We present a modal theory of self-trapping spatially incoherent light
beams in any general nonlinear media. We find that a self-trapped inco
herent beam induces a multimode waveguide which guides the beam itself
by multiply populating the guided modes. The self-trapping process al
ters the statistics of the incoherent beam, rendering it localized. We
find the conditions for self-trapping (''existence region'' in parame
ter space) and the correlation function of the incoherent self-trapped
beam.