PLASMA EMISSION FROM ISOTROPIC LANGMUIR TURBULENCE - ARE RADIO MICROBURSTS STRUCTURELESS

The brightness temperature of radio emission through the fundamental and second harmonic plasma processes is determined for isotropic Langmuir waves of low-energy density in order to account for the microbursts at meter-dekameter wavelengths. The probable cause for low levels of Langmuir turbulence is the presence of isotropic density fluctuations in the corona which isotropize the beam-generated Langmuir waves. Motivated by,the fact that the microbursts at meter-decameter wavelengths have characteristics similar to those of normal type III bursts, we determined the energy density of Langmuir waves attainable from the beam-plasma instability in the presence of isotropic density fluctuations. Since the electron density fluctuations isotropize the beam-generated plasma waves, the head-on collision of plasma waves becomes efficient to produce the second harmonic plasma emission. For reasonable beam parameters, the brightness temperature of the fundamental never exceeds 10(6) K, while the second harmonic covers the observed range of microburst brightness temperatures (0.5 x 10(6)-1.0 x 10(7) K). Thus, the microbursts are predominantly at second harmonic. This leads to an important conclusion that the microbursts are structureless, similar to a population of normal type III bursts of low polarization with no fundamental-harmonic structure.