EVALUATION OF EMISSION MECHANISMS AT OMEGA-PE USING ULYSSES OBSERVATIONS OF TYPE-III BURSTS

We present the first observational tests of existing theories for the generation of type III radio bursts emitting at the fundamental plasma frequency, omega(pe). This study is based on local radio emission and in situ wave phenomena associated with four interplanetary type III radio bursts observed by the unified radio and plasma wave experiment on the Ulysses spacecraft. Intense Langmuir wave peaks with energy densities and rapid time variations indicative of Langmuir solitons are observed for some events. Low-frequency waves below 500 Hz are not observed. For each event, brightness temperatures derived from radio observations are compared with those predicted by various mechanisms for the c onversion of Langmuir waves to electromagnetic radiation. The theories tested here are: (1) scattering of Langmuir waves by thermal ions; (2) wave-wave interactions, i.e., merging and decay processes involving Langmuir and low-frequency waves, (3) strong turbulence phenomena involving Langmuir solitons; and (4) direct coupling between Langmuir and electromagnetic waves due to density gra dients. The mechanism of scattering on thermal ions may be ruled out as a major source of electromagnetic radiation since it yields brightness temperatures well below observed type III values. Wave-wave interactions yield brightness temperatures four to six orders of magnitude greater than observed values. The strong turbulence mechanism and the direct coupling mechanism predict brightness temperatures less than two orders of magnitude too large.