Reflecting on the Findings of the Ulysses Spacecraft

More than 100 years ago it was realized that storms in the Earth's magnetic field often followed about 3 days after disturbances seen on the Sun. It was suggested [Chapman and Ferraro, 1931] that the Sun occasionally emitted bursts of charged particles that took a few days to reach Earth and trigger the geomagnetic storms. Bierman [1951] noted that the plasma tails of comets were at all times pushed in the direction of the Sun-comet line, and he argued that this indicated a continuous, rather than occasional, outward radial flow from the Sun. Such a flow was not consistent with models of a solar atmosphere, the corona, in hydrostatic equilibrium. Parker [1958] predicted that indeed the corona could expand continuously, giving rise to a supersonic solar wind. Gringauz [1961] and coworkers in the Soviet Union and Neugebauer [1962] and coworkers on the U.S. Mariner 2 spacecraft established the existence of a continuously present solar wind in the ecliptic plane.

The superconducting solar wind carries with it from the Sun a frozen-in magnetic field. Changes in the solar wind pressure and related magnetic disturbances impinge on the Earth's magnetosphere, giving rise to changes that propagate down to the ionosphere and atmosphere. Such changes can be large enough to cause severe radio communication problems and even to trigger blackouts in high-latitude power distribution grids. Understandably then, the solar wind - in which the Earth is immersed at all times - is of considerable significance to our increasingly technology-dependent society.

Before Ulysses, direct study of the solar atmosphere had been possible only in the Sun's equatorial regions. The joint ESA/NASA Ulysses spacecraft, launched in October 1990, used the gravity of Jupiter to pull itself into an orbital plane perpendicular to the ecliptic. It reached its highest southern solar latitude (80.2°) in September 1994 and will pass by the north pole in mid-1995 (Figure 1).

Fig. 1. The Ulysses trajectory.

Most of the instruments carried by Ulysses study the solar wind, the magnetic field carried out from the Sun by the solar wind, and the arrival of cosmic radiation, which is obstructed or modulated by the outflowing solar wind [Wenzel et al., 1992].

Solar Wind

Ulysses encountered the well-known behavior of gusty in-ecliptic solar wind flow until 13°S was reached in mid-1992. It then began to experience a remarkable 26-day periodic pattern that persisted for about a year until a latitude of 35°S was passed (Figure 2) [Bame et al., 1993]. Each time the Sun rotated, bringing the wavy heliospheric current sheet (HCS) close to the spacecraft, wind velocities around 400 km/s were recorded. Thirteen days later, halfway through a solar rotation, the spacecraft found itself immersed in a 750 km/s flow, presumably from a large coronal hole encompassing south solar polar regions. After August 1993 and 35°S, Ulysses entered a region of continuous, relatively smooth flow that departed little from 750 km/s [Phillips et al. 1994]. This flow regime persisted all the way to the highest latitude achieved (80.2°). At 40° latitude the flow regime is unchanged as Ulysses heads back toward the equatorial regions. It should be remembered that Ulysses has found these variations with latitude at a time close to solar minimum. At solar maximum the picture is expected to be very different.

Fig. 2. Upper Panel: Solar wind velocity. In early 1992, the wavy current sheet was crossing the spacecraft every 26 days as the Sun rotated. In May of 1993, Ulysses had reached 30 degrees latitude and found itself clear of the current sheet. From this time on, it experienced only high-velocity solar wind and magnetic fields in one direction. Lower Panel: fraction of the time each 26 days during which positive magnetic field was encountered. Positive (outward) fields originate in the northern solar hemisphere at the 1992-1993 phase of the solar cycle.

As the wind velocity V increased, its density decreased, so that if short-term fluctuations are ignored, the product V remained roughly constant throughout the complete latitude range.

It had been suspected for some time that there are distinct types of solar wind. The difference between the slow in-ecliptic wind and the fast coronal hole wind is evident in the charge state of solar wind ions. In the in-ecliptic wind, ions such as magnesium, with low first ionization potential (FIP), are favored over ions such as oxygen with high FIPs. On the other hand, in the fast wind arriving directly from coronal holes, both low and high FIP ions are present in a proportion closer to that believed to exist down in their birthplace in the solar photosphere [Geiss et al., 1994].

