ESA Science & Technology11-Jul-2005 15:01:04

Science Results

The double cusp

Tango, Rumba and Samba detect the polar cusp with the CIS ion instrument

Recently analysed data from three Cluster Ion Spectrometry (CIS) experiments on different spacecraft have revealed remarkable changes over time and space as the satellites slowly sail over the Earth's north polar region.

During their flight over the Earth's dayside, the CIS instruments have recorded dramatic variations in the energy levels and population of positively charged particles (protons) arriving from the Sun. Over lower latitudes, outside the cusp, CIS typically measures large numbers of protons with very high energies - more than 30 000 electron-volts (eV). However, as Cluster continues its northward journey towards the pole and enters the cusp, the proton population is characterised by lower energy, typically 0 to 5000 eV.

On 30 August 2001, the Cluster spacecraft were arranged like a string of pearls, each one following a very similar orbit. The Rumba (Cluster 1) and Tango (Cluster 4) spacecraft entered the northern cusp within one minute of each other, at around 15:30 UT (16:30 CET). The CIS experiments on board both satellites recorded the expected decrease in protons as they traversed the cusp. Since the Samba (Cluster 3) spacecraft was lagging some 6000 kilometres behind the others, its instruments did not register cusp entry until three quarters of an hour later at 16:17 UT (17:17 CET).

At first, the CIS instrument on Samba showed the typical decrease in protons, but this trend was suddenly reversed 28 minutes later at 16:45 UT (17:45 CET), when there was a sudden jump in the number and energy levels of the protons recorded. After a brief plateau, the proton levels then dropped again. The Cluster scientists describe this type of feature as a double cusp. with the support of ground-based observatories and modeling, this observation is explained by a rapid motion of the polar cusp toward the North pole, induced by the turning of the interplanetary magnetic field from South during Rumba and Tango crossings to North during Samba crossing.

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