Type III radio bursts and excitation of Langmuir waves by energetic electrons

Context. Solar activity occurs not only in terms of the well-known 11-year Sun spot cycle but also in terms of short-lived phenomena as radio bursts. For instance, type III radio bursts are the most common phenomenon of this activity in the Sun's radio radiation. In dynamic radio spectra, they appear as short-lived stripes of enhanced radio emission rapidly drifting from high to low frequencies. They are regarded as the radio signature of beams of energetic electrons travelling along magnetic field lines in the corona. The radio emission is thought to be plasma emission, that is to say the radio emission happens near the electron plasma frequency and/or its harmonics. Plasma emission means, that energetic electrons excite Langmuir waves, which convert into radio waves. Aims. Initially, energetic electrons are injected in a small region in the corona. Due to their spatio-temporal evolution, they develop a beam-like velocity distribution function (VDF), which is able to excite Langmuir waves. The aim of the paper is to study the spatio-temporal behaviour of the generation of Langmuir waves under coronal cirumstances and its effect on type III radio bursts. Methods. The generation of Langmuir waves is treated by means of the Maxwell-Vlasov equations. The results are discussed by employing plasma parameters usually found in the corona, for instance at the 150 MHz level. Results. The Langmuir waves associated with the type III bursts are not generated by a monoenergetic electron beam, but by a population of energetic electrons with a broad velocity distribution. Hence, the Langmuir waves are produced by different parts of the energetic electron population at different times and positions. Conclusions. In the case of type III bursts, the velocities derived from their drift rates in dynamic radio spectra are not the velocities of electrons, which generate the onset of the type III burst at a given frequency. That can lead to an apparent accelerated motion of the type III radio burst source.