  The penetration of an $ac$ magnetic signal into a type II superconductor
residing in the Shubnikov phase depends on the pinning properties of Abrikosov
vortices. Within a phenomenological theory, the so-called Campbell penetration
depth $\lambda_{\rm \scriptscriptstyle C}$ is determined by the curvature
$\alpha$ at the bottom of the effective pinning potential. Preparing the sample
into a Bean critical state, this curvature vanishes and the Campbell length
formally diverges. We make use of the microscopic expression for the pinning
force density derived within strong pinning theory and show how flux
penetration on top of a critical state proceeds in a regular way.
