Ulysses' rapid crossing of the polar coronal

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 103, NO. A2, PAGES , FEBRUARY 1, 1998 Ulysses' rapid crossing of the polar coronal hole boundary D. J. McComas, P. Riley, and J. T. Gosling Space and Atmospheric Sciences Group, Los Alamos National Laboratory, Los Alamos, New Mexico A. Balogh and R. Forsyth Imperial College of Science and Technology, London, England 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 The spacecraft, which was at 1.35 AU and -19 ø north heliographic latitude, moving northward in its orbit, remained the fast solar wind from then through summer 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 detail. The crossing is marked by a region of enhanced pressure, typical of stream interaction regions, which extends -2x107 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 (<8X10 4 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 x10 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 ' to -750 km s 'l, 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. 1. Introduction Prior to the flight of the joint NASA/European Space Agency Ulysses Mission over the poles of the Sun, there were no direct in situ observations of the high-latitude solar wind. Ulysses results over the past several years have shown that at least for the declining phase of the solar cycle, the highlatitude solar wind, arising from the polar coronal holes has a ubiquitous and extremely steady, high speed and low density compared with the typical low-latitude solar wind which is a mixture of plasmas arising from both holes and from the solar streamer belt [e.g., Phillips et al., 1994]. Figure 1 displays a plot of the solar wind speed measured by Ulysses as a function of heliolatitude as it transited its elliptical polar orbit about the Sun. This plot shows how constant the speed is throughout the high-latitude regions compared with the much more variable properties of the wind at low latitudes. High-speed, low-density flows, commonly called highspeed solar wind streams, have also been routinely observed at low latitudes [e.g., Snyder et al., 1963]. High-speed streams at low latitudes originate both from limited extent, low-latitude coronal holes and from large dipole tilts and the extension of polar coronal holes to lower latitudes. An example of a highspeed stream at low latitudes can be seen on the left side of Copyright 1998 by the American Geophysical Union. Paper number 97JA /98/975A Figure 1 at--12 ø. Such observations have provided glimpses of the high-latitude solar wind even in low heliolatitude observations. Previous studies have examined the latitudinal variation of the solar wind from a very limited latitude range near the ecliptic plane. Schwenn et al. [1978] combined Helios and IMP observations from <1 AU and found that the thickness of the boundary between fast and slow winds was <10 ø in latitude. These authors derived gradients in the flow speed of >30 km s ' deg '. Subsequently, Mitchell et al. [1981] examined observations from a number of spacecraft between one and five AU, near the ecliptic plane. Their results agreed with the findings of Schwenn et al. [1978] and indicated that the transition from slow to fast winds occurs over -5 ø in latitude. Stream interfaces between fast and slow solar winds [Belcher and Davis, 1971] at the edge of coronal holes were predicted to be discontinuous even before such interfaces were observed in space [Dessler and Fejer, 1963]. Measurements in the early 1970s from IMP 6-8 provided a statistical data set for examining discontinuous stream interfaces. Gosling et al. [1978] found that about one third of the high-speed streams observed in the ecliptic plane at 1 AU contained such sharp boundaries near their leading edges. These boundaries, which separate what was originally slow, dense solar wind from originally faster, less dense wind, also (1) are the sites of shear in the solar wind flow, (2) occur at or near the maxima in the pressure, (3) are marked by increases in the electron and proton temperatures, and (4) are typically separate regions of differing solar wind composition as indicated by discontinuous changes in the alpha to proton ratios. 1955

