On 13 September 1994, the joint ESA NASA Ulysses mission passed a major milestone on its journey of exploration through the third dimension of the Sun's environment, the heliosphere. Nearly four years after its launch by the Space Shuttle 'Discovery', the European-built spacecraft reached the most southerly point on its out-of-ecliptic orbit, 80.2 degrees south of the Sun's equator, at a distance of 2.3 AU (345 million km) from the Sun. Although it will take scientists many months to unravel fully the new and exciting data acquired by Ulysses, several important results have already emerged.
The international Ulysses mission, a joint undertaking by ESA and NASA, is currently surveying the unexplored region of space above the poles of the Sun. Following a unique trajectory (Fig. 1) that allows it to escape the confines of the ecliptic plane (in which the Earth and most of the planets orbit the Sun), the Ulysses spacecraft and its payload of interplanetary particle and field instruments (listed in Table 1) are returning data of exceptional quality. Data coverage throughout the mission to date has been very high, exceeding 95% on average (Fig. 2). This comprehensive data set, representing one of the most complete records of interplanetary phenomena ever obtained, is enabling scientists to accomplish successfully the primary goal of the mission, namely the first-ever study of the Sun's environment from the equator to the poles.
Even before embarking on the key, out-of-ecliptic phase of its mission, Ulysses had already demonstrated its excellent scientific capabilities. Launched in October 1990 from the Space Shuttle 'Discovery', Ulysses needed a gravity assist by the giant planet Jupiter to achieve its final, high-inclination orbit. En route to Jupiter, Ulysses collected a detailed set of in- ecliptic interplanetary measurements (see ESA Bulletin No. 67), while the two-week passage through Jupiter's magnetosphere in February 1992 also produced new and exciting data (see ESA Bulletin No. 72).
This article focuses on the results obtained during the first high-latitude pass.
Figure 1. The Ulysses flight path viewed from above the ecliptic plane. The north and south polar passes, defined to be the segments of the trajectory above 70 degrees solar latitude, and the position of Ulysses at its most southerly point are shown.
Figure 2. Data coverage during 1994, showing the percentage of real-time (1024 bit/s) and playback (512 bit/s) data
Table 1. The Ulysses scientific investigations
The launch energy provided by the Space Shuttle and three powerful upper-stage rockets, combined with the gravity-assist manoeuvre at Jupiter, placed the Ulysses spacecraft in a Sun-centred, elliptical orbit inclined at 80 degrees with respect to the Sun's equator. Important design requirements for the mission were to maximise the time spent at high solar latitudes and to achieve the highest possible latitude. Owing to the relative positions of the Earth, Sun and Jupiter at the time of the planetary swingby, a south-going out-of-ecliptic trajectory best met these requirements.
On 26 June 1994, 28 months after leaving Jupiter, Ulysses began its passage over the Sun's southern polar cap. The Ulysses polar passes are defined to be the segments of the trajectory corresponding to solar latitudes greater than or equal to 70 degrees in either hemisphere. The south polar pass lasted 132 days, equivalent to five solar rotations. During this time, the distance from the spacecraft to the Sun decreased from 2.8 to 1.9 AU (1 AU = 150 million km). The spacecraft reached its most southerly point, 80.2 degrees south of the solar equator, on 13 September 1994, at a distance of 2.3 AU from the Sun. An overview of the Ulysses polar passes is presented in Table 2.
Table 2. Key dates during the Ulysses mission
Solar wind and magnetic field
The polar passes of Ulysses take place near the minimum in the current activity cycle of the Sun. The structure of the corona near solar minimum is dominated by the appearance of large coronal holes, cool regions in the Sun's corona, at the north and south poles with relatively few transient disturbances. From remote-sensing observations over many years (utilising, for example, the scintillation of distant radio sources), it was expected that Ulysses would encounter fast solar wind from the coronal holes over the poles. Fast streams of solar wind are also observed in the ecliptic at times when coronal holes extend to low latitudes. Observations from Ulysses, the first ever to be made in-situ in the solar wind flowing from the polar caps, have confirmed this expectation.
