A New Scenario for Type III Solar Radio Emission

Abstract

This paper describes a new scenario for type III solar radio emission. While the conventional theories emphasize Langmuir waves propagating along the ambient magnetic —eld, the present model stresses extraordinary-mode Bernstein waves with quasi-perpendicular wave vectors. The present model relies on plasma inhomogeneity and wave re—ection to produce the electromagnetic radiation, in contrast to the conventional theories, which rely on nonlinear wave-wave and wave-particle interactions. The essence of the model is that Bernstein waves with frequencies above the X-mode cutoU frequency are —rst excited. Initially these waves can only propagate toward regions with higher density and magnetic —eld. During this process, quasi-electrostatic Bernstein waves spontaneously convert to electromagnetic waves and rapidly reach the local X-mode cutoU frequency. Subsequently, these waves are re—ected and propagate away from the source region. The escaping radiation is observed as the type III emission. It is pointed out that in general, the observed radiation should possess one band in the dynamic spectrum. However, under certain conditions, the present theory is also capable of explaining the appearance of a pair of bands similar to those commonly called the ii fundamental ˇˇ and ii harmonic ˇˇ components in the liter- ature. The present model resolves some of the inconsistencies between the usual plasma emission hypoth- esis and the observed pair emissions. Subject headings: Sun: radio radiationwaves