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 r adio bursts observed by the unified radio and plasma wave experiment o n the Ulysses spacecraft. Intense Langmuir wave peaks with energy dens ities and rapid time variations indicative of Langmuir solitons are ob served for some events. Low-frequency waves below 500 Hz are not obser ved. For each event, brightness temperatures derived from radio observ ations are compared with those predicted by various mechanisms for the conversion of Langmuir waves to electromagnetic radiation. The theori es tested here are: (1) scattering of Langmuir waves by thermal ions; (2) wave-wave interactions, i.e., merging and decay processes involvin g Langmuir and low-frequency waves, (3) strong turbulence phenomena in volving Langmuir solitons; and (4) direct coupling between Langmuir an d electromagnetic waves due to density gradients. The mechanism of sca ttering on thermal ions may be ruled out as a major source of electrom agnetic radiation since it yields brightness temperatures well below o bserved type III values. Wave-wave interactions yield brightness tempe ratures four to six orders of magnitude greater than observed values. The strong turbulence mechanism and the direct coupling mechanism pred ict brightness temperatures less than two orders of magnitude too larg e.