On 30 September 1995, five years after launch, Ulysses completed the first phase of its highly successful exploratory mission to study the Sun's environment from the unique perspective of a solar polar orbit. The ESA-built spacecraft is the first ever to investigate the region of space above the Sun's poles, passing over the south pole in mid-1994, and the north pole one year later. The scientific data returned by the nine experiments on board Ulysses have literally added a new dimension to our view of the Sun's environment, the heliosphere. The breadth of scientific topics addressed by the mission is truly impressive, ranging from detailed measurements of the solar wind and its magnetic field, to the properties of interstellar gas and the isotopic composition of cosmic-ray nuclei. Most of these seemingly diverse phenomena co-exist in the heliosphere, and a major contribution of the Ulysses observations is the very specific constraints they place on our physical description of this environment. In this article, we summarise the key findings of the mission to date, and discuss the exciting results to be expected from the second phase of Ulysses' out-of-ecliptic journey. The novel operational challenges of the mission and the lessons learned to date are discussed in the companion article in this issue by A. McGarry and N. Angold.
The Sun's expanding outer atmosphere, the solar wind, carves out a bubble in the surrounding interstellar medium which we call the heliosphere. The pole-to-pole exploration of the heliosphere by the Ulysses mission, an international collaboration between ESA and NASA, has been a spectacular success, not only for its scientific discoveries and technological achievements, but also for the excellent collaborative spirit within the project and science teams.
Launched by the Space Shuttle Discovery in October 1990, Ulysses' primary objective was to characterise, for the first time, the properties of the Sun's environment at all latitudes from the equator to the poles, and at distances ranging from 1 to 5.4 astronomical units (1 astronomical unit (AU) is the Sun-Earth distance, 150 million km). The scientific investigations address a wide range of heliospheric phenomena, including the solar wind, the heliospheric magnetic field, energetic particles and cosmic rays, natural radio emissions, interstellar gas and dust, and gamma-ray bursts.
The timing of the launch, and the requirement to use Jupiter's gravitational field to catapult the spacecraft into its final solar-polar orbit, resulted in the high-latitude part of the mission taking place under near-quiet solar conditions. Phasing the mission this way was the preferred option scientifically, although as a result of several lengthy delays, the actual launch date was ultimately determined by circumstance rather than design.
In the seven years since launch, Ulysses has provided an unprecedented perspective of the heliosphere around solar minimum. As noted above, the quality of the results has been enhanced by the excellent international cooperation between NASA and ESA and by the involvement of investigators from the larger space science community. The Ulysses observations obtained so far have resolved a broad range of questions in the space sciences, due in large part to the unique orbit of the spacecraft. They have also raised questions unanticipated from our previous knowledge of the heliosphere, and provided a firm base on which to continue our exploration. The purpose of this article is firstly to highlight the major findings of Ulysses' 'solar minimum mission', and secondly to look ahead to the equally exciting opportunities offered by the 'solar maximum mission'.
The scientific accomplishments of Ulysses to date have been reported in more than 700 publications covering a wide range of solar, heliospheric and astrophysical phenomena (see also ESA Bulletins 67, 72 and 82). These accomplishments include the following:
In the following sections, we will describe a number of these findings in more detail.
Global heliospheric structure
One of the main goals of the Ulysses mission was to determine the global structure of the heliosphere, in particular with regard to the distribution of solar-wind plasma, and its 'frozen-in' magnetic field. The latitude survey carried out by Ulysses, the first ever, has resulted in the following picture (Fig. 1) of the heliosphere at solar minimum. The three-dimensional structure is characterised by a basic north-south symmetry, and is dominated by the presence of the fast solar wind from polar and high-latitude regions that expands to occupy a large fraction of the heliospheric volume. The slow wind, shown by Ulysses to be the 'exception' rather than the 'rule' (as was previously thought), is confined to low latitudes. Observations made by Ulysses during its rapid pole-to-pole transit near perihelion have revealed that the transition from slow to fast wind is surprisingly abrupt. This is graphically illustrated in Figure 2, which shows a polar plot of the solar-wind speed measured by Ulysses as a function of heliolatitude.
