Following its successful launch on 29 December 2003 and the commissioning of the spacecraft systems and the instruments, the first Double Star spacecraft (TC-1) was declared ready to start the nominal operation phase by the Commissioning Review Board.

The Board, consisting of members from the Chinese National Space Agency (CNSA) and ESA, reviewed the performance of the spacecraft, instruments and ground segment of the Double Star mission during a meeting held at Villafranca (Spain) on 11 March 2004.

Spacecraft and Instrument Status
The spacecraft and instruments are working nominally and return data as expected in response to Observational Requests.

Ground Segment
The ground segment, which includes the Chinese stations in Beijing and Shanghai and the ESA ground station at Villafranca, has been tested extensively and the downlink performances are better than expected.

The three operation centres, the European Payload Operation System at RAL (UK), the Chinese Double Star Science Operation Centre and Double Star Operation and Management Centre are routinely operating both European and Chinese instruments.

The European Data Disposition System at IWF (Austria) forwards raw data to the European users. Finally the Austrian, Chinese, French and United-Kingdom data centres have successfully exchanged and distributed preliminary science data.

Orbital Information
Double Star currently has its apogee in the solar wind. It typically crosses the magnetosphere, the magnetosheath (shocked solar wind) and the solar wind during a full orbit lasting 27 h.

The effect of ASPOC can clearly be observed in the electron spectrograms (bottom panel) in the diagram below. When ASPOC is activated, between 19:00-00:00 UT and 14:00-19:00 UT, the PEACE sensor measures the full electron spectrum from the surrounding medium, while when ASPOC is off, 01:00-13:00, photoelectrons coming from the spacecraft dominate at low energy (below 10 eV, large red band at the bottom of the plot).

Science Results
A very interesting event observed recently was a crossing of the bow shock observed on 25 February by the HIA instrument (ion sensor) and the STAFF instrument (magnetic fluctuations sensor).

Due to its unique orbit, with an apogee around 13.5 Earth radii of geocentric distance, TC-1 at times skims along the bow shock. TC-1 therefore has a better chance to observe the bowshock for relatively long intervals than other spacecraft that have visited this region.

On Figure 3, before 07:05 UT the spacecraft was in the magnetosheath, then between 07:05 and 07:13, the spacecraft was very close to the bow shock, and later between 07:13 and 07:17 it was in the solar wind. During this event, the ions show a clear energy increase with time, observed around 07:12 UT.

These ions from the solar wind are thought to have been reflected at the bow shock so that they now partly move back toward the Sun and are accelerated by the shock in the process.

This very clear energy dispersion should help us to find their origin and understand better the role of the bow shock in accelerating particles to high energies.

The strong burst of low frequency waves that are simultaneously observed by the STAFF experiment might play a role in this process. Such new knowledge will have wide significance since similar processes are thought to energise particles in many astrophysical environments.

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