2 1956 MCCOMAS ET AL.: ULYSSES' RAPID CROSSING N. Pole Speed (km s -t) ø 45 ø 500 Equator I Peri - Aphelion I _ 1000 I Equator t s -500 _45 ø -45 ø S Pole Figure 1. Polar plot of solar wind speed as a function of heliolatitude for the out-of-ecliptic phase of Ulysses mission from February 1992 through January While discontinuous stream interfaces are easily observed and studied at low heliolatitudes, the complete transition from slow, streamer belt solar wind to fast, polar coronal hole flow is best examined using higher-latitude Ulysses observations. This is because low latitude crossings occur only when the high-speed polar flow had pushed locally down to low latitudes. Such limited extent regions are necessarily nonplanar, and the distance that a low-latitude spacecraft has passed into or through the full transition is largely unknown. The high-speed excursion at ~-12 ø is an example of this sort of encounter. In contrast, the two crossings between slow and fast solar winds during the Ulysses rapid latitude scan in early 1995 are qu,litatively different than previously ones owing to the substantial meridional spacecraft motion and the complete transition between the two types of solar wind. The fast/slow crossings in the Ulysses fast latitude scan are clear in Figure 1 at roughly +20 ø. The first crossing (-20 ø) is complicated by a coronal mass ejection which is embedded in and traveling out along the slow-fast transition boundary [Gosling et al., 1995a]. The second crossing, on the other hand, provides a uniquely simple and rapid transition from the low latitude, lower speed solar wind to the high latitude, high-speed northern polar wind. In this study we examine this second crossing in detail and discuss its implications for understanding the boundary of the polar coronal hole in general. 2. Observations The data used in this study are derived from Ulysses solar wind plasma observations from the Los Alamos solar wind ion and electron spectrometers, called the solar wind observations over the poles of the Sun (SWOOPS) experiment [Barne et al., 1992], and from magnetic field observations from the Imperial College/JPL magnetometer [Balogh et al., 1992]. The solar wind electron and ion spectrometers utilize electrostatic analyzers that measurelectrons and ions with energies from 0.81 to 862 ev and 255 ev/q to 34.4 kev/q, respectively. The plasma moments have been calculated from integration of the electron and ion distribution functions, providing independent parameters for the proton, alpha particle, core electron, and halo electron components. Throughou this study, we use the RTN coordinate system where the R unit vector points radially outward from the Sun along the Sun-spacecraft line, T is the cross product of the solar rotation axis direction with R and gives a vector that lies generally in the direction of planetary motion around the Sun, and N (= R x T) completes the set. Angles are measured in RTN coordinates as follows: 0 is measured as the north-south angle in the R-N plane from +R toward +N, is measured as the east-west angle in the R-T plane from +R toward +T. Plate 1 displays eleven days of solar wind velocity observations around the time of Ulysses' crossing from the low-latitude band of solar wind variability into the fast polar wind flow. The top panel shows the proton and alpha speeds and the bottom panel gives the ½ and 0 angles. It is interesting to note that while the proton and alpha speeds are similar in the low-speed wind, they diverge with increasing speed and are roughly separated by the local Alfven speed as it typical for fast streams observed near the ecliptic plane [Marsch et al., 1982]. The stream interface is indicated by the vertical line at ~1610 UT on day 87. Characteristic of stream interfaces, the leading edge of the speed increase is quite abrupt and flow deflections indicate a shear in the flow across it. In contrast, the overall transition to the high-speed region takes many days. The smooth, regular shape of this larger transition in speed suggests that the structure of this particular crossing was likely stable, allowing Ulysses to examine the spatial characteristics of the full transition region, rather than simply observing temporal variations and irregular motions of the boundary with respect to the spacecraft.

3 ß ß MCCOMAS ET AL.: ULYSSES' RAPID CROSSING Alpha Speej '.-.'".5,',':"' t '.',, ' :-" ß, ,;t -i:.,. /,,.,. ': Proton Speed :I' ' 50O 400 o 300,., I,,,,,,,,,,,,,, Theta, " I ;':. ß ' '. s :' ', : ' ' '. '.: :.., '. ' '"."-,-'-... ß :.. - i I 0 I Ill I : 'i, ' I : : ' I ' ' ' I ' : ' I ' : Z 94 Day of Year Plate 1. (top) Solar wind proton and alpha speeds and (bottom) proton flow angles for 11 days encompassing Ulysses crossing from the low-latitude, slow solar wind into the fast solar wind from the polar coronal hole. Discontinuous stream interfaces are expected to be tangential discontinuities (TDs)[e.g., Gosling et al., 1978; Pizzo, 1991]. At a TD both the flow velocity and the magnetic field have zero normal components and the plasmas on the two sides of the discontinuity are in overall (plasma plus magnetic) pressure equilibrium. In this study, we first assume that the interface is a TD and use the magnetic field to determine its orientation. We then test this result by examining both the pressur equilibrium and plasma flows immediately adjacent t o the calculated boundary. As will be seen below, both of these independent measures are consistent with the interpretation of this interface being a tangential discontinuity. In order to determine the orientation of the boundary, we examined the highest time resolution magnetic field data over several intervals. Figure 2 displays the magnetic field components and magnitude for the interval from UT. As can be seen in the field magnitude (bottom panel), the transition occurs in two steps; region B is taken at the intermediate level. Regions A and C were chosen to be just outside of the transition. Each of the intervals contains 86 1-s averaged magnetic field vectors. We next examined normals derived from the cross product of each upstream and downstream magnetic field vector in the three intervals, taken pairwise: A-B, B-C, and A-C. To remove the ambiguity in these cross products, we assume that all normals point into the outward (+R) hemisphere. Plate 2 displays the range of polar and azimuthal angles of possible normals derived by calculating these normals from every

