Jet Propulsion Laboratory, Pasadena, CA USA. Abstract. In the 25 months ... Ulysses will achieve its highest southern heliolatitude of 80 S in. September 1994.
ULYSSES
SOLAR
W I N D O B S E R V A T I O N S TO 56 ~ S O U T H
D.J. McCOMAS, J.L. PHILLIPS, S.J. BAME, J.T. GOSLING Los Alamos National Laboratory, Los Alamos, NM USA B.E. GOLDSTEIN and M. NEUGEBAUER Jet Propulsion Laboratory, Pasadena, CA USA
A b s t r a c t . In the 25 months since Jupiter flyby, the Ulysses spacecraft has climbed southward to a heliolatitude of 56~ This transit has been marked by an evolution from slow, dense coronal streamer belt solar wind through two regions where the rotation of the Sun carried Ulysses back and forth between streamer belt and polar coronal hole flows, and finally into a region of essentially continuous fast, low density solar wind from the southern polar coronal hole. Throughout these large changes, the momentum flux normalized to 1 AU displays very little systematic variation. In addition, the bulk properties of the polar coronal hole solar wind are quite similar to those observed in high speed streams in the ecliptic plane at 1 AU. Coronal mass ejections and forward and reverse shocks associated with corotating interaction regions have also been observed at higher heliolatitudes, however they are seen less frequently with increasing southern heliolatitude. Ulysses has thus far collected data from 20 ~ of nearly contiguous solar wind flows from the polar coronal hole. We examine these data for characteristic variations with hefiolatitude and find that the bulk properties in general show very little systematic variation across the southern polar coronal hole so far. 1. I n t r o d u c t i o n The Ulysses spacecraft has been returning essentially continuous measurements of the interplanetary particles and fields environment since turn-on shortly after launch in October 1990. Ulysses will achieve its highest southern heliolatitude of 80 ~ S in September 1994. At this writing, Ulysses plasma data are available through 11 March 1994 when Ulysses was at a heliolatitude of 55.6 ~ S. The solar wind plasma instruments on Ulysses are curved plate electrostatic analyzers which independently measure the distributions of electrons and ions as functions of spatial look direction and energy per charge (E/q) (Bame et al., 1992). The ion instrument covers an energy range from 255 eV/q to 34.4 keV/q over all look directions that the super-Alfvenic solar wind beam can come from. The electron instrument measures particles incident from >95% of 4It sr continuously from 1.6 to 862 eV. Two previous studies of the Ulysses plasma observations have examined the latitudinal evolution of bulk solar wind properties (Bame et al., 1993; Phillips et al., 1994), In brief, Ulysses has transited from observing primarily slow, dense solar wind from the coronal streamer belt to fast, low density solar wind from the southern polar coronal hole. In this study we examine and extend the analysis of this evolution in the observed solar wind and provide a more detailed analysis of the 20 ~ of observations since Ulysses entered the polar coronal hole.
Space Science Reviews 72: 93-98. 9 1995 KluwerAcademic Publishers. Printed in the Netherlands.
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D.J. McCOMAS ET AL.
2. Observations Figure 1 displays 6-hour averages of the solar wind proton speed, proton density, proton flux, and proton mass flux/amu observed by Ulysses between Jupiter and 56 ~ S. The three bottom panels are normalized by the heliocentric distance squared to adjust to 1 AU equivalent values. The triangles along the top axis of the top panel indicate the heliocentric distance of the Ulysses spacecraft, while those along the bottom indicate the heliolatitude in 5 ~ increments from 6~ at Jupiter to 56~ The solar wind speed and density display significant variations with heliolatitude; these variations can be characterized by four distinct regions. At low heliolatitudes (
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Heliolatitude [o S] F i g u r e 2. O b s e r v a t i o n s o f s e l e c t e d solar w i n d p l a s m a p a r a m e t e r s s i n c e U l y s s e s b e c a m e c o n t i n u o u s l y i m m e r s e d in the polar hole flow at a heliolatitude o f 36 ~ S. Linear fits to these parameters are overplotted on each curve; fit parameters are given in Table 1.
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D.J. McCOMAS ET AL.
4.
Conclusions
The Ulysses solar wind instruments are fulfilling their primary objective of charting the solar wind properties out of the ecliptic plane, over the poles of the Sun. Thus far in the mission, Ulysses has achieved a heliolatitude of 56 ~ S and has transited from slow, dense solar wind flows from the coronal streamer belt to fast, less dense solar wind from the southern polar coronal hole. Throughout these changes in average speed and density, the momentum flux was quite constant on average. Ulysses observed two other regions at intermediate heliolatitudes where it repeatedly sampled both higher and lower heliomagnetic latitudes as the rotation of the Sun carried both streamer belt and polar hole types of solar wind back and forth across the spacecraft. By examining the peak vaules in the polar hole portions of these rotations, we demonstrated that the solar wind speed from the southern polar coronal hole appears to have been relatively constant over the 19 months that Ulysses has been at least intermittently sampling it. In addition, the bulk properties of the solar wind from the polar coronal hole are generally quite similar to those normally observed in high speed streams in the ecliptic plane at 1 AU. A more detailed analysis of the last 20 ~ of essentially continuous polar hole flow indicates that the bulk properties of the solar wind from this region are relatively constant and do not display strong variations with increasing heliolatitude; this is particularly true of the proton flux which displays the least average percentile change over the polar hole observations thus far. Nonetheless, small characteristic variations, such as the nearly 2 km s -1 deg-1 increase in the average bulk solar wind speed, may be real. If the solar wind continues to increase with heliolatitude at this rate, an average solar wind speed of over 800 km s -1 would be observed at the maximum Ulysses heliolatitude of 80 ~ S in September 1994. The next several months of Ulysses observations hold the key to what the polar solar wind is really like. Acknowledgements. This work was carried out under the auspices of the United States Department of Energy with support from the National Aeronautics and Space Administration.
References Bame, S.J., et al.: 1992, 'The Ulysses solar wind plasma experiment', Astron. and Astrophys., Suppl. Ser., 9 2, 221. Bame, S.J., et al. : 1993, 'Ulysses observations of a recurrent high speed solar wind stream and the heliomagnetic streamer belt', Geophys. Res. Lett., 2 0, 2323. Feldman, W.C., et al. : 1977, Plasma and magnetic fields from the Sun, The solar output and its variation, ed., O.R. White, Col. Assoc. Univ. Press, Boulder. Gosling, J.T. et al.: 1993, Latitudinal variation of solar wind corotating interaction regions: Ulysses, Geophys. Res. Lett., 2 0, 2789. Gosling, J.T. et al. : 1994a, The speeds of coronal mass ejections in the solar wind at mid heliographic latitudes: Ulysses, submitted to Geophys. Res. Lett. Gosling, J.T. et aI. : 1994b, A forward-reverse shock pair in the solar wind driven by overexpansion of a coronal mass ejection:Ulysses observations, Geophys. Res. Lett., 2 1, 237. Phillips, J.L., et aL: 1994, 'Ulysses at 50 ~ south: constant immersion in the high-speed solar wind', submitted to Geophys. Res. Lett.
