The observations of the quiet Sun at meter and decameter wavelengths show that its brightness temperatures can be $1 order of magnitude lower than the expected values of 10 6 K and the apparent diameters can be very large. We examine whether this unusual behavior is due to refraction in the smoothly varying coronal plasma and scattering by random density fluctuations using an improved Monte Carlo simulation technique. We use a three-dimensional model for the electron density, N e , and a power-law spectrum for the density fluctuations with an exponential index of . We consider two cases: (1) ¼ 11/3 (Kolmogorov spectrum), relative level of density fluctuations ¼ ÁN e /N e ¼ 0:1, and scale heights ranging from the inner scale l i ¼ 684 (N e cm À3 ) À1/2 to outer scale l o ¼ 10 6 l i , and (2) ¼ 3 (flat spectrum), ¼ 0:02, and scale heights ranging from 50 to 75 km. We consider the fluctuations to be anisotropic with axial ratios of $10. The Kolmogorov type of fluctuations yield slightly lower brightness temperatures in comparison with those of a flat spectrum. The brightness temperature distributions, east-west diameters, and central brightness temperatures at various frequencies indicate that the refraction and scattering are probably the underlying reasons for the unusual behavior of the quiet-Sun radio emission. This study clearly demonstrates that Monte Carlo techniques can be very effective in extracting the coronal electron temperatures accurately from the radio data, provided the information about the density distributions and density fluctuations is known by some independent methods.

We present the Ulysses UniÐed 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 Ðrst 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 *f/f D 6%, in comparison to that of the harmonic (H) component with *f/f D 38%. Based on energy considerations, it is shown that whistlers are not generated by any nonlinear e †ects 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 is well W L /n e T e D 10~5 above the electrostatic decay threshold D7 ] 10~7. The occurrence of Langmuir waves as intense spikes with spatial scales of 300È3000 Debye lengths with peak energy densities well above the modulationalinstability threshold of D2 ] 10~7 suggest that the Langmuir waves are also saturated by strong turbulence processes. The implications of these observations for the emission mechanisms at and at f pe 2f pe and for beam stabilization processes are discussed.

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widths of these broad peaks agree well with the predicted widths of envelope solirons. We show that the orientation of the Langmuir field structures is random with respect to the ambient magnetic field, indicating that they are probably isotropic structures that have evolved from initially pancake-like solirons. These observations suggest that strong turbulence processes, such as the modulational instability or the OTSI, stabilize the electxon beams that produce type III bursts.

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