Frequency Fine Structures of Type III Bursts Due to Localized Medium-Scale Density Structures Along Paths of Type III Beams

Predictions from large-scale kinetic simulations are presented for the effects on coronal type III bursts of localized, medium-scale, enhanced density structures superposed on the coronal background along the paths of type III beams. The simulations show that these density structures can produce pronounced frequency fine structures in type III spectra. Flux intensifications and reductions of f _p and 2f _p emission relative to those for the unperturbed background corona occur at frequencies corresponding to the density structures, where f _p is the local electron plasma frequency. Frequency fine structures that are intense, slowly drifting, and narrowband, and thus resemble the characteristics of stria bursts, are predicted for the 2f _p emission. The 2f _p results are consistent with the qualitative proposal of Takakura and Yousef (Solar Phys. 40, 421, 1975) for the interpretation of stria/type IIIb bursts. However, the predicted f _p emission is much weaker than the 2f _p emission and generally below observable levels, and the predicted frequency fine structures do not always show stria characteristics. The predictions are thus inconsistent with the qualitative suggestion of Takakura and Yousef and the interpretations of many observers that stria bursts occur more often in f _p than in 2f _p emission. The significant discrepancies for f _p emission between our numerical calculations and the qualitative proposition of Takakura and Yousef (1975) are mainly caused by: i) differences in the detailed emission processes, ii) neglect of scattering of f _p emission off small-scale density fluctuations by Takakura and Yousef (1975), and iii) other simplifications made in both works. Possible improvements to the simulations are discussed, including improvements to the emission processes and the coronal and beam conditions (e.g., beam speed), in order to produce realistic stria/type IIIb bursts in f _p emission.