IPST, University of Maryland
Plasma Emission by Nonlinear Electromagnetic Processes
The plasma emission is the radiation mechanism responsible for solar type II and type III radio bursts. These radio bursts are important in that they may be used in the space weather forecasting effort, which is a research topic of contemporary importance. The detection of radio bursts from the Sun in the meter wavelengths were first carried out in the 1940s. Identification and classification of different types of solar radio emissions into type I, II, and III, were subsequently done in the 1950s. Subsequently, further classification and identification of type IV and V radio bursts were added in the 1960s. The first theory of plasma emission, which was put forth to explain the type II and III radio bursts, came in 1958 by the Soviet scientist Vitaly Ginzburg, and further developed by others in the 1970s and 1980s, and well into the 1990s. As a result, the standard paradigm was established in which, the complicated radio emission process first involves energetic electrons produced at the solar active region during the flare. As the energetic electrons stream outward following open magnetic field lines, they interact with the dense background solar wind plasma. The beam-plasma interaction excites electrostatic Langmuir waves via the well-known bump-on-tail instability, followed by nonlinear processes of wave decay and scattering, eventually leading to the backward-propagating Langmuir waves as well as low-frequency ion-acoustic waves. The generation of EM radiation is a byproduct of the nonlinear processes. Partial conversion of electrostatic wave energy, primarily residing in the form of Langmuir turbulence, to transverse EM wave energy is called the plasma emission process, and the resultant radiation emission occurs at the plasma frequency and/or its harmonic(s). Such an elaborate process can, in principle, be demonstrated on the basis of EM weak turbulence theory, the fundamental equations thereof and their derivation can be found in the literature and in the papers by the present author. However, the complete numerical solution of the entire set of EM weak turbulence equations has not been done until quite recently, when the present author and his colleagues [Ziebell, Yoon, et al., 2014, 2015] numerically solved the complete equations for the first time. Until then, various approximations and simplifications have been made. This talk will overview the solar radio emission phenomenon and the recent theoretical development.