  In present paper, we describe a theoretical model of generation of harmonic
emissions of type III solar radio bursts. The goal of our study is to fully
take into account the most efficient physical processes participating in
generation of harmonic electromagnetic emission via nonlinear coupling of
Langmuir waves in randomly inhomogeneous plasma of solar wind ($l+l^{'}
\rightarrow t$). We revisit the conventional mechanism of coalescence of
primarily generated and back-scattered Langmuir waves in quasihomogeneous
plasma. Additionally, we propose and investigate another mechanism that
generates the harmonic emission only in a strongly inhomogeneous plasma: the
nonlinear coupling of incident and reflected Langmuir waves inside localized
regions with enhanced plasma density (clumps), in the close vicinity of the
reflection point. Both mechanisms imply the presence of strong density
fluctuations in plasma. We use the results of a probabilistic model of
beam-plasma interaction and evaluate the efficiency of energy transfer from
Langmuir waves to harmonic emission. We infer that harmonic emissions from a
quasihomogeneous plasma are significantly more intense than found in previous
studies. The efficiency of Langmuir waves conversion into electromagnetic
harmonic emission is expected to be higher at large heliospheric distances for
the mechanism operating in quasihomogeneous plasma, and at small heliocentric
distances - for the one operating in inhomogeneous. The evaluation of emission
intensity in quasihomogeneous plasma may also be applied for type II solar
radio bursts. The radiation pattern in both cases is quadrupolar, and we show
that emission from density clumps may efficiently contribute to the visibility
of harmonic radio emission.