4 1958 MCCOMAS ET AL.' ULYSSES' RAPID CROSSING Su. f N Stream ati l x///interface Re /"/ / T Spacecraft,,,,r' / '._ Moti Magnetic "' Field R Normal (.81,.42,-.41) Figure 2. High-resolution magnetometer measurements covering 15 min before and after the stream interface tangential discontinuity. Regions A, B, and C (used to determine the boundary orientation) are immediately preceding, during, and immediately following the interface, respectively. permutation of data points in each of the three intervals. The centroids of each of the three pairings are indicated by the crosses; the closeness of these centroids demonstrates that regions bounded by these windows are nearly parallel. Table 1 summarizes the results of this analysis. Of the three pairs of combinations, the A-C pairs give the smallest amount of scatter and provides a centroid that is near the middle of all three distributions. For the purposes of this study we adopt the A-C derived normal [0.81, 0.42, -0.41] which has q and 0 angles of 28 ø and -24 ø, respectively. Thus the stream interface is aligned nearly along an Archimedian spiral and tilted in the meridional plane in the sense expected for a corotating interaction region driven by flow from the northern polar hole [e.g., Pizzo, 1991; Gosling et al., 1995b]. Figure 3 schematically displays this stream interface configuration. In order to calculate the real gradients in solar wind properties across the slow-fast transition, the spacecraft motion with respect to the boundary must be determined. This relative motion is simply the solar wind velocity at the interface minus the spacecraft velocity vector. We use the solar wind velocity vector (417.5, 22.1, ) km/s, taken as the average of the two most central ion measurements at an average time of 1610:41 UT, precisely in the center of the A-C interface region, and the spacecraft motion vector (7.5, -7.2, 30.7) km/s. Subtracting these two vectors provides a total motion of the structure over the spacecraft of (410, 14.9, 13.2) km/s. 180! I I I I IE0 60 ß! B-C A-C o -6o -ao o ao 6o 0 (o) Plate 2. Polar-azimuthal angle plot of the ranges of interface normals derived by using all possible single vector pairs from regions A and B (red), B and C (blue), and A and C (yellow). The three crosses show the centroids of these three regions; for this study we adopt the A-C centroid value. 9O

