  We present a theory of the pinning of dynamic spin density wave (SDW)
fluctuations in a d-wave superconductor by local imperfections which preserve
spin-rotation invariance, such as impurities or vortex cores. The pinning leads
to static spatial modulations in spin-singlet observables, while the SDW
correlations remain dynamic: these are the `Friedel oscillations' of a spin-gap
antiferromagnet. We connect the spectrum of these modulations as observed by
scanning tunnelling microscopy to the dynamic spin structure factor measured by
inelastic neutron scattering.
