Nonlinear Langmuir waves during Type III solar radio bursts

Abstract

Type III solar radio bursts are thought to be associated with intense levels of electron-beam-excited Langmuir waves. The nonlinear evolution of these waves, in time and in two spatial dimensions, due to their coupling to other waves is studied numerically. For parameters appropriate to one-half the earth-sun distance, nonlinear effects are found to be important, as in previous one-dimensional work. However, a new and important phenomenon, two-dimensional soliton collapse, is found to occur. This collapse, induced directly by the wave-packet nature of the beam-excited waves, produces two-dimensional wave spectra extending over a much broader range of wave numbers than has been predicted by inhomogeneous quasi-linear theory. The results compare favorably with certain aspects of recent observations. The background magnetic field is neglected; while substantially justified for the present parameters, this neglect may require reexamination at locations closer to the sun.