OBSERVATIONS BY ULYSSES OF A PURE ELECTRON EVENT AND ITS ASSOCIATED TYPE-III RADIO-EMISSION OF 15 DECEMBER 1990

We describe an interplanetary electron event associated with a type III radio burst observed by Ulysses at 1.37 AU. This event was observed when the spacecraft crossed a region of strong compression between a fast and a slow solar wind stream, characterized by a relatively high density and a complex behaviour of the plasma density, magnetic field and energetic electron anisotropy. The coronal source of the event is identified and the structure of the interaction region is discussed; in particular, it contains a stream interface and a relatively thin overdense region in which one component of the magnetic field varies rapidly and the energetic electrons temporarily loose their anisotropy. The associated type III radio burst is observed at frequencies as low as the local plasma frequency, without evidence of simultaneous excitation of Langmuir waves.