  Measuring the $ac$ magnetic response of a type II superconductor provides
valuable information on the pinning landscape (pinscape) of the material. We
use strong pinning theory to derive a microscopic expression for the Campbell
length $\lambda_{\rm \scriptscriptstyle C}$, the penetration depth of the $ac$
signal. We show that $\lambda_{\rm \scriptscriptstyle C}$ is determined by the
jump in the pinning force, in contrast to the critical current $j_c$ which
involves the jump in pinning energy. We demonstrate that the Campbell lengths
generically differ for zero-field-cooled and field-cooled samples and predict
that hysteretic behavior can appear in the latter situation. We compare our
findings with new experimental data and show the potential of this technique in
providing information on the material's pinscape.
