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Astron. Astrophys. 316, 396-405 (1996)

Ulysses spacecraft observations of radio and plasma waves: 1991-1995

R.J. MacDowall1, R.A. Hess2, N. Lin3, G. Thejappa4, A. Balogh5, and J.L. Phillips6

1 NASA, Goddard Space Flight Center, Code 695, Greenbelt, Maryland, 20771, USA
2 Hughes STX Corporation, Lanham, Maryland, 20706, USA
3 School of Physics and Astronomy, University of Minnesota, Minneapolis, Minnesota, 55455, USA
4 Department of Astronomy, University of Maryland, College Park, Maryland, 20740, USA
5 The Blackett Laboratory, Imperial College, London SW7 2BZ, UK
6 Los Alamos National Laboratory, MS D466, Los Alamos, New Mexico, 87545, USA

Received 22 March 1996 / Accepted 16 July 1996

Abstract. The radio and plasma wave investigation of the Ulysses spacecraft mission provided nearly continuous observations of wave electric fields (less than 1 MHz) and magnetic fields (less than 450 Hz) from spacecraft launch in late 1990 through perihelion in 1995 and beyond. The Ulysses spacecraft reached heliographic latitudes of img1.gif , providing a unique opportunity to study wave activity in fast solar wind emanating from polar coronal holes and to examine the differences relative to waves in the streamer belt. These data led to the discovery that many magnetic field depressions (magnetic holes) are populated by waves at the electron plasma frequency; such phenomena occur much more frequently in high-latitude, fast solar wind. At high latitudes, Ulysses frequently detected the waves conventionally assumed to be ion-acoustic waves; however, the electron-proton temperature ratio at these latitudes is always less than one, suggesting that the waves must be some other mode. Electrostatic and electromagnetic waves at lower frequencies also evidenced marked changes as Ulysses recurrently entered and exited the streamer belt during the ``fast latitude scan'' phase of its orbit. These variations with heliographic latitude, as well as variations with distance from the Sun, are presented and interpreted.

Key words: plasmas - waves - solar wind - interplanetary medium

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