From July 1992 until April 1993, the solar wind flow at Ulysses was dominated by the appearance of a single high-speed stream once per solar rotation, with slower solar wind in between (Fig. 3). The fast stream was traced back to an equatorward extension of the southern polar coronal hole, while the slower wind originated in the so-called 'coronal streamer belt' that encircles the Sun's magnetic equator. Starting in May 1993, this recurrent pattern underwent a change. While the dominant high-speed stream remained visible in the data, the speed of the wind in the inter-stream regions increased, significantly reducing the peak-to-valley excursions. As a consequence of its increasingly southern position at this time, Ulysses was no longer exposed to solar wind from inside the streamer belt, only to wind from the boundary region between the belt and the coronal hole and to fast wind from the hole itself.
Once above 40 degrees latitude, Ulysses became totally immersed in fast solar wind from the polar coronal hole flowing continuously at an average speed of 750 km/s. These conditions persisted throughout the south polar pass, continuing at least up to the end of 1994. However, given the much more rapid change in spacecraft latitude during the pole-to-pole segment of the trajectory than during the intial climb out of the ecliptic, it is to be expected that soon Ulysses will once more encounter a recurrent pattern of fast and slow solar-wind streams similar to that seen at lower latitudes prior to the south polar pass.
The profile of solar-wind speed shown in Figure 3 is a very useful 'road map' of Ulysses' first excursion to high latitudes. Many of the phenomena studied exhibit features that can be related to the same broad regions found in the solar-wind data.
The continuous exposure to fast solar wind over a period of many months has enabled Ulysses to study the characteristics of high-speed flows in unprecedented detail, leading to a very clear understanding of the fundamental differences between fast and slow wind. Fast wind from the poles originates in a region of the solar atmosphere that is several hundred thousand degrees cooler than the 1.8 million degree source region of the slower wind at the equator. Fast wind also has a different chemical composition from slow wind, being richer in elements such as oxygen that are relatively hard to ionise.
Ulysses' measurements at middle latitudes, where both slow and fast wind were sampled once per solar rotation, have also shown that the boundaries between these two kinds of solar wind are quite sharp and well-defined, even at the relatively large distance of Ulysses (Fig. 4). Even more surprising is the degree to which the 'temperature boundaries' and the 'composition boundaries' observed by Ulysses match, since the former must be established in the corona, whereas the latter are created in the chromosphere, below the solar atmosphere. This apparent relationship between conditions in the corona and processes in the chromosphere is expected eventually to shed light on the still-unanswered question of how the solar wind is created.
A surprising phenomenon identified by the solar-wind plasma experiment on board Ulysses was a new class of so- called 'coronal mass ejections '(CMEs) in the fast-moving solar wind at high latitudes. CMEs are large bubbles of gas, often having masses of 10 to 13 kg (equivalent to 100 000 large aircraft carriers!), propelled into space from the corona by magnetic forces at the Sun. CMEs near the ecliptic are known to 'plough into' slow solar wind ahead of them, creating a shock wave in the plasma, rather like a supersonic aircraft in the Earth's atmosphere. The high-latitude CMEs observed by Ulysses behave quite differently from their ecliptic cousins. They travel at the same high speed as the polar solar wind in which they are embedded and expand rapidly (apparently as a result of high internal pressure). The rapid expansion of the high-latitude CMEs drives a pair of shock waves, one towards and one away from the Sun. Since it has been demonstrated that CMEs are the main culprits for causing major magnetic storms on Earth (which in turn can disrupt technological systems like electrical power grids and satellites in orbit), it is important to gain a full understanding of these manifestations of the restless Sun.
