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Astron. Astrophys. 316, 384-395 (1996)

A comparison of interplanetary coronal mass ejections at Ulysses with Yohkoh soft X-ray coronal events

L.A. Weiss1, J.T. Gosling1, A.H. McAllister2, A.J. Hundhausen2, J.T. Burkepile2, J.L. Phillips1, K.T. Strong3, and R.J. Forsyth4

1 Los Alamos National Laboratory, Los Alamos, NM 87545, USA
2 High Altitude Observatory / NCAR, Boulder, CO 80307, USA
3 Lockheed Solar and Astrophysics Laboratory, Palo Alto, CA 94304, USA
4 Imperial College, Blackett Laboratory, London, SW7 2BZ, UK

Received 9 February 1996 / Accepted 14 March 1996

Abstract. Coronal Mass Ejections (CMEs) observed at several AU by the Ulysses spacecraft are mapped radially back to the Sun and compared with Yohkoh Soft X-ray Telescope (SXT) images of the corona in an effort to identify correlated events. Correlations between the observations were difficult to make during the ecliptic phase of the Ulysses mission when the satellite footprint was at low heliographic latitudes and the Sun was particularly active. During its traversal to high southerly latitudes (February 1992 - September 1994), however, the correspondence became clearer for two reasons: 1) the radial velocity profiles of the high-latitude CMEs were better preserved since they were less likely to be driving shocks or to have interacted with high-speed streams; and 2) solar activity decreased, making it easier to discern individual and/or low-intensity events in the SXT images. We describe five Ulysses-observed CMEs which correlated with spatially and temporally isolated coronal X-ray events in the Yohkoh SXT images, concentrating on similarities and differences between their solar wind and coronal structures. Two of the five events appeared to have been initiated concurrently with active region (AR) flares; the other three involved the restructuring of low-intensity, polar crown arcades. Significantly, however, all five events exhibited an "LDE" signature, though only the two AR events generated a detectable signal above the GOES integrated background X-ray flux. The characteristics of the interplanetary CMEs were not well correlated with their coronal X-ray signatures: similar-looking coronal events produced very different interplanetary field structures, and different-looking coronal signatures evolved into remarkably similar structures at Ulysses. Although we suspect that all of the events may have had an initially helical field structure, only three of the events displayed coherent field rotations characteristic of nearly force-free flux ropes (two of these were associated with polar crown arcades and one with an AR flare). It appears that the most important factor in determining the magnetic field evolution of a CME in interplanetary space is its plasma beta, but that it is very difficult to predict the interplanetary beta based on the post-eruption coronal X-ray signature.

Key words: Sun: corona - solar wind - Sun: magnetic fields - plasmas

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