DYNAMICS AND EFFICIENCY OF TYPE-III SOLAR RADIO-EMISSION

Existing calculations of nonlinear coupling coefficients for fundament al and harmonic emission via three-wave interactions are refined and u sed to calculate the conversion efficiency of Langmuir energy into ele ctromagnetic waves when the Langmuir waves have the observed bursty fo rm. Resulting field strengths for harmonic emission are found to be co nsistent with typical ISEE 3 observations at 1 AU. Fundamental emissio n at 1 AU is found to proceed only when stimulated by the presence of a source of ion sound waves. However, it is argued that electrostatic decay of Langmuir waves can supply the necessary waves provided the dr iving electron beam is sufficiently fast. Under these conditions, the predicted fundamental field strengths can account for both the highest and typical fields observed; they also dominate the predicted harmoni c fields, consistent with observations. This mechanism is also consist ent with previous observations that fundamental emission generally occ urs early in type Ill events, when the beam is fastest. For typical pa rameters it is shown that neither fundamental nor harmonic emission sa turates its respective source instability, contrary to previous assump tions. However, saturation cannot be ruled out under particularly favo rable conditions.