Magnetic Field

In its journey south, Ulysses encountered field directions typical of the northern heliosphere more than 50% of the time until 13°S was reached. This is the latitude at which the 26-day solar wind variations began (Figure 2). During subsequent solar rotations, less and less northern heliosphere field was seen until 35°S was reached, where the solar wind 26-day oscillations ended. The field encountered there was of one polarity only and was directed toward the Sun [Smith et al., 1993].

Zeeman splitting of solar spectroscopic lines, and the variations measured as the Sun directs its poles 7° toward and away from the Earth-Sun line in the course of a year, enable Earth-based observers to determine that the Sun has a dipole field like that of a bar magnet at its surface. A dipole field is entirely radial at its poles and is also strongest at its poles. Ulysses has found no signature of a dipole field over the south solar pole, which lies at radial distances around 2.2 AU. The radial field strength, rather than increasing as field lines are concentrated within a funnel over the pole, remains uniform as a function of latitude. It appears that field line transverse magnetic pressure is transmitted by means of solar wind plasma to establish an equilibrium condition. In this condition volumes with higher magnetic pressure cannot continue to exist.

It has also been discovered that at high solar latitudes, transverse magnetic (Alfvén) waves are continuously present. These have wavelengths around 0.25 AU and velocities around 50 km/s and are only sometimes evident in the ecliptic plane. They may play a significant role in modulating cosmic rays and may be related to the heating required to generate the solar wind.

The interplanetary field carried out from the Sun by the solar wind is wound into an Archimedian spiral by the rotation of the Sun. Up to 45° latitude the spiral, or azimuthal, angle of the interplanetary field agreed with Parker theory predictions and a solar sidereal rotation period of 25.4 days. However, at high latitudes, the average field direction deviates from that predicted by Parker theory by 10-20°. The average field is more radial - less tightly wound - than predicted. The large amplitude Alfvén waves mentioned above are believed to affect the average field direction.

Cosmic Rays

In the present solar minimum phase of the solar cycle Ulysses has found only a small increase in cosmic ray intensity at 80° S and essentially no increase up to latitudes of 60°. It has to be concluded that the process by which the solar system modulates the cosmic ray arrival is much more spherically symmetrical than commonly believed [McKibben et al., 1994]. Alfvén waves at high latitudes may make it more difficult for the cosmic rays to enter.

Ulysses has seen a 26-day modulation of the cosmic radiation in phase with the 26-day variations in solar wind speed (Figure 2). A remarkable feature is that this periodic modulation continued beyond 70° S and long after the solar wind variation was last seen by Ulysses at 35° S [Kunow et al., 1994]. The effect, evident up to very high energies (>>1 GeV), was unexpected and awaits explanation. About 20 years ago, anomalous cosmic rays were detected for the first time. The anomaly was that a helium component appeared at energies that were too low to penetrate heliospheric magnetic fields as deduced from cosmic ray proton measurements. The observation could be explained, however, if it was assumed that the anomalous helium was singly charged and thus more able to penetrate magnetic barriers than the normal doubly charged helium in the cosmic radiation. It was suggested that neutral particles from interstellar space penetrated the solar system without opposition, became only singly ionized as they approached the Sun, were picked up by the magnetic field in the solar wind, and were accelerated to cosmic ray energies perhaps near the boundary of the heliosphere.

Ulysses has achieved the first ever direct detection of neutrals arriving from interstellar space [Witte et al., 1993] and has recorded too, pick-up and acceleration of singly ionized atoms in the solar wind, [Gloecker et al., 1994] thus confirming the theory of anomalous cosmic ray production.

Ulysses has been very successful in exploring high southern latitudes in the solar atmosphere. Some predictions have been confirmed and some surprises have been encountered. All of the instruments continue to enjoy good health and the north polar passage in mid-1995 is eagerly awaited. So too is the second 6-year orbit, which will bring the spacecraft back over the solar poles at a time of maximum solar activity, when the solar atmosphere is expected to be very different from its present configuration.

References

Witte, M., H. Rosenbauer, M. Banaszkiewicz and H. Fahr, The Ulysses neutral gas experiment: Determination of the velocity and temperature of the interstellar neutral helium, Adv. Space Res., 13, 121, 1993.

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