Figure 1. Schematic showing new aspects of the heliosphere at solar minimum as revealed by Ulysses. Among the phenomena depicted are: (a) fast solar wind filling the bulk of the heliosphere, separated from the near-equatorial slow wind by a relatively sharp boundary; (b) interstellar neutral gas, the source of pick-up ions which in turn form the anomalous cosmic-ray component (ACR); (c) co-rotating interaction regions (CIR), giving rise to periodic variations in the intensity of energetic particles and cosmic rays up to high latitudes; (d) gamma-ray bursts; (e) the heliospheric magnetic field, with enhanced fluctuations at high latitudes scattering incoming cosmic rays, and field lines connecting high and low latitudes; (f) interstellar dust grains
Figure 2. Polar plot of solar wind speed, as measured by the SWOOPS experiment on board Ulysses, versus latitude. Time runs clockwise starting in the lower right-hand quadrant. Shaded areas represent the region of the heliosphere dominated by slow solar wind (yellow), and the region accessible to space probes in the ecliptic (green)
Note that nearly all other space missions have been confined to the narrow region of the heliosphere at low latitudes dominated by slow wind, and that Ulysses has provided the first direct, detailed view of the 'true' solar wind flowing from the polar coronal holes. This fast-flowing solar wind has been found by Ulysses to be relatively constant near solar minimum, with a speed of approximately 750 km/sec. On the other hand, while clearly much less variable than the low-latitude slow wind, closer inspection has shown that even the fast wind is far from quiescent. New observations from the SOHO spacecraft present a picture of the solar atmosphere that, even at solar minimum, is highly dynamic and clearly reflected in the Ulysses solar-wind data. For example, Ulysses discovered so-called 'microstreams', which may be related to the polar plumes observed to emanate from polar coronal holes.
The Ulysses magnetic-field measurements indicate that the radial field component, and consequently the magnetic flux, are independent of latitude. This has led to the unanticipated conclusion that the magnetic-field pressure controls the solar-wind flow near the Sun, driving a non-radial expansion by more than a factor of 5 from the polar coronal holes. Another characteristic property of the fast high-latitude wind that has been revealed by Ulysses is the continual presence of large-amplitude transverse waves in the magnetic field. The wave amplitudes typically equal or exceed the magnitude of the background field and, as such, are an example of strong turbulence. The waves are observed over a wide range of periods; the longer period variations probably originating in the 'random walk' of field lines at the Sun, which in turn is a signature of the non-quiescent solar surface.
Although the spiral magnetic-field structure predicted by the earliest models of the solar wind is preserved to the highest latitudes, a significant departure from this so-called 'Parker' field configuration has been discovered. A likely explanation is that the observed deviations are the result of the large-amplitude waves referred to above.
Ulysses observations of solar radio bursts, which are relatively rare at solar minimum, provide a capability for remote-sensing of large-scale magnetic-field structures. As the electrons responsible for the radio emission stream out along the magnetic field lines, they provide an 'image' of the field lines in the inner heliosphere.
In particular, the high-latitude observations of these radio bursts from Ulysses alone provide the 2-D image shown in Figure 3. Significant distortions of the field lines - for example, by a transient structure in the solar wind - can be readily detected in the trajectory of the radio burst.
Figure 3.'Snapshot' of the spiral shape of the interplanetary magnetic field, as obtained from radio observations from Ulysses. This view from the north ecliptic pole is based on Ulysses radio measurements made as it was passing over the Sun's south pole. The white symbols represent the actual observations of the location of outward-moving streams of electrons, ejected from the Sun on 25 and 30 October 1994. The numbers indicate the frequency of radio emission, so that '940' represents emission at a radio frequency of 940 kHz. A yellow arrow points out the location of the Sun where a solar flare on 25 October 1994 ejected the electrons tracked by Ulysses on that date. The spiral blue lines illustrate the shape of the magnetic field as predicted from theory for a constant solar wind speed (Courtesy of NASA/GSFC)
The north-south symmetry discussed earlier is associated with the heliospheric current sheet (HCS) that separates oppositely directed magnetic fields in the two hemispheres and defines the Sun's magnetic equator. The corresponding magnetic axis passes through the Sun's polar caps; it is the tilt of this axis relative to the Sun's rotation axis which causes alternating slow (low-latitude) and fast (high-latitude) solar-wind streams to sweep over an observer in the ecliptic. Because of the radial outflow of the solar wind, the fast streams eventually run into the slower wind ahead, forming so-called 'co-rotating interaction regions' (CIRs). These are regions of compressed solar-wind plasma, which, as the name suggests, co-rotate with the Sun. At distances beyond 1 AU, CIRs are often bounded by forward and reverse shock waves. As will become apparent, the investigation of CIRs themselves, and their influence on the energetic particles and cosmic rays that populate the heliosphere, forms a major theme in the scientific output of Ulysses.
Energetic particles and cosmic rays
Prior to Ulysses, it was known that the shocks associated with CIRs are able to accelerate low-energy charged particles. Characteristic increases in particle intensity are frequently observed when a CIR sweeps past a spacecraft once per solar rotation. At much higher energies, the same CIR can act as a temporary shield against incoming cosmic rays, causing the observed intensity to decrease rather abruptly, then slowly recover.
