such waves, and so the location of the upper edge of the acoustic cavity of the modes – the upper turning point – likewise depends only on frequency, with the upper turning point of low-frequency modes being deeper in the solar interior than the upper turning point of high-frequency modes. The radius of the lower turning point depends on the angle of trajectory and therefore $l$, with low-$l$ modes turning deeper within the solar interior than high-$l$ modes.

Helioseismology can be split into two categories: global and local. Global helioseismology studies the natural resonant acoustic oscillations of the solar interior that are able to form standing waves in the entire 