The commissioning phase of the Cluster mission came a step nearer completion recently when the last three instruments (CIS, RAPID and PEACE) on the Rumba spacecraft were switched on. Once they complete commissioning in early December, engineers will subject the 11 instruments on each spacecraft to an interference campaign in order to check out whether they affect each other's measurements.

An artist's concept the Cluster 2 satellites flying in formation. Photo: Dornier Satellitensysteme GmbH

Even before the full complement of experiments has been brought on line, various instruments on the four spacecraft have begun to collect their first data. Results from PEACE, FGM, STAFF, EFW and the five WEC experiments clearly show that the Cluster quartet made their first crossings of the magnetopause on November 9. This was a surprise for the Cluster science team, since it was about one week earlier than predicted.

The magnetopause is the boundary between the Earth's zone of magnetic influence - the magnetosphere - and the region of interplanetary space that is dominated by the solar wind. Scientists expect the magnetosphere to be mainly populated by a modest number of high energy electrons and protons, while the solar wind contains many more subatomic particles at much lower energies. There is also a sudden shift in the direction of the magnetic field at the magnetopause.

"PEACE detected a high flux (count) of electrons that we believe marked the magnetopause," said Cluster Project Scientist, Philippe Escoubet. "Magnetometer and wave consortium experiment data also observed many changes in the magnetic field, indicating that we crossed the magnetopause many times."

"At that time, the magnetosphere was being hit by a solar storm and it was fluctuating back and forth like a rapidly inflating and deflating balloon," he explained. "The very large compressions of the magnetosphere, particularly between 07:00 and 09:00 UTC (08:00 - 10:00 CET), enabled us to detect the magnetopause earlier than expected."

Electric field data from EFW on two spacecraft also show the motion of the spacecraft in and out of the magnetosphere. At the peak of the solar storm, both spacecraft were exposed to the solar wind for more than 2 hours until the compression of the magnetosphere decreased and the spacecraft moved back inside the Earth's magnetic bubble.

Despite the severity of the solar storm - the fourth most powerful such event recorded since 1976 - there was no long-lasting damage to the Cluster quartet other than a small loss of efficiency in the spacecraft's solar arrays.

"We were affected by a single event upsets in the Solid State Recorder and had to reset the wave instruments on Salsa and Samba," said Philippe Escoubet. "However, the spacecraft and payload remain in good shape."

The Cluster quartet are currently in an approximately tetrahedral formation, about 600 km apart and rotating at 15 rpm. Their orbital separations will be adjusted and refined to create the perfect tetrahedron during several manoeuvres in December and early next year.