Like ships on a never-ending expedition around the world, ESA’s four Cluster spacecraft continue to explore the mysterious magnetic environment that surrounds the Earth – a stormy sea filled with electrified particles instead of water. As the quartet surveys the planet’s poles, they are discovering the secrets of the northern cusp – a funnel-shaped opening in the magnetic field.

The key to Cluster’s new discoveries is the multi-spacecraft observations that enable scientists to study near-Earth space in unprecedented detail. 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.

The particles become slower and less energetic until they barely register any energy levels at all (no more than 10 eV). By the time Cluster leaves the cusp, hardly any protons are recorded.

“This is because the protons from the Sun enter the cusp more easily on its sunward side,” explained Philippe Escoubet, Cluster project scientist.

“The particles slow down and become less energetic over the pole of the planet,” he said. “This is also where particles mainly move away from the Earth and the Sun. They travel down the magnetic tail, so only a few protons enter on the antisunward side of the cusp.”

Sometimes, this general pattern – numerous high energy protons near the equator diminishing to almost nothing beyond the polar cusp – is radically altered. In particular, CIS and other instruments on Cluster have recorded a strange ‘double cusp’ feature.

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. However, Rumba stayed inside the cusp for some one and a half hours, considerably longer than Tango – an indication that the region was highly mobile and rapidly shifting location.

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.

“We don’t yet know why the proton count rises at 16:45 or the cusp is so much larger for Rumba,” said Jean-Michel Bosqued, CIS co-investigator at CESR/CNRS in Toulouse, France. “However, there seems to have been a lot of motion in the magnetic field at that time – it was very dynamic.”

“We can see that the cusp was much wider than normal during the passage of Rumba, but the reason is not clear,” he added. “Either the cusp was really much larger than normal or it was slowly moving in same direction as the satellite.”

What about the double cusp?

“Again, we are not yet sure, but it would seem likely that the funnel-shaped cusp swivelled in space,” he said. “This would mean that the Samba spacecraft entered the cusp in the normal way, then the magnetic field shifted position, causing the spacecraft to take measurements once again near the daytime boundary. It’s as if Samba flew through the cusp twice.”

Further observations of this double cusp phenomenon should help to clarify what is happening in the turbulent, unpredictable ocean of particles in near-Earth space.

For further information please contact:
Dr Jean-Michel Bosqued
CESR/CNRS, Toulouse, France
Tel: +33 56-155-6673

Dr Philippe Escoubet
Cluster project scientist
ESTEC, The Netherlands
Tel: +31 71-565-3454