The energy transfer due to weak nonlinear interactions in random wave fields is reinterpreted in terms of a hypothetical ensemble of interacting particles, antiparticles, and virtual particles. In the particle picture, the interactions can be conveniently described by Feynman diagrams, which may be regarded either as branch diagrams of the perturbation expansion or as collision diagrams. The derivation of the transfer expressions can then be reduced to a few general rules for the construction of the diagrams and the associated collision cross sections. The representation follows closely the standard treatment of nonlinear lattice vibrations, but the particle picture differs from the usual phonon interpretation of lattice waves. It has the unreMistie property that the energies and number densities of antiparticles are negative. This is offset by simpler interaction rules and a closer correspondence between the perturbation graphs and collision diagrams. The method is illustrated for scattering processes in the oceanic wave guide involving surface and internal gravity waves, horizontal currents (turbulence), seismic waves, and bottom irregularities.
Nonlinear interactions between surface gravity waves have been studied byPhillips [1960], Hasselmann [1962; 1963a, b], Longuet-Higgins [1062], Benney [1962], Bretherton [1064], among others. Computations of the energy transfer for a random wave field were found to be in qualitative agreement with the observed decay of ocean swell [Snodgrass et al., 1965]. 2 K. HASSELMANN
Scattering from gravity waves into elastic waves is one of the principal sources of microseisms. The conversion can occur either through direct nonlinear interactions between gravity waves [Longuet-Higgins, 1950] or through the interaction of gravity waves with an inhomogeneous ocean bottom [Wiechert, 1904]. Both processes have been quantitatively confirmed by measurements byHaubrich et al. [1963] [Hasselmann, 1963c]. The generation of long gravity waves with periods in •he minute range ('surf bea•' [Munk, 1949; Tucker, 1950]) may be a•tributed to a nonlinear gravity wave interaction plus a bottom interaction [Longuet-Higgins and Stewart, 1962; Hasselmann et al., 1963; Gallagher, 1965]. Sca•ering between surface and in•ernal gravity waves has been investigated by Ball [1964], Thorpe [1966], and Kenyon [1966]. Kenyon has computed •ransfer ra•es for both gravity wave and Rossby wave interactions. Nonlinear interactions between surface gravity waves and quasi-s•eady curren•s or •urbulence have been s•udied by Longuet-Higgins and Stewart [1961] and Phillips [1959]. Sca•ering processes very similar •o •hese geophysical examples have also been considered in o•her fields.
In a fundamental paper on •he hea• conduction in solids, Peierls [1929] investigated •he •ransfer effects due •o secular interactions between random lattice vibrations. Nonlinear interactions between ligh• waves and lattice vibrations, firs• investigated in •he classic papers of Brillouin [1922] and Raman [1928], have la•ely received renewed ...