ESA Science & Technology - Highlights from 8th Cluster Workshop

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:

• Working Group 1: Bow shock processes, structure, and dynamics
  (organizers: H. Kucharek, M. Scholer, E. Moebius)
• Working Group 2: Physics of the inner magnetosphere
  (organizers: V. Jordanova, P. Puhl-Quinn, H. Matsui)
• Working Group 3: Reconnection processes at the magnetopause and in the tail
  (organizers: C. Mouikis, L. Kistler, C. Farrugia)
• Working Group 4: Cusp dynamics and structure and auroral processes
  (organizers: H. Laakso, C. Farrugia, E. Lund)

Out of numerous presentations in the workshop sessions we present a summary on selected science topics of each of the individual working groups. More detailed information can be obtained from the workshop web-page.

Earth's Bow Shock: Origin of Ion Beams Revealed

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 is called the Earth bow shock. Within this layer the solar wind becomes rapidly decelerated, the interplanetary magnetic field and plasma density increase, 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 Θ_Bn between the interplanetary magnetic field and the shock normal. In the (quasi-) parallel region where Θ_Bn is smaller than 45º a more or less isotropic (diffuse) ion distribution is found, and in the (quasi-) perpendicular region for Θ_Bn > 45º a collimated ion beam emerges and reflected gyrating ions are seen within one gyro-radius of the shock.

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

Figure 1: Magnetic field (top panel) and ion distribution (lower panel) in velocity space downstream, in the ramp, and upstream of the bow shock

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.

Figure 2: A schematic of the SAPS electric field at ionspheric altitudes (for example as measured by the Defense Meterological Satellite Program [DMSP]) and in the magnetosphere (for example as measured by Cluster at a distance of L=4). The figure is in the post-dusk meridional plane.

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 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.

Figure 3: SAPS event

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. More detailed information on these measurements can be found here and by following the link from figure 3.

Science presentations in Working Group 2 also included reports on:

• ring current morphology and dynamics
• first use of the curlometer technique in the ring current employing multi-spacecraft CLUSTER-FGM data
• radiation belts variability
• plasmaspheric density structures
• fundamental properties and role of ELF/VLF waves near the plasmapause
• theoretical and simulation results compared with datasets from Cluster and other satellites.

For a complete list of talks please visit the workshop web page at http://atlas.sr.unh.edu/cluster8/

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)

Figure 4: Schematic that shows nested plasmoid structures observed by the Cluster tetrahedron.

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.

Figure 5: Statistical average of the magnetic wave power at 1-10 Hz in the cusp region as measured by the STAFF instrument on Cluster, as a function of distance from the Earth toward the Sun (X) and distance from the Sun-Earth line (R). Power increases from blue to red. Credit: N. Cornilleau-Wehrlin (CETP, Velizy, FR) and E. Budnik (CESR, Toulouse, FR).

Contact Details

Bow shock processes, structure, and dynamics

H. Kucharek, Univ. of New Hampshire, Durham, NH ; tel. 603-862-0322
E. Moebius, Univ. of New Hampshire, Durham, NH ; tel. 603-862-0322
M. Scholer, Max-Planck-Institut für extraterrestrische Physik, Garching, Germany ; tel. 49-89-300003821

Physics of the inner magnetosphere

V. Jordanova, Univ. of New Hampshire, Durham, NH ; tel. 603-862-0322
P. Puhl-Quinn, Univ. of New Hampshire, Durham, NH ; tel. 603-862-0322
H. Matsui, Univ. of New Hampshire, Durham, NH ; tel. 603-862-0322

Reconnection processes at the magnetopause and in the tail

C. Farrugia, Univ. of New Hampshire, Durham, NH ; tel. 603-862-0322
L. Kistler, Univ. of New Hampshire, Durham, NH ; tel. 603-862-0322
C. Mouikis, Univ. of New Hampshire, Durham, NH ; tel. 603-862-0322

Cusp dynamics and structure and auroral processes

C. Farrugia, Univ. of New Hampshire, Durham, NH ; tel. 603-862-0322
H. Laakso, ESA/ESTEC, Noordwijk, The Netherlands ; tel. 31-71-565 3319
E. Lund, Univ. of New Hampshire, Durham, NH ; tel. 603-862-0322