Evidence for Langmuir Collapse in the Source Regions of Interplanetary Type III Radio Bursts

             G. Thejappa 

The Unified Radio and Plasma Wave (URAP) and WAVES  
experiments on the Ulysses and WIND spacecraft provide first observational 
evidence for the Langmuir collapse processes in the type III burst source 
regions. These observations are in the form of simultaneous occurrence 
of high frequency ion-acoustic waves (4-7 kHz) and Langmuir waves ($\sim 
25$ kHz) in local type III burst source regions. The wave-wave interaction 
process, namely the electrostatic decay instability probably is not 
responsible for the observed ion acoustic waves, since their wavenumbers 
are too large to resonate with the beam excited Langmuir 
waves.  On the other hand, the high time resolution  observations
show that  Langmuir waves occur as highly modulated coherent structures of 
very small spatial scales $\sim (10-20)$ Debye lengths ($\lambda_D$), with
peak intensities well above strong turbulence thresholds. More over, the  
wavelengths of the ion acoustic waves are of the order of the scale 
sizes of the millisecond field structures of Langmuir waves.  
These observations indicate that the short wavelength ion acoustic waves 
are most probably are those waves, radiated by the 
burnt-out cavitons, i.e., remnants of Langmuir collapse processes. The implications 
of these observations for the beam stabilization and emission 
mechanisms of type III radio bursts will be discussed.