EVIDENCE FOR STRONG AND WEAK TURBULENCE PROCESSES IN THE SOURCE REGION OF A LOCAL TYPE-III RADIO-BURST

We present the Ulysses Unified Radio and Plasma Wave Experiment (URAP) observations of a local type III radio burst and the associated plasm a wave activity generated in the vicinity of the spacecraft. These obs ervations represent the first clear evidence of (1) the occurrence of ion-acoustic waves as well as whistlers in close association with Lang muir waves, (2) the coexistence of strong turbulence processes (modula tional instability and soliton formation) and weak turbulence processe s (electrostatic decay instability), (3) the frequency ratio of the fu ndamental (F) and harmonic (H) components as 1:1.67, and (4) a very na rrowband F component with relative bandwidth of Delta f/f similar to 6 %, in comparison to that of the harmonic (H) component with Delta f/f similar to 38%. Based on energy considerations, it is shown that whist lers are not generated by any nonlinear effects involving Langmuir wav es; instead, along with Langmuir waves, they are excited directly by t he type III electron beam. On the other hand, ion-acoustic waves are g enerated by the Langmuir waves through the electrostatic decay process , since (1) the observed frequencies agree with the predicted values, (2) the ion-acoustic occupation numbers are well below those of Langmu ir waves, and (3) the normalized peak energy density of Langmuir waves W-L/n(e)T(e) similar to 10(-5) is well above the electrostatic decay threshold similar to 7 x 10(-7). The occurrence of Langmuir waves as i ntense spikes with spatial scales of 300-3000 Debye lengths with peak energy densities well above the modulational-instability threshold of similar to 2 x 10(-7) suggest that the Langmuir waves are also saturat ed by strong turbulence processes. The implications of these observati ons for the emission mechanisms at f(pe) and at 2f(pe) and for beam st abilization processes are discussed.