N° 30–1993: Ulysses breaks latitude record

Ulysses is gathering important new information concerning the Sun and its environment. Its prime mission objective is to carry out the first systematic exploration of the inner part of the heliosphere - the region of space carved out of the interstellar medium by the solar wind - at all latitudes from the solar equator to the poles.

The spacecraft, launched by the space shuttle Discovery on 6 October 1990 in the framework of an ESA-NASA collaborative venture, underwent a gravity assist manoeuvre at Jupiter in February 1992 and is now in a highly inclined solar orbit that will bring it over the south pole of the Sun in September 1994. At that time, Ulysses will establish a new record as it climbs to its maximum latitude of just over 80 degrees.

The spacecraft and its scientific instruments are in excellent condition and the data coverage since launch has been consistently close to 100% thanks to the dedicated efforts of the joint ESA-NASA Mission Operations Team and NASA's Deep Space Network.

Although the most exciting phase of the mission - the study of the Sun's polar regions - will only begin in mid-1994, Ulysses has already produced a wealth of new scientific results. These include :

 

• - The first direct detection of neutral helium atoms arriving from interstellar space.
• - The measurement of micron-sized dust grains arriving from interstellar space.
• - The first measurement of singly-charged H, N, O and Ne ions which entered the heliosphere as interstellar neutral atoms and were then ionised.
• - The highest-resolution measurements to date of the isotopic composition of cosmic ray nuclei (e.g. C, N, O, Ne, Si and Mg).

In addition to the above, the traversal of Jupiter's magnetosphere at the time of the fly-by enabled the Ulysses investigators to acquire new and highly valuable data concerning this very complex and dynamic plasma environment. Among the more exciting results to emerge are the possible entry into the polar cap of Jupiter's magnetosphere near the time of closest approach, and the unexpectedly strong influence of the solar wind deep in the magnetosphere during the outbound passage.

With the Jupiter fly-by safely accomplished, the scientific focus is now directed towards phenomena related to the increasing latitute of the spacecraft. Already, there is strong evidence that Ulysses is now in the domain of the southern polar magnetic field. By a fortunate coincidence, just as Ulysses was reaching 32 degrees South, the magnetic field and plasma analyser scientific teams were seeing the first evidence of the effect of increasing latitude on the magnetised solar wind. The two complementary sets of observations reveal the passage of Ulysses into a magnetic unipolar regime corresponding to latitudes above the sunspot belt, having permanently crossed the boundary separating northern and southern magnetic fields.

Following the flight over the Sun's southern pole, Ulysses' orbit will bring the spaceprobe swinging back towards the equatorial regions, heading for its second high-latitude excursion in mid-1995, this time above the North polar regions.

"By the end of September 1995, Ulysses will have put our knowledge of the Sun and its environment in a completely new perspective", says Dr. Marsden, ESA's Project Scientist for Ulysses.

"Only by studying the way the Sun influences the space around it in a global manner can we hope to understand its influence on our local interplanetary environment". An example of this "local" influence is the disturbance experienced by technical systems in Earth orbit and on the ground (e.g., telecommunications systems) at the time of major solar flares.

Note to the Editors:

Near the Sun's equatorial regions, the interplanetary magnetic field alternately points toward and away from the Sun during a single solar rotation which gives rise to so-called magnetic sectors. The existence of these sectors is interpreted as the effect of a vast current sheet, tilted with respect to the Sun's rotation axis, which separates oppositely-directed magnetic fields from the North and South polar caps. The recent Ulysses magnetic field observations reveal a single polarity, i.e. the disappearance of magnetic sectors. This implies that Ulysses is now above the current sheet. The polarity of the observed fields corresponds to that of the Sun's south polar cap as would be expected.

Concurrent with the magnetic sector disappearance, the Ulysses solar wind plasma instrument, which detects solar wind ions and electrons, shows a persistently fast solar wind flow. High speed solar wind is thought to issue from coronal holes (regions of reduced X-ray brightness and density in the Sun's corona), and it is known that extended coronal holes cover each of the polar caps at this phase of the solar cycle. The Ulysses observations are interpreted as indicating that the spacecraft has now entered the region above the southern polar coronal hole.