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Astron. Astrophys. 316, 406-412 (1996)

Type III radio bursts observed by Ulysses pole to pole, and simultaneously by wind

Y. Leblanc1, G.A. Dulk1,2, S. Hoang1, J.-L. Bougeret1, and P.A. Robinson3

1 CNRS-URA 264 and DESPA, Observatoire de Paris, F-92195 Meudon, France
2 Department of Astrophysical, Planetary and Atmospheric Sciences, University of Colorado, Boulder, CO 80309, USA
3 Department of Theoretical Physics, University of Sydney, Sydney, NSW 2006, Australia

Received 16 February 1996 / Accepted 17 June 1996

Abstract. We consider 555 type III bursts observed by Ulysses and/or Wind while Ulysses traversed a large range of longitude and latitude. We find:

1) The percentage of bursts seen by both spacecraft was 59.5%. It was higher, about 80%, when Ulysses was to the south and east of the Sun as seen from Wind. It decreased to about 50% when Ulysses was near the ecliptic behind the Sun, and it remained near 50% as Ulysses moved to the north and west.

2) The distribution of low frequency cutoffs img1.gif of type III radiation is very similar for bursts seen by Ulysses and those seen by Wind, whereas the distribution of the in-situ plasma frequency img2.gif at the two spacecraft is very different. Many bursts descend to close to img3.gif kHz at Wind but few descend to the lower img2.gif img4.gif kHz at Ulysses. We confirm earlier findings that img1.gif is rarely lower than about 20 kHz.

3) Statistically, the low frequency limit depends strongly on the burst intensity, being about four times lower for strong bursts than for weak bursts.

We consider three hypotheses for the close relation between intensity and img1.gif : proximity of the burst source to one spacecraft or the other, directivity and propagation effects, and density and speed of the fast electron stream. We conclude that, while the first two may be important for some bursts, for many others the character of the electron stream is the dominant factor in establishing both the burst intensity and the lowest frequency attained.}

Key words: Sun: radio radiation - activity - corona

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