Evidence for Strong and Weak Turbulence Processes in the Source Region of a Local Type III Radio Burst

Abstract

We present the Ulysses Unified Radio and Plasma Wave Experiment (URAP) observations of a local type III radio burst and the associated plasma wave activity generated in the vicinity of the spacecraft. These observations represent the first clear evidence of (1) the occurrence of ion-acoustic waves as well as whistlers in close association with Langmuir waves, (2) the coexistence of strong turbulence processes (modulational instability and soliton formation) and weak turbulence processes (electrostatic decay instability), (3) the frequency ratio of the fundamental (F) and harmonic (H) components as 1:1.67, and (4) a very narrowband F component with relative bandwidth of Δf/f ~ 6%, in comparison to that of the harmonic (H) component with Δf/f ~ 38%. Based on energy considerations, it is shown that whistlers are not generated by any nonlinear effects involving Langmuir waves; instead, along with Langmuir waves, they are excited directly by the type III electron beam. On the other hand, ion-acoustic waves are generated 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 Langmuir waves, and (3) the normalized peak energy density of Langmuir waves WL/neTe ~ 10-5 is well above the electrostatic decay threshold ~7 × 10-7. The occurrence of Langmuir waves as intense spikes with spatial scales of 300-3000 Debye lengths with peak energy densities well above the modulational-instability threshold of ~2 × 10-7 suggest that the Langmuir waves are also saturated by strong turbulence processes. The implications of these observations for the emission mechanisms at fpe and at 2fpe and for beam stabilization processes are discussed.