5 MCCOMAS ET AL.' ULYSSES' RAPID CROSSING 1959 Table 1. Stream Interface Normal Orientations A-B B-C A-C ø -20 ø -24 ø 23 o 28 ø 28 ø Plate 3 displays the measured solar wind velocity vector (black, blue, and red traces) for March 28, 1995, in the moving reference frame of the interface (I/F). The green trace shows the speed of the wind normal to the interface as determined by the dot product of the motion vector with the interface normal vector. Note that this speed is zero at the two points immediately adjacent to the interface and quite small and flowing away from it for some time on either side. This result supports our previous assumption that the interface is a TD. We assume that the structure is planar on the scale of the crossing such that all gradients are parallel to the normal direction. The spacecraft moves through the interface at an angle 36 ø from the normal (see Figure 3) so that actual gradients are 24% steeper than they appear along the spacecraft track. The velocity of the spacecraft normal to the structure is Ivl cos 36 ø = 333 krn/s. The use of this normal speed is very good near to the actual boundary where it was determined and probably becomes progressively less accurate at larger distances. Plates 4-7 and Figure 4 display various plasma and field parameters as a function of distance normal to the tangential discontinuity on two spatial scales. We have chosen not to remove the small humber of glitches observable in these highest time resolution data because several of them are uncertain and may represent real small-scale structures. Figure 4 displays the helium abundance ratio as a function of this distance over 2.5x107 km (top) and 2x106 km (bottom) on either side of the interface. Immediately before the interface, the helium abundance was lower than average. At the interface, the abundance ratio jumped to a value of -4.3%, typical of the much more constant ratios observed in the high-latitude solar wind. The transition here occurs between two individual ion measurements, which are four minutes apart. Hence the spatial scale of this transition is <8x10 n km. Plate 4 shows the proton and alpha ion densities and core and halo electron densities in the same format as Figure 4. The bulk densities (protons and core electrons) display an abrupt change across the interface as does the alpha ratio. In contrast, the alpha and halo electron densities show little, if any, variation across the interface. This figure demonstrates that it is the variation in proton density across the interface that drives the jump in the alpha ratio seen in Figure 4. Temperatures for the proton, alpha, core electron, and halo electron components are shown in same format in Plate 5. Both of the ion temperatures begin to rise right at the interface but continue to rise over substantially longer spatial scales (-106 km). On the other hand, the core and halo eldctron temperatures are nearly constant across the interface, in contrasto that found in the statistical study of Gosling et al. [1978]. Plate 6 displays the ion, electron, magnetic, and total pressures in the same format as the previous plots. Plate 6, top, shows that the stream interface is embedded in a larger region of enhanced plasma and magnetic pressures. This enhancement extends ~107 km on either side of the interface, similar to the region of flow deflections shown in Plate 1. This spatial scale is a decade larger than the scale of the variation of the ion temperatures seen in Plate 5 but is comparable to the scale of the full density variation. In addition to this large-scale pressure enhancement, the bottom panel shows that the interface itself is in pressur equilibrium with the ion and electron pressures dropping while the magnetic pressure rises. Again, the spatial scale for these changes is less than one measurement sampling interval or 8x10 n km. For completeness, Plate 7 displays the ion, electron, and total betas for this crossing. Since beta is simply the ratio of the plasma and magnetic pressures, this information is contained within the previous plot. None-the-less, it is useful to plot these ratios as well. The top panel shows that all of the betas are highly variable. On average, the electron pressure and beta are larger than the ion pressure and beta on the slowspeed wind side of the interface while the opposite is true on the high-speed side. Right at the interface, both the electron and ion betas drop substantially, again on the shortest spatial scale observable here, <8x10 n km. Interestingly, the betas immediately downstream of the interface drop below one for several x 105 km, indicating that the total pressure is dominated by the plasma just upstream of the interface and the magnetic field just downstream. Intervals of beta <1 are very unusual in the Ulysses high-latitude observations [e.g., McCornas et al., 1995]. 3. Discussion In this paper we have examined the boundary of the polar coronal hole in detail at the location where Ulysses made its fast latitude transit from the slow, low-latitude solar wind into ca -E -4-6 A B C 16:00 16:05 16:10 16:15 16:aO 16:a5 as-mar-1995 Figure 3. Schematic diagram of the stream interface in RTN coordinates. Note that the local magnetic field lies in the plane of the stream interface and maps back to the slow-fast transition boundary near the Sun. The spacecraftransits the interface at an angle of 36 ø with respecto the normal.

6 MCCOMAS ET AL.: ULYSSES' RAPID CROSSING 120 Vr-Vr(I/F) --:... Vt-Vt(I/F) 80 Vn-Vn(I/F) V dot n ii 40! i I[ 0 J I,J _ i J!! i lo UT - March ' 'i... I ] t... [, j... r Vr-Vr(I/F) Vt-Vt(i/F) 80 Vn-Vn(I/F) clot n I I I I! UT- March Plate 3. Velocity vectors in the frame of the moving interface (I/F) (top) for all of March 28, 1995, and (bottom) a blowup of the 2 hours surrounding the TD. The speed of the solar wind normal to the interface (Von) is also shown (green), while the normal was derived using the magnetometer data alone. Note that this value is nearly zero on both sides of the discontinuity.

