Ulysses' rapid crossing of the polar coronal hole boundary

Abstract

The Ulysses spacecraft crossed from the slow dense solar wind characteristic of the solar streamer belt into the fast, less dense flow from the northern polar coronal hole over a very short interval (several days) in late March 1995. The spacecraft, which was at 1.35 AU and ∼19° north heliographic latitude, moving northward in its orbit, remained in the fast solar wind from then through summer 1996. This boundary crossing is unique in that the combination of the spacecraft motion and rotation of the structure past the spacecraft caused Ulysses to move smoothly and completely from one regime into the other. In this study we examine this crossing in detail. The crossing is marked by a region of enhanced pressure, typical of stream interaction regions, which extends ∼2 × 107 km across. We find that the transition between the slow and fast regimes occurs on several temporal, and hence spatial, scales. On the shortest scale (<8×104 km) the stream interface is a tangential discontinuity where the proton and core electron densities and ion and electron pressures all drop while the magnetic pressure jumps to maintain a rough pressure balance. The alpha to proton ratio also jumps across the stream interface to reach the comparatively constant polar hole value of ∼4.3%. On larger scales (a few ×106 km) the proton and alpha temperatures rise to their high‐speed wind values. Finally, on the largest scale (∼108 km) the solar wind speed ramps up from ∼400 km s−1 to ∼750 km s−1, typical of polar hole flows. While it seems likely that the stream interface maps back to a sharp boundary near the Sun, the large region of increasing flow speed suggests that there is also an extended gradient in solar wind source speed close to the Sun.