Observations from Ulysses have confirmed that the large-scale structure of the magnetic field in the polar regions is, on average, organised according to the model predictions made by the father of the solar wind, Prof. Gene Parker, more than three decades ago. In this model, the field is shaped by the combined effects of the solar wind, which carries the field, flowing radially outward, and the rotation of the Sun to which the footpoints of the field lines are anchored (Fig. 5). The field is wound into a spiral which is tighter at the equator than at the poles. There are, however, significant and in many cases unexpected variations on all time scales (Fig. 6). Detailed study of these variations, which can be interpreted in terms of a variety of both dynamic and spatial structures in the solar wind, have revealed a striking similarity to features observed on occasion in fast solar wind in the ecliptic and much closer to the Sun (0.3 AU) by the Helios spacecraft. Both sets of observations point to solar-wind plasma that has undergone relatively little change during transit from the Sun. The surprise is that the polar solar wind retains this unevolved character out to distances of 2 AU or more. The observations also suggest that the fast solar wind seen in the ecliptic has its origin at higher latitudes, again indicating the influence of non-radial effects.
A surprising result to emerge from the observations of the heliospheric magnetic field over the poles concerns the strength of the field. It was expected that the Ulysses data acquired at high latitudes would contain evidence of a dipole- like field (similar to a bar magnet) with a clear concentration of magnetic flux corresponding to a south magnetic pole. This expectation was based on an extrapolation of the Sun's surface (photosferic) magnetic field, as measured routinely from the Earth using spectroscopic techniques. The surface field at solar minimum clearly resembles a dipole with its axis tilted by 10-20 degrees with respect to the Sun's rotation axis. Scientists believed that an imprint of this field would be carried out by the magnetised solar wind. What Ulysses found, however, was a rather uniform field with no concentration of magnetic flux at high latitudes. Clearly, scientists have to re-think their ideas concerning the way in which the Sun's surface magnetism is carried into the solar wind. One possibility is that magnetic stresses acting close to the solar surface are able to redistribute the field.
Figure 3. Solar-wind speed as a function of time and latitude since Jupiter flyby as measured by the SWOOPS experiment on board Ulysses, together with an X-ray image of the Sun from the Japanese Yohkoh satellite (Courtesy of the Los Alamos National Laboratory and ISAS)
Figure 4. Data from the SWICS experiment on board Ulysses showing measurements of fast and slow solar wind. Note the correlation between the inferred source temperature in the corona and the relative abundance of magnesium to oxygen, both of which are anti-correlated with solar-wind speed. (Courtesy of J. Geiss, University of Bern)
Figure 5. Simplified representation of the heliospheric magnetic field as predicted prior to the Ulysses mission
Figure 6. Data from the magnetometer experiment on board Ulysses, showing the high degree of variability in the field measured over the south pole. (Courtesy of T. Horbury, Imperial College London)
Ever since a mission to explore the third heliospheric dimension was conceived, scientists have been intrigued by the possibility of being able to detect a more complete sample of cosmic- ray particles - high-energy nuclei thought to be created in supernova explosions - over the solar poles. The reasoning is rather simple: since the heliospheric magnetic field at the poles is much less tightly wound by solar rotation and presumably less disturbed than near the equator, cosmic- ray particles (which are electrically charged and therefore bound to follow the magnetic field) ought to have an easier access to the inner heliosphere over the poles. In that case, particles entering the heliosphere through such 'cosmic-ray funnels' would reach a solar-polar orbiting spacecraft like Ulysses with very little loss in energy. This in turn would allow scientists to study the properties of the cosmic rays (e.g their composition and energy distribution) over a much broader energy range than is possible in the ecliptic, where the tightly-wound magnetic field and turbulent solar wind form an effective barrier to low-energy cosmic rays.
In fact, although Ulysses detected an increase in the cosmic-ray flux over the south pole compared with the fluxes measured in the ecliptic, the increase was much smaller than expected (Fig. 7), particularly at low energies. It is now thought that the irregularities in the magnetic field seen over the pole are able to scatter the incoming cosmic-ray particles, making the funnel less effective.