7 ._._ MCCOMAS ET AL.: ULYSSES' RAPID CROSSING E o E lo 20 Distance to Interface [Mkm] Distance to Interface [Mkm] Figure 4. Helium to proton number ratio as a function of normal distance from the interface for the (top) 5X107 km and (bottom) 4x106 km surrounding the TD. Across the interface, many of the plasma properties change abruptly. In particular, the proton and core electron densities drop. The pressures also change across the interface with the electron and ion pressures dropping and the magnetic field pressure jumping such that the discontinuity remains roughly in pressure balance. As a consequence, the electron, ion, and total betas drop across the interface from typical values at these distances in the solar wind of a few to atypical values <1. Both the proton and alpha temperatures begin to rise right at the interface, but unlike the discontinuous changes discussed above, these temperatures continue to rise toward their final high-speed values over distances of a few x 106 km. The region of compression, and hence high plasma and magnetic pressures, associated with the interaction region has an even larger scale (-2 x 107 km). This region also contains substantial plasma flow deflections, again as expected for a stream interaction. Unlike most of the rest of the parameters, which only change on the increasing speed side of the discontinuity, the pressure enhancements and flow deflections are roughly symmetric and bracket the interface. Finally, over the largest spatial scale, the solar wind speed initially climbs quickly from -400 km s ' near to the interface and then rises more slowly as it almost asymptotically approaches the typical high latitude speed of-750 km s 4. Our single derived normal direction is undoubtedly not entirely accurate for this several-day interval. None the less, the result that this transition corresponds to a scale of-108 km, much larger than any of the other scale sizes, seems clear. Plate 8 shows the solar wind velocity vector components measured in the frame of the interface (similar to Plate 3) over the entire span of the variation. The region of enhanced pressures associated with the stream interaction region is indicated by the vertical lines, as is the tangential discontinuity at the stream interface. Clearly, the region of the speed increase is appreciably larger than the interaction region and extends only on the downstream side of it. The speed increase, which could also be fit with a single curve, can be well characterized simply by two linear gradients (heavy lines overplotted on the radial speed). On the spatial scale of the stream interaction region, ~2x107 km, the gradient is ~ 9 km s 4 Mkm 4. If one simply converts this thickness and gradient to angular values at Ulysses' distance of 1.35 AU, the result is -6 ø and -32 km s 4 deg 4, which are similar to the values found by Schwenn et al. [1978] and Mitchell et al. [1981]. In addition to just a quickly changing portion of the transition, however, here we find that there is also a much broader region over which the speed more slowly increases up the fast, polar hole wind. This transit provides a unique opportunity for examining such an interface near to the Sun (1.35 AU) owing to the fact that the spacecraft passed completely through the comparatively smooth transition. The interface is identified by a tangential discontinuity embedded in a stream interaction region. We determined the orientation of the TD, and hence, of the stream interface, using measured high-resolution magnetic field vectors. This analysis was shown to be robust by calculating the normal with a large number of vector combinations and demonstrating that the results clustered in a small range of directions. This direction is also in qualitative agreement with the expected direction based on a physical understanding of the development and evolution of corotating stream interaction regions. Using the independent plasma to its final high-latitude values. The spatial scale and gradient for this second portion are km and km s ' Mkm ', roughly 5 times larger and more slowly changing than in the measurements, we confirmed that the interface was in pressure stream interaction region. The steeper slope for the balance and had no flow across it, as expected for a TD. This study has demonstrated that the slow/fast interface is characterized by several temporal and hence spatial scales. The shortest scale is that of the tangential discontinuity itself. High-resolution magnetometer observation show a transition that takes several minutes to complete while the 4-min cadence of the ion plasma observations set an upper bound on this thickness of <8x104 km. Using IMP observations with 2-min resolution, Gosling et al. [1978] set an upper bound of half of that given here (<2x104 km) for the discontinuities in their study. In the solar wind at ~1 AU, proton gyroradii are -103 km, much smaller than either of these upper bounds. interaction region portion is almost certainly attributable to the fact that the plasma within the interaction region has been compressed by steepening of the stream edge. Closer to the Sun, the wind could have had a single, more linear gradient comparable to that observed after the interaction region (-1.6 km s 4 Mkm 4) at 1.35 AU. Alternately, perhaps there is some way that the solar wind source is uniform across the polar hole but expansion of the field lines toward the equator limits the amount of acceleration compared with that along the straighter, higher latitude fields. It is interesting to note that while most of the plasma and field properties are modified on some spatial scale by the slow

8 1962 MCCOMAS ET AL.: ULYSSES' RAPID CROSSING 1 oo 10! t i i I, np nalpi a n-core n-halo Distance to Interface [Mkm] 1 o0 np nalpha n-core ii n-halo,,,, Distance to Interface [Mkm] Plate 4. Similar to Figure 4, but showing the proton, alpha, core electron, and halo electron densities.

