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Ulysses Status Report - February 2005

Status Report From: European Space Agency
Posted: Tuesday, March 15, 2005

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Mission Status

The spacecraft and its scientific payload are in good health and no anomalies have occurred during the reporting period. Nevertheless, because of the large distance from the Sun and the decreasing radioisotope thermoelectric generator (RTG) output, the general spacecraft thermal environment is currently well below the lower limits originally foreseen for the mission.

The most critical item in this respect is the hydrazine fuel of the Reaction Control Subsystem, which must be prevented from freezing if at all possible. This places additional constraints on spacecraft operations, limiting the flexibility in assigning measurement opportunities to those experiments that are not part of the core payload. On a personal note, the Ulysses Mission Operations Manager (P. Beech) retired at the end of 2004, after almost 20 years with the project.

On 1 February, Ulysses will be at a radial distance of 5.3 AU from the Sun, and heliographic latitude 17.5° south of the solar equator.

Operations and Archive

All science operations during the reporting period have been nominal. Payload operations are being conducted according to the pre-determined power-sharing plan. Data return has remained excellent (97% on average over the course of the mission, with an average of 98.2% during the last 5 years). The ESA Ulysses archive is accessible via the World Wide Web at URL: http://helio.esa.int/ulysses.

Science Highlights

The unique perspective offered by Ulysses' orbit naturally lends itself to multi-spacecraft studies of transient solar wind features. Good examples of such studies are the so-called SOHO-Ulysses "quadrature" campaigns conducted when Ulysses is located off the limbs of the Sun as seen from Earth. Remote-sensing observations from SOHO can then be used to track disturbances leaving the Sun in the direction of Ulysses, and the in-situ measurements at Ulysses reveal the evolution of these structures as they travel outwards in the heliosphere. A prerequisite is the ability to identify the same parcel of plasma at both locations.

The quadrature campaign in November 2002, at which time Ulysses was at 4.3 AU and 27° N off the west limb of the Sun, was particularly successful in this regard. For the first time, it was possible to identify the same very hot plasma remotely at the Sun with SOHO and in situ at Ulysses. Four large coronal mass ejections (CMEs) were observed by SOHO leaving the Sun in the general direction of Ulysses over a period of several days at the end of November. By the time they reached Ulysses some 15 days later, the interplanetary counterparts of these CMEs (ICMEs) had apparently merged to form a single large solar wind structure that drove a strong interplanetary shock. The plasma of this merged structure contained unusually large enhancements in highly ionised iron ions (charge state Fe16+), indicating a high-temperature source. Such high charge states are often seen in the solar wind and have been identified with ICMEs.

The data from SOHO/UVCS also showed high Fe charge states, in particular in the aftermath of the 26 November CME. In this case, it was very hot plasma at 6-10 million degrees Kelvin that was apparently produced high in the solar atmosphere, above 1.5 solar radii. The most likely source of such hot plasma was a reconnection event occurring in post-flare loops.

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