A topic of great interest during the first polar pass has been the variation in the energetic- particle fluxes with latitude. Prior to Ulysses, it was generally expected that fluxes of solar and interplanetary energetic particles would be low over the poles near solar minimum, principally because of the lack of high-latitude acceleration sites. Surprisingly, the recurrent increases in particle intensity observed at low latitudes in association with corotating shock waves formed by the interaction of long-lived fast and slow solar-wind streams, continued to be seen up to 70 degrees latitude, even though the shocks themselves were not detected at the location of the spacecraft. During the south-polar pass itself, and almost up to the end of the year, the fluxes showed very little variation, remaining essentially at background levels (Fig. 8).
One possible explanation of the Ulysses energetic-particle results is as follows. Co-rotating shocks, i.e. particle acceleration sites, do form at high latitudes, but at greater distances from the Sun than Ulysses. The recurrent flux enhancements observed at moderately high latitudes presumably originated at these more distant locations. However, a source of low-energy particles is also required as input to the acceleration process. An obvious candidate in this regard would be a solar flare. If no solar flare (or other) source is present, the acceleration process will probably be less efficient. In fact, no energetic flares occurred on the Sun during the Ulysses south-polar pass, which would provide one explanation for the lack of recurrent particle increases over the pole. Observations over the north pole will help to substantiate this picture.
Figure 7. Cosmic-ray data from the Ulysses/COSPIN HET experiment showing the smaller-than-expected increase in the number of cosmic-ray particles arriving over the south pole compared with model predictions. (Courtesy of R.B. McKibben, University of Chicago)
Figure 8. Energetic-particle data from the Ulysses/COSPIN LET experiment showing the absence of periodic enhancements above 70 degrees latitude during the south polar pass. (Courtesy of T. Sanderson, ESA Space Science Dept.)
Another area of research using Ulysses high-latitude data that has proved to be very fruitful is the study of interstellar pick-up ions. These particles flow into the heliosphere as neutral atoms of interstellar gas, are subsequently ionized, and 'picked up' by the outflowing solar wind. Unique results in this field have been obtained by the solar-wind ion composition spectrometer on board Ulysses (fig. 9). Oxygen, nitrogen and neon pick-up ions of interstellar origin have been detected for the first time, permitting estimates of the relative atomic abundances of the interstellar gas. In addition, by using simultaneous measurements of the fluxes of doubly-ionized helium of both solar-wind and interstellar-pick-up origin, a new method has recently been developed for determining the absolute abundance of neutral helium in the local interstellar medium. Preliminary analysis indicates a value close to 0.01 atoms/cubic centimeters.
Figure 9. Interstellar pick-up ions detected by the SWICS experiment on board Ulysses. (Courtesy of J. Geiss, University of Bern, and G. Gloeckler, University of Maryland)
Having crossed the ecliptic, Ulysses is now en route to the north polar regions, which it will begin to explore on 19 June. Given the wealth of new data - and several unexpected puzzles - that have been generated during the exploration of the south pole, the Ulysses investigators are eager to see what surprises are in store above the Sun's north pole. Whatever is found, Ulysses has already altered our view of the heliosphere for ever.
Looking even further ahead, the prospects for continuing the mission after the north polar pass are very good. The ESA Science Programme Committee has approved the Agency's participation in the mission until 2001, corresponding to a full second orbit of the Sun. NASA has also expressed its intention to continue the mission, on the understanding that the NASA budget-approval procedures operate on a shorter-term basis than in ESA.
A technical evaluation has shown that the spacecraft is capable of operating until the end of 2001, albeit with certain constraints during the last few months of its lifetime. The limiting factor is the power output of the radioisotope thermoelectric generator (RTG), which will have decreased to the point where it can no longer supply sufficient electrical power to maintain the attitude-control fuel above its freezing point.
Scientifically, the second solar orbit is highly desirable, since the polar passes in 2000 and 2001 (see Fig. 10 and Table 2) will occur when the Sun is at its most active. This will permit scientists to extend their survey of the three-dimensional heliosphere to cover the full range of solar activity conditions.
Figure 10. The flight path of the Ulysses spacecraft during its second orbit of the Sun. Also shown are the polar passes in 2000 and 2001
Table 2. Key dates during the Ulysses mission