9 MCCOMAS ET AL.: ULYSSES' RAPID CROSSING :' T-alpha I ' "T-core I II I..., O Distance to Interface [IVlkm] 10? E lo s ' "T-p T-alpha T-core I,,,, I,,,, t,,,,,...,,,,, I,,, T-halo Distance from Interface [Mkm] Plate 5. Similar to Figure 4, but showing the proton, alpha, core electron, and halo electron temperatures.

10 MCCOMAS ET AL.: ULYSSES' RAPID CROSSING l 'I' P_ion [PPa"i... P_ele [ppa] P_mag [ppa] P total [ppa]! l ß.,,,, I,,...,,. t,, lo 20 Distance from Interface [Mkm] P_ion [ppa] l: P_ele [ppa] I' P_mag [ppa]! P total [ppa] ] ø Distance from Interface [Mkm] Plate 6. Similar to Figure 4, but showing the ion, electron, magnetic, and total pressures.

11 ,., MCCOMAS ET AL.: ULYSSES' RAPID CROSSING 1965 ' beta-ion... beta-ele... beta-total i 10 0 I! z Distance from Interface [Mkm], '' '' I'' '' I'' ''1''' ' beta-ion... beta-ele ' beta-total i i ii iiii Distance from Interface [Mkm] Plate 7. Similar to Figure 4, but showing the ion, electron, and total betas.

12 1966 MCCOMAS ET AL.: ULYSSES' RAPID CROSSING lo o Distance to TD [107 km] Plate 8. Velocity vectors in the frame of the moving interface as a function of normal distance from the D for the interval covering the full solar wind speed rise to the polar coronal hole values. Note that the stream interaction region is much smaller and accounts for only a fraction of the total speed rise. to fast transition discussed here, a few are not. In particular, both the core and halo electron temperatures remain relatively constant across the transition. The lack of heating of the electrons observed here is unusual when compared with most of the stream interfaces observed near 1 AU [Gosling et al., 1978]. The rise in the ion temperatures with no significant change in the electron temperature also means that the core electron temperature and pressure dominate that of the ions in the slow solar wind with the opposite being the case in the fast wind. In addition, the minor (alpha and halo electron) densities did not change appreciably across this region. It seems surprising that the alpha density is unaffected by the transition when the proton density is so strongly affected. Since the proton density drops across the interface, the plasma composition, as indicated by the alpha ratio, also jumps across the interface, reaching its typical high-latitude value of -4.3%. Somehow, the alphas seem to be incorporated into both the fast and slow solar wind in roughly equal numbers in this case. Since fast and slow winds probably arise from quite different source regions, this result may suggest that the acceleration of heavier ions (alphas) is less sensitive to the particulars of the source region than is the acceleration of the protons. The high-energy, halo electrons, which are largely noninteractive with the bulk of the plasma, have similar densities and temperatures on the two sides of the slow-fast interface. This suggests that coronal heating of this highenergy tail, which produces an outward flowing beam or strahl (which is thought to subsequently populate the rest of the halo), is similar between the fast and slow wind sources. Our results are consistent with a heating source high enough in the corona that the underlying magnetic topologies (e.g., the sources of fast and slow solar wind) are no longer critical. Since the interface between the slow and fast wind observed in interplanetary space by Ulysses must map back to a boundary between the sources of such winds at the Sun, the range of spatial scales shown here are suggestive of boundary structure close to the Sun. While the plasma gradients steepen in the stream interaction region, the sharp boundary at the stream interface is strong evidence for a quite sharp boundary between the sources of fast and slow solar wind near the Sun [e.g., Gosling et al., 1978]. The larger region of increasing flow speed outside of the stream interaction region suggests

