Mode Coupling in the Solar Corona. VI. Direct Conversion of Langmuir Waves into o-mode Waves

Abstract

Fundamental plasma radiation can result from direct coupling of Langmuir waves into o-mode waves in an inhomogeneous magnetized plasma. The coupling occurs for Langmuir waves propagating nearly along the direction of increasing plasma density, and the resulting o-mode waves propagate towards lower plasma densities. An analytic theory for the coupling (which is of z-mode waves into o-mode waves) is developed using approximate solutions of the Booker quartic equation. The results are shown to agree with the results of numerical calculations for the 'Ellis window' in 'the generation of the z trace in the terrestrial ionosphere. This 'direct conversion' of Langmuir waves into o-mode waves is an alternative mechanism for fundamental plasma emission, possibly replacing or supplementing the more familiar scattering from the charge clouds around thermal ions. Possible applications of 'direct conversion' to various solar radio bursts are explored, but for none is it found to be more favourable than alternative mechanisms.