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Solar electron beams associated with radio type III bursts: propagation channels observed by Ulysses between 1 and 4 AU
Received 8 January 1998 / Accepted 18 March 1998
Solar and interplanetary type III (TIII) radio bursts are produced by solar electron beams of flare origin streaming away in the interplanetary medium. The radio emission process implies the excitation of Langmuir waves by the electron beam which are converted by non linear processes into transverse electro-magnetic waves. The study of the beam properties reveals information about their propagation regime in the interplanetary medium. The local plasma condition observed in the vicinity of the radio emission zone are important in understanding the Langmuir waves excitation process and their conversion into electromagnetic radiation as well as the propagation of the electron beams. Interplanetary TIII have therefore long been studied for either of theses aspects but only for few isolated cases could the three aspects of such events be studied together. This paper presents for the first time a complete study of the three aspects of nine interplanetary TIII events observed by the radio, particle and plasma experiments aboard the Ulysses spacecraft between 1.3 and 4.3 AU. The main result of this study is to establish that the interplanetary medium contains well beyond 1 AU 'propagation channels' previously identified around 1 AU. Those plasma structures are rooted in solar corona and seem to channel the propagation of solar electron beams. They have been identified in each of the nine events studied here; Langmuir waves and almost scatter-free propagation of the solar electrons were observed while the spacecraft was crossing these formations. Plasma properties of the 'propagation channels' have been studied: their main characteristic is a very low level of magnetic field fluctuation. This property may be at the origin of a stabilization process of the plasma inside the channels explaining why they are maintained on such large distances as well as a key element in understanding the particle propagation regime and Langmuir waves excitation observed inside. The implications on the medium distance (1-5 AU) heliosphere are also discussed.
Key words: Sun: corona Sun: particle emission Sun: radio radiation interplanetary medium
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© European Southern Observatory (ESO) 1998
Online publication: June 12, 1998