13 MCCOMAS ET AL.: ULYSSES' RAPID CROSSING 1967 that there is also a real gradient in solar wind source speed on the high-speed side of the discontinuity close to the Sun. Acknowledgments. This work was carried out under the auspices of the US Department of Energy with support from NASA under the Ulysses project. The Editor thanks G. Erd0s and K. W. Ogilvie for their assistance in evaluating this paper. References Balogh, A., T.J. Beek, R.J. Forsyth, P.C. Hedgecock, R.J. Marquedant, E.J. Smith, D.J. Southwood, and B.T. Tsurutani, The magnetic field investigation on the Ulysses mission: Instrumentation and preliminary scientific results, Astron. Astrophys., Suppl. Ser., 92, , Bame, S.J., D.J. McComas, B.L. Barraclough, J.L. Phillips, K.J. Sofaly, J.C. Chavez, B.E. Goldstein, and R.K. Sakurai, The Ulysses solar wind plasma experiment, Astron. Astrophys., Suppl. Ser., 92, , Belcher, J.W., and L. Davis Jr., Large-amplitude Alfven waves in the interplanetary medium, 2, J. Geophys. Res., 76, 3534, Barraclough, B.L., J.T. Gosling, J.L. Phillips, D.J. McComas, and B.E. Goldstein, He abundance variations in the solar wind: Observations from Ulysses, in Solar Wind 8, edited by D. Winterhalter et al., pp , Dessler, A.J., and J.A. Fejer, Interpretation of Kp index and M-region geomagnetic storms, Planet. Space Sci., 11,505, Gosling, J.T., J.R. Asbridge, S.J. Bame, and W.C. Feldman, Solar wind stream interfaces, J. Geophys. Res., 83, , Gosling, J.T., S.J. Bame, W.C. Feldman, D.J. McComas, J.L. Phillips, B.E. Goldstein, M. Neugebauer, J. Burkepile, A.J. Hundhausen, and L. Acton, The band of solar wind variability at low heliographic latitudes near solar activity minimum: Plasma results from the Ulysses rapid latitude scan, Geophys. Res. Lett., 22, , 1995a. Gosling, J.T., W.C. Feldman, D.J. McComas, J.L. Phillips, V.J. Pizzo, and R.J. Forsyth, Ulysses observations of opposed tilts of solar wind corotating interaction regions in the northern and southern solar hemispheres, Geophys. Res. Lett., 22, , 1995b. Marsch, E., K.-H. Muhlhauser, H. Rosenbauer, R. Schwenn, and F.M. Neubauer, Solar wind helium ions: Observations of the Helios solar probes between 0,3 and 1 AU, J. Geophys. Res., 87, 35-51, McComas, D.J., B.L. Barraclough, J.T. Gosling, C.M. Hammond, J.L. Phillips, M. Neugebauer, A. Balogh, and R.J. Forsyth, Structures in the polar solar wind: Plasma and field observations from Ulysses, J. Geophys. Res., 100, , Mitchell, D.G., E.C. Roelof, and J.H. Wolfe, Latitude dependence of solar wind velocity observed >1 AU, J. Geophys. Res., 86, , Phillips, J.L., A. Balogh, S.J. Bame, B.E. Goldstein, J.T. Gosling, J.T. Hoeksema, D.J. McComas, M. Neugebauer, N.R. Sheeley Jr., and Y.-M. Wang, Ulysses at 50 ø south: Constant immersion in the highspeed solar wind, Geophys. Res. Lett., 21, , Pizzo, V.J., The evolution of corotating stream fronts near the ecliptic plane in the inner solar system, 2, Three-dimensional tilted-dipole fronts, J. Geophys. Res., 96, , Schwenn, R., M.D. Montgomery, H. Rosenbauer, H. Miggenrieder, K.H. Muhlhauser, S.J. Bame, W.C. Feldman, and R.T. Hansen, Direct observation of the latitudinal extent of a high-speed stream in the solar wind, J. Geophys. Res., 83, , Snyder, C.W, M. Neugebauer, and U.R. Rao, The solar wind velocity and its correlation with cosmic-ray variations and with solar and geomagnetic activity, J. Geophys. Res., 68, , A. Balogh and R. Forsyth, Blackett Laboratory, Imperial College, London SW7 2BZ, England. J. T. Gosling, D. J. McComas, and P. Riley, Space and Atmospheric Sciences Group, Los Alamos National Laboratory, NIS-1, MS D466, Los Alamos, NM ( dmccomas@lanl.gov) (Received February 28, 1997; revised May 13, 1997; accepted May 15, 1997.)