Cluster is a four spacecraft-part mission, carrying 11 identical instruments on each spacecraft; it is designed to study the Earth's magnetic field or "magnetosphere", and the plasma environment in the near Earth region. This mission, in orbit since the summer of 2000, allows for the first time, three-dimensional measurements of key regions of space surrounding the Earth.

The eighth biannual Cluster joint workshop between the European Space Agency (ESA) and the National Aeronautics and Space Administration (NASA) was held on the UNH campus in Durham, New Hampshire from, 29 September to 1 October 2004.

This was the first such meeting to be held in the United States for the European-led mission. During the three-day workshop the scientists discussed several hot topics in four working groups:

The Earth's magnetic field is a gigantic shield that prevents the solar wind from entering the upper atmosphere. The solar wind becomes decelerated from supersonic speed and deflected around the Earth's magnetic field, which constitutes an obstacle, called the magnetosphere.

The first outermost boundary is formed in a distance of about 15-20 Earth radii- it is called the Earth bow shock. Within this layer the solar wind rapidly decelerates, the interplanetary magnetic field and plasma density increases, and strong currents are formed.

A very prominent feature at the Earth's bow shock is the presence of back streaming accelerated ions. Different ion distributions are seen depending on the angle TBn between the interplanetary magnetic field and the shock normal.

In the (quasi-) parallel region where TBn is smaller than 45�, a more or less isotropic (diffuse) ion distribution is found, and in the (quasi-) perpendicular region for TBn > 45�, a collimated ion beam emerges, and reflected gyrating ions are seen within one gyro-radius of the shock.

Although significant progress has been made in our understanding of the ions in the foreshock region (through numerical simulations and observations), the underlying production mechanisms are still being debated. The origin and the basic production mechanism of the field-aligned ion beams was one of the science topics tackled in Working Group 1.

Multi-spacecraft observations with Cluster and new instrument capabilities have provided important clues on the source and the basic production mechanism of these field-aligned beams. Figure 1 (not shown) shows a composite plot during the crossing at 18:48 UT on 31 March 2001, from downstream to upstream, including a snapshot in the shock ramp.

The shock transition is clearly visible as the rapid change in the magnetic field in the upper panel.

In the lower panel the ion distributions, with their velocities parallel and perpendicular to the magnetic field (mean magnetic field orientation indicated by arrows), are shown for the three shaded time periods: downstream, in the ramp, and upstream of the shock. Downstream (left) velocity space is filled with ions up to a parallel velocity of 1000 kms-1, while the ion beam in the upstream region (right) has a peak speed of �1500 kms-1, where velocity space is empty downstream.

The shock ramp (center) is filled with gyrating ions, from which the distribution extends into the parallel velocity region that is occupied by the beam further upstream.

These observations upstream and downstream of the quasi-perpendicular bow shock indicate that field-aligned beams most likely result from effective scattering in pitch-angle during ion reflection in the shock ramp.

At least in this low Mach number shock, leakage of thermalized ions from the downstream region apparently cannot be the source, as the volume in velocity space occupied by beam ions is empty downstream of the shock.

This is strong evidence that processes right in the shock ramp must produce the ion beams. While propagating upstream these beams create waves via ion-ion beam instabilities. These waves in turn are then part of the turbulence in the Earth's foreshock region.

Several other interesting topics, such as shock ramp thickness, waves associated with field-aligned beams, downstream thermalization, Short Large Amplitude Magnetic Structures (SLAMS), as well as e-folding distances have been addressed in this working group.

Cluster Observations of Sub-Auroral Magnetosphere-Ionosphere Coupling

New Cluster observations of the coupling between the sub-auroral ionosphere and the magnetosphere of the Earth were discussed in Working Group 2. Coupling between these regions is possible via the approximately dipolar magnetic field of the inner-magnetosphere.

This coupling intensifies during periods of strong geomagnetic activity in the pre-midnight local time sector, as it is the location of intense mingling of cold, dense plasma in the plasmasphere with hot plasma from the magnetotail (for example: the plasma comprising the ring current and the radiation belts).

Energy transferred from these magnetospheric populations into the ionosphere leads to the formation of sub-auroralairglow, or, SAR arcs.

Another distinct feature of this coupling is the formation of a sustained large poleward electric field in the ionosphere (which maps into a radially outward field in the magnetosphere). Figure 2 (not shown in this article) illustrates these electric fields in an evening meridional plane. Both electric field orientations produce westward drift of ions around the Earth.