Surprisingly, Ulysses discovered that the recurrent effects in energetic-particle and cosmic-ray intensity extend to much higher latitudes than the CIRs themselves. Under the influence of the magnetic configuration of the Sun, the angle between the Sun's rotational and magnetic axes changes with the solar cycle. Near solar minimum, the magnetic and rotational axes are nearly aligned, so that the CIRs are restricted to relatively low heliographic latitudes (typically less than 30 degrees).
As shown in Figure 4, the series of recurrent increases and decreases starting at low latitudes clearly extends to regions far beyond the latitude range of the CIRs or their associated shocks. The Ulysses observations could be explained more easily than previously thought if the particles are able to move across the mean magnetic field to higher latitudes. An alternative explanation is that the differential rotation of photospheric foot points of heliospheric magnetic field lines interacts with the non-radial near-Sun expansion of the solar wind to produce field lines that deviate drastically from the traditional constant-latitude spirals (see Fig. 5). The result is to bring field lines from high latitudes near the Sun to low latitudes at 15 AU or more from the Sun, thereby connecting Ulysses to the acceleration region of the energetic particles. This remarkably different magnetic topology is not detectable near the ecliptic plane; it could only be discovered by observations at high heliolatitudes. If confirmed by further work, this theory, based exclusively on Ulysses data, will revolutionise our understanding of the heliospheric magnetic field and cosmic-ray transport.
Figure 4. Cosmic-ray and energetic-particle measurements made by the COSPIN experiment on Ulysses showing the recurrent pattern (26-day period) of low-energy increases and high-energy decreases in intensity persisting up to the highest latitudes. The corresponding solar-wind data show periodic effects only up to latitudes of ca. 35 degrees
Figure 5. Ulysses magnetic-field measurements have caused theorists to revise their models of the heliospheric magnetic field. Shown here is the high- latitude field configuration proposed by L. Fisk (a), compared with the traditional Archimedes spiral model (b) due originally to E. Parker
A major goal of Ulysses was to investigate the physics of propagation of energetic particles in the heliosphere. In particular, the question was posed as to whether or not cosmic rays would have easier access to the inner heliosphere along relatively straight magnetic field lines expected over the poles of the Sun. Ulysses showed that the cosmic-ray intensity increased by less than a factor of two from the equator to the poles, implying nearly equal difficulty of access. A possible explanation is found in the Ulysses discovery that large-amplitude waves in the magnetic field, which obstruct cosmic-ray propagation, are a characteristic feature of the high-latitude fast solar wind.
Interstellar gas and pickup ions
Ulysses has, for the first time, observed a variety of atoms entering the heliosphere from interstellar space. With a new technique to directly detect low-energy (>30 eV) atomic helium, the local angular distribution of He was measured and, from these observations, the velocity vector and kinetic temperature of the interstellar neutral helium at the boundary of the heliosphere have been determined with unprecedented precision. The velocity vector describes the motion of the Solar System through the surrounding Local Interstellar Cloud (LIC). Furthermore, from the neutral and pickup helium (see below) observed locally with Ulysses, it was possible for the first time to infer the neutral helium density in the LIC (0.0155 per cm3).
Ulysses discovered a vast heliospheric population of ions (so-called 'pickup' ions) produced from interstellar atoms and dust grains by photo-ionisation and charge exchange with the solar wind. Measurements of these pickup ions (H, He, 3He, 4He, C, N, O and Ne) on Ulysses have led to important new results, including the following:
The dust detector on the Ulysses spacecraft is the first to directly detect dust at high ecliptic latitudes. The primary pre-flight objective was to obtain new information on the latitude and radial distribution of interplanetary meteoroids. This objective included separating the cometary and asteroidal populations from each other as well as measuring the beta-meteoroid population (meteoroids that are leaving the Solar System) which presumably originate via collisions from these two populations.
Other important objectives were to measure the flux of interstellar grains (if they existed in the Solar System), to measure dust in the Jovian magnetosphere as Ulysses flew by Jupiter, and to look for previously unknown dust populations. All of these pre-flight objectives have been accomplished, with the exception of additional work to better determine the asteroidal versus cometary contributions to the zodiacal cloud. The Ulysses data, of course, suggest new problems to solve that were not anticipated before launch.
The dust experiment has discovered a flux of interstellar grains passing through the Solar System. The detections occur at the rate of about one detection every 3 or 4 days. Streams, or bursts, of dust were also discovered to be emanating from the Jovian system. An analysis of all Ulysses data has identified a total of 11 such streams, and revealed that the dust grains are electrically charged and their trajectories are bent by the solar-wind magnetic field.
