Nearly one hundred cases of such multiple high-speed ion beams were found over three years of data collected by the satellites of the European Space Agency Cluster mission. Altogether these observations fall in agreement with predictions of a theory published 14 years ago, that has recently been revisited.

From the Sun to the nightside of the Earth
The Earth's magnetic field deflects most of the solar wind, a continuous flow of plasma expelled by the Sun. However, there are two regions located around the magnetic poles, known as polar cusps, where solar particles can precipitate along magnetic field lines towards the Earth magnetic poles.

At each pole, a significant fraction of these particles bounces back along the magnetic field lines and populates the plasma mantle, a plasma layer located on the night side of the Earth, inside the magnetosphere along its boundary. Under the action of electromagnetic forces, plasma contained in the mantle drifts equatorward, across the tail lobe and the plasma sheet boundary layer (PSBL) towards the current sheet region located along the tail axis.

There, according to a model developed in the late 80's/early 90's, there exist a few localized regions in the current sheet called "resonant locations" where ions acquire kinetic energy along the magnetic field. The amount of energy acquired depends on how far in the tail the resonance location lies. Such accelerated ions are beamed back to Earth, from the magnetic equator, into the PSBL, towards the poles, in a kind of elongated parabolic flight along terrestrial magnetic field lines. Satellite observations of these ion beams inside the PSBL allow predictions of this model to be tested.

High speed ion beams towards the Earth
On 1 September 2003, the four Cluster satellites were flying in close formation in the magnetotail when they entered the southern PSBL. As they crossed it, two distinct beams of protons were detected streaming Earthward along the magnetic field with velocities of V//1~1000kms-1 and V//2~2400kms-1 respectively. These ion beams were simultaneously observed during a period of around three minutes by three Cluster satellites. These beams correspond to a characteristic double-peak signature in the data collected by the Cluster Ion Spectrometry instrument onboard each satellite.

Due to the way plasma is transported in the magnetotail, the same acceleration source cannot be responsible for the two beams. Their simultaneous observation in the PSBL is thus a strong indicator of, at least, two independent sources in the magnetotail. What was observed on 1 September 2003 is not a unique case. More than ninety "double-peak" events were found over three years of Cluster data, during quiet and moderately disturbed geomagnetic periods and covering a large range of velocities.

A 14 years old theory backed by in-situ data
A theory published in 1993 by Professor Maha Ashour-Abdalla (UCLA, Los Angeles, USA), Dr. Berchem (UCLA), Dr. Buchner (Max Planck Institute, Berlin, Germany) and Prof. Zelenyi (IKI, Russian Academy of Sciences, Moscow), recently revisited (Zelenyi et al. 2006), predicted a universal scaling law for the velocity ratio of ion beams accelerated at distinct spatial sites in the magnetotail. The unique data set collected by the Cluster satellites allowed Professor Lev Zelenyi and co-workers to statistically check this scaling law.

"Each time, the ratio of velocities (V//1/V//2) of the two ion beams measured fell in agreement with the universal scaling law predicted by Prof. Ashour-Abdalla and co-workers in 1993," wrote Zelenyi in a study published in Geophysical Research Letters in 2006.

A recent study published in the Journal of Geophysical Research in May 2007 focuses on the spatio-temporal characteristics of these ion beams or beamlets to estimate their typical duration and size in the direction perpendicular to the equatorial plane (Z-axis). Previous studies based on single satellite missions reported typical beamlet duration ~1-2 minutes and no scale size could be inferred.

"Our statistical studies based on simultaneous observations by three satellites of the Cluster mission show for the first time that the typical beamlet duration is ~5-15 minutes and its typical spatial extent in the Z-direction is ~1300-4500 km. Cluster observations have led to a new understanding of this phenomenon," says Dr. Elena Grigorenko (IKI, Russia) lead author of this study.

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