Study of Coronal Type III Solar Radio Bursts

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The investigators will fully assess their newly developed concept regarding the mechanism that generates "type III" radio emission in the solar corona. The main effort is to produce a detailed ray tracing analysis of the Bernstein-to-fast wave mode-conversion process, as well as a concrete demonstration of the pitch-angle scattering process of energetic electrons, the relation between the level of intrinsic hydromagnetic turbulence and the degree of pitch-angle scattering. The analysis will also consider other crucial observational aspects related to type III emissions such as the low-frequency cutoff problem and the relevance of the mechanism to type V emissions. The scientific objective of the research program is to investigate the radio emission process, particularly the type III bursts in the solar corona, in terms of an instability mechanism excited by a beam of energetic electrons propagating along the ambient magnetic field in the presence of intrinsic hydromagnetic waves. This mechanism involves the excitation of thermal electron cyclotron harmonic modes, known as the Bernstein modes in the literature. The excitation of these waves occurs at the expense of free energy in the electrons whose momentum distribution function possesses positive perpendicular slope. Such a feature results from the pitch-angle scattering of streaming electrons by low-frequency hydromagnetic turbulence, as they traverse along the open magnetic field line. The excited Bernstein waves are then mode-converted into free-space X and O modes, and observed as the fundamental and harmonic type III bursts. This model is very different from the standard emission mechanism for type III bursts, which rely on beam-generated Langmuir waves, which are nonlinearly converted to electromagnetic waves. There are many difficulties with the standard emission mechanism, especially when applied near the corona. The new mechanism appears to be promising with respect to many of these problems.