The study of the Sun and its environment by Ulysses has an obvious importance to stellar astrophysics, providing the only possible detailed analysis of the interaction of a typical star with its surroundings. Ulysses has also made important contributions to astrophysical studies that reach far beyond the heliosphere.
For example, the gamma-ray burst (GRB) experiment provides a distant point in space to obtain arcminute positions for cosmic gamma-ray bursts and to observe solar-flare X-radiation stereoscopically. It has become the flagship in the 3rd Interplanetary Network of gamma-ray burst detectors. This network includes, or has included, Pioneer Venus Orbiter, the Russian GRANAT spacecraft, WIND, Yohkoh, the Compton Gamma-Ray Observatory, and the Italian SAX mission, among others. The synergy which exists among these missions, as well as the German ROSAT and Japanese ASCA X-ray observatories, has permitted identification of a possible quiescent X-ray counterpart to a gamma-ray burst source and determination of the position of a recent GRB to 0.76 square arcminutes. The latter observation has led to the first possible detection of an optical counterpart to a gamma-ray burst source.
Ulysses has also carried out cosmic-ray studies significant for astrophysics. So far, the measurements have provided isotopic abundance measurements of unequalled precision for many of the more abundant elements in the cosmic radiation, including the first fully-resolved, good-statistics measure-ments of the isotopes of iron and nickel which have already eliminated some models for heavy-element nucleosynthesis and for the origin of cosmic rays. However, the quality of the measurements for many species is still limited by statistics; continued collection of events is required not only for the refinement of the isotopic composition of the more abundant elements already measured, but also to permit measurements of less-abundant elements such as Cl and Ar which have important implications for nucleosynthesis and propagation theory.
The Ulysses measurements of the ratio of helium isotopes (3He/4He) in the local interstellar gas, the first of their kind, have made it possible to compare present-day light-element abundances in the Local Interstellar Cloud with their values at the time of the formation of the Solar System. The abundances were found to have remained essentially unchanged, placing new constraints on models of galactic chemical evolution. These data have also led to important refinements in another fundamental cosmological parameter, providing information on conditions that prevailed in the Big Bang.
The mission of Ulysses is to explore and define the heliosphere in three dimensions. For the solar-minimum heliosphere, as detailed above, this mission has been accomplished, changing our view of the heliosphere and stimulating a wide variety of theoretical and modelling efforts. Since the heliosphere is a dynamic structure which undergoes large variations in both large- and fine-scale structure over the period of a solar magnetic cycle (22 years), it is critical that observations also be obtained near solar maximum. This will be accomplished during Ulysses' second solar orbit (Fig. 6), the so-called 'Ulysses Solar Maximum Mission'. It is certain that new insights will be gained as we continue to observe the effects of increasing solar activity, changing coronal structure and, ultimately, the reversal of the solar magnetic polarity from the vantage point of Ulysses at high latitude in the coming solar maximum.
Figure 6. The Ulysses orbit, showing the second set of polar passes in 2000/2001. The spacecraft is currently close to aphelion at a distance of 5.3 AU from the Sun
In addition to heliospheric studies, Ulysses' role in astrophysical investigations will continue to yield rewards during the Solar Maximum Mission. Ulysses' discoveries of matter from the local interstellar cloud and identification of vast regions of neutral matter in the inner heliosphere are giving us a new view of the phenomena and conditions beyond our Solar System; a view which will become much better defined through continuing observations. The determination of accurate locations for gamma-ray bursts will continue through the second orbit, and the continued collection and identification of rare nuclear species from the galactic cosmic radiation will increase the accuracy with which we know the composition of the only sample of matter available for analysis that has probed conditions in large regions of space outside the Solar System in the Galaxy.
Many of the topics of the Ulysses Solar Maximum Mission can only be addressed fully when Ulysses revisits the polar regions in 2000-2001. Nonetheless, observations in the intervening period, i.e. 1998-99, are critical to placing the later observations in context. Furthermore, we do not know when many of the processes related to the onset of solar maximum will begin to appear at the location of Ulysses.
Key topics for the next phase of the Ulysses Mission include:
Achieving these goals will involve collaborations with many other spacecraft and ground-based projects, but the unique high-latitude perspective of Ulysses and its integrated instrument payload are invaluable assets.
For most of the questions described above, the Ulysses Solar Maximum Mission represents the only opportunity in our lifetimes to provide robust answers. The capability to contrast the observations from solar minimum and solar maximum is crucial to placing the data in perspective. It is certain that the additional data obtained during the rise to and at solar maximum, in combination with the earlier Ulysses data, will more than double our understanding of the three-dimensional heliosphere. The goals relating to the interstellar medium, as well as the astrophysical objectives, also represent opportunities that may never be repeated.