Ulysses: the mission over the poles of the Sun

R.G. Marsden & K.-P. Wenzel

Ulysses homepage http://helio.estec.esa.nl/ulysses/

Ulysses is an exploratory mission carried out jointly by ESA and NASA to study the properties of the interplanetary medium and solar wind in the inner heliosphere as a function of heliographic latitude. The European-built Ulysses spacecraft was launched by the Space Shuttle on 6 October 1990; a Jupiter gravity-assist manoeuvre, executed in February 1992, deflected the probe into its final high inclination heliocentric orbit. Major milestones in the mission were reached in September 1994 and July/August 1995 when Ulysses passed over the south and north polar regions of the Sun, respectively.

During the high latitude phase of its prime mission, Ulysses spent a total of 235 days at solar latitudes greater than 70°, enabling detailed measurements of the solar wind and related phenomena to be carried out in this uncharted region. At the highest latitude reached (80.2° south and north of the Sun's equator), Ulysses was 2.3 AU (south) and 2.0 AU (north) from the Sun (1 AU = 150 million km). In October 1995, the highly successful mission entered a new phase, embarking on the so- called Second Solar Orbit, which will bring Ulysses back over the Sun's polar regions in 2000/2001. In contrast to the polar passes of the prime mission, which occurred near solar minimum, the high latitude segments of the Second Solar Orbit will be characterised by much more active conditions.

Spacecraft operations, conducted by the joint ESA-NASA Mission Operations Team at JPL, have for the most part proceeded smoothly during the period covered by this report. One of the very few problems of any significance to perturb scientific operations has been the predicted occurrence of the nutation-like disturbance that affects Ulysses at certain times. Two such episodes, totalling almost a year, had to be dealt with. Although highly effective, the operational measures developed to control the nutation stretched NASA and ESA ground facilities used for the task to the limit, as well as the operational staff at JPL and the ground stations. It is to the credit of the Operations Team that the level of data coverage has consistently exceeded 95% for the majority of the mission to date.

The general picture to have emerged from the Ulysses data regarding the global, 3-D structure of the solar wind at, or near, solar minimum is one of the high latitude solar wind originating from the polar coronal holes being characterised by a fast, uniform flow, with an average speed of 750 km/s. Slow to medium speed wind streams originating in the coronal streamer belt are confined to a relatively narrow range of latitudes on either side of the heliographic equator. This band, in which the majority of solar wind variability is observed, also contains the heliospheric current sheet (HCS) that separates the oppositely directed large-scale magnetic fields originating in the two hemispheres. Although the streamer belt and HCS are approximately aligned with the heliographic equator at solar minimum, they both exhibit significant warping, thereby allowing stream interactions to occur. At the time Ulysses made its rapid pole-to-pole passage in 1994/95, the band of solar wind variability at the position of the spacecraft (1.4 AU) occupied some 43° in latitude, from -22° to +21°. This region was noticeably narrower compared with the situation in 1993, when Ulysses left the slow to medium solar wind at -35° latitude on its way to the south polar regions, consistent with the evolution of the streamer belt towards solar minimum.

Now that Ulysses has completed polar passes in both hemispheres, an obvious question to be answered is: do the data reveal significant north-south asymmetries? In fact, modest north-south asymmetries have been identified in a number of the data sets. For example, the average solar wind speed at latitudes >40° is ~15-25 km/s higher in the north than in the south. The cosmic ray flux observed by Ulysses was higher at a given heliographic latitude in the north than in the south by up to 50%. However, not all data show north-south asymmetries. For example, the magnitude of the radial magnetic field, when corrected for heliocentric distance, is the same in both hemispheres. The origin of the apparent asymmetries and/or symmetries may well be related to the topology of the HCS at the time Ulysses made its fast latitude scan. In particular, the tilt of the HCS, coupled with the different longitudes at which Ulysses entered and exited the streamer belt, could possibly account for the observations.

The ESA Project Scientist provides scientific advice to the operations team on all mission aspects. He provides the liaison with experimenters on scientific matters, is SWT co-chairman and is also a member of the Joint Working Group, which constitutes the main ESA/NASA executive body for this cooperative mission. The non-scientific aspects of the Ulysses mission in the Science Directorate are managed by K.-P. Wenzel. The Project Scientist is represented at JPL by the resident ESA Science Coordinator, D.E. Page.

Figure 3.2.4: Solar wind speed measured by Ulysses' SWOOPS experiment covering the period from Jupiter flyby to October 1996 (courtesy D. J. McComas).

During the reporting period, four Science Working Team (SWT) meetings were held, two in Europe and two at JPL. The Project Scientist participated in a NASA Reconfirmation Review in March 1995. Ulysses was one of several ongoing heliospheric science missions to undergo such a review, whose goal was to reduce the overall cost of science and operations to NASA, while at the same time providing wider opportunities for scientific participation in these missions in the future.

With regard to the latter, one of the recommendations in the Review Board's report (which in the case of Ulysses was very positive) was to establish a Ulysses Guest Investigator Programme. An AO for this programme, which is coordinated by the Project Scientist, was issued by ESA in mid-1996. Nine proposals were submitted and the results of the selection procedure are expected to be announced early in 1997. Approved Guest Investigators will normally be attached to one or more of the existing PI teams.

As in the past, the Project Scientist was involved in the organisation of a number of special sessions at international scientific meetings that focused on Ulysses results and the study of the high latitude heliosphere. Results from Ulysses also featured prominently at two Workshops. The first, in March 1996, was devoted to 'Corotating Interaction Regions (CIRs)', and the International Space Science Institute hosted the second ('3D Modulation of Cosmic Rays') in September 1996.

In addition to numerous individual papers, three coordinated publications have appeared in print during the reporting period: in Science (results from the south polar pass), Geophysical Research Letters (observations from the pole- to-pole transit) and Astronomy & Astrophysics. The last collection of papers, for which the Project Scientist acted as Guest Editor, focuses mainly on the results from the northern polar pass.

Ulysses experiment PIs retain proprietary rights for publication of their data up to 1 year, after which appropriate data sets are submitted to ESA and NASA for archiving. The principal public archives are the recently created ESA Archive for Ulysses data in SSD, which will be the central repository and distribution point in Europe during the mission's operational lifetime, and the National Space Science Data Center (NSSDC) at NASA Goddard Space Flight Center. Flow of data into the public archives began in late 1994 and was greatly expedited by electronic transfers of digital data over the Internet from many Ulysses experiment teams and from the ESA Ulysses Data System. Online data stored at the ESA Archive for Ulysses data can be accessed electronically by Anonymous FTP (anonymous@helio.estec.esa.nl) or through the World Wide Web
(URL http://helio.estec.esa.nl/ulysses/archive/). Ulysses data sets are also stored on CD-ROMs for deep archiving and for distribution upon request.