  We introduce the mode connectivity as a measure of the number of eigenmodes
of a wave equation connecting two points at a given frequency. Based on
numerical simulations of scattering of electromagnetic waves in disordered
media, we show that the connectivity discriminates between the diffusive and
the Anderson localized regimes. For practical measurements, the connectivity is
encoded in the second-order coherence function characterizing the intensity
emitted by two incoherent classical or quantum dipole sources. The analysis
applies to all processes in which spatially localized modes build up, and to
all kinds of waves.
