Type III Radio Bursts
A major goal of the URAP experiment is to study the characteristics of solar type III radio bursts, which are the most frequently observed solar radio burst at frequencies < 1 MHz. In the Figure, type III bursts start at approximately 4:00, 14:30, 15:15, and 22:00, at the highest frequency. These bursts are generated when suprathermal electrons (velocity ~ 0.05 to 0.3 c, where c is the speed of light) are ejected from solar active regions and then travel outward along open magnetic field lines through the corona and interplanetary medium. Along their path these electrons excite plasma oscillations (Langmuir waves) at the electron plasma frequency (fpe); these electrostatic waves are then partially converted into radio waves at fpe and/or 2fpe. Thus the progress of the electrons into regions of decreasing density produces radio waves progressing to successively lower frequencies. The absolute low-frequency limit of the radio waves is imposed by the local fpe, below which the radio waves cannot propagate. In general, however, the type III frequencies do not descend to fpe.