The ionospheric observation of enhanced (> 500 ms-1) sub-auroral westward drift in the dusk to midnight sector has been coined "sub-auroral polarization streams", or SAPS. These observations are quite extensive, and the statistical description of this phenomenon is well-established. Magnetospheric observations of SAPS, however, are rare.

To understand the underlying coupling requires simultaneous, magnetically conjugate observations of SAPS at ionospheric and magnetospheric locations.

The Cluster Electron Drift Instrument (EDI), has been used to find SAPS electric fields in the inner magnetosphere. Case studies of events conjugate with ionospheric observations of the Defense Meteorological Satellite program (DMSP) have been presented in the Working Group 2 sessions. These observations indicated that SAPS-type electric fields were observed only after the storm time injection of energetic particles.

Cluster observations of Earthward moving plasmoids in the Earth's magnetotail Plasmoids and/or flux ropes are important features associated with various kinds of eruptive processes in astrophysical plasmas, notably with the occurrence of magnetospheric substorms and solar Coronal Mass Ejections (CMEs).

In the Earth's magnetotail, such loop-like magnetic structures are generated as the result of X-type reconnection and are commonly understood (according to the single plasmoid model) to travel tailward (anti-sunward). Cases of Earthward (sunward) moving plasmoids were presented and discussed in the Working Group 3 (WG3)

A schematic of the inferred orientation and the motion of the observed plasmoid with respect to the four Cluster tetrahedral configuration is shown in figure 3 (not shown). The XZ plane extended plasmoid with a significant duskward speed will lead the Cluster spacecraft cross the plasmoid structure in the order of C1 -> C4 -> C3 -> C2.

The fleet of Cluster spacecraft crossed the plasmoid structure in a "first in, last out" order indicative of the non-planar nested structure of the plasmoid. The large separation distance (around 1 Re) of the Cluster satellites in October 2002 provided constraints on the size and shape of the plasmoid structure. The scale size of the plasmoid observed on 28 October 2002 between 19:30 UT and 20:00 UT was calculated as 5.04 Re.

These observations support the model of multiple-plasmoid formation in the course of a substorm event. The observation of a sequence of plasmoids and TCRs (Traveling Compression Regions) is an indication of the repeated development of NENLs (Near Earth Neutral Lines). The plasmoids can occur intermittently and repeatedly.

The repetition rate may depend on the conditions of the solar wind and the IMF. The significance of multiple plasmoids/flux ropes in the plasma sheet is that their formation can be understood in terms of simultaneous reconnection at multiple X-lines in the magnetotail.

Multiple X-lines due to tearing mode reconnection could produce magnetic islands. The resulting overall field configuration is that of a magnetic island without a core field in the center. If the tail field lines are not anti-parallel, the islands will contain a field component along the island's axis to form a flux rope.

These observations also raise some interesting questions regarding the "fate" of Earthward moving plasmoids in the plasma sheet. The Earthward moving plasmoids pushed up against the geomagnetic field will probably dissipate quickly since the orientation of their magnetic fields is favorable for reconnection with the geomagnetic closed field lines in the near Earth.

In particular, multiple Earthward-moving plasmoids/flux ropes could be possible triggers for the substorm injections observed at geostationary orbit.

Other science topics addressed in this working group included: the current sheet structure and particle and field properties in the magnetic reconnection diffusion region, component � anti-parallel reconnection, the importance of O+ in reconnection/substorm dynamics, the observation of reconnection related slow-mode shocks in the near Earth magnetotail, the observation of magnetopause dynamics, momentum transfer, FTE structures from Cluster in conjunction with other datasets and comparisons with simulations.

Ultra-Low Frequency Waves in the Cusp
Working Group 4 focused on the cusp and auroral regions. These sessions featured presentations on the structure and dynamics of the cusp region as well as new results on ion acceleration in the cusp and above the aurora.

The results presented include some preliminary statistical surveys of Cluster data from these regions. The presentations showed the advantages of the different separation distances, which have been used during the Cluster mission.

A statistical survey of data from the STAFF instrument has found that significant ultra-low frequency (1-10 Hz) wave power is almost always present in the cusp. Using the four Cluster spacecraft at 100 km separation to measure wavelengths and propagation directions of these waves, scientists have positively identified the wave modes present in the cusp at these frequencies.

The above chart shows the average distribution of electromagnetic wave power in this frequency range in the cusp, with power increasing from blue to red. This distribution is shown superimposed on the average configuration of magnetic field lines near the Earth. The figure shows that wave activity is more intense within the cusp and that activity increases with distance from the Earth.

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