ESA Science & Technology - Publication Archive

On 14 January 2001, the four Cluster spacecraft passed through the northern magnetospheric mantle in close conjunction to the EISCAT Svalbard Radar (ESR) and approached the post-noon dayside magnetopause over Greenland between 13:00 and 14:00 UT. During that interval, a sudden reorganisation of the high-latitude dayside convection pattern accurred after 13:20 UT, most likely caused by a direction change of the Solar wind magnetic field. The result was an eastward and poleward directed flow-channel, as monitored by the SuperDARN radar network and also by arrays of ground-based magnetometers in Canada, Greenland and Scandinavia.

Magnetic reconnection has a crucial role in a variety of plasma environments in providing a mechanism for the fast release of stored magnetic energy. During reconnection the plasma forms a 'magnetic nozzle', like the nozzle of a hose, and the rate is controlled by how fast plasma can flow out of the nozzle. But the traditional picture of reconnection has been unable to explain satisfactorily the short timescales associated with the energy release, because the flow is mediated by heavy ions with a slow resultant velocity. Recent theoretical work has suggested that the energy release is instead mediated by electrons in waves called 'whistlers', which move much faster for a given perturbation of the magnetic field because of their smaller mass. Moreover, the whistler velocity and associated plasma velocity both increase as the 'nozzle' becomes narrower. A narrower nozzle therefore no longer reduces the total plasma flow-the outflow is independent of the size of the nozzle. Here we report observations demonstrating that reconnection in the magnetosphere is driven by whistlers, in good agreement with the theoretical predictions

Eds. Schmidt, R. and Guyenne, T.D.

The operations concept for the Cluster-II mission has had to evolve with respect to the original Cluster baseline, due mainly to changes in the spacecraft design and the reduction in the number of ground stations from two to only one for routine mission-operations support. The solutions adopted have allowed the overall impact on the ground segment and mission operations to be minimised, whilst still maintaining the scientific data return at the original level.

The Cluster-II mission is designed to study the near-Earth space environment in three dimensions. This will be the first scientific mission with four identical spacecraft flying together in the Earth's environment. The relative distance between the four spacecraft will vary between 200 and 18 000 km, according to the scientific region of interest. Cluster-II and the Solar and Heliospheric Observatory (SOHO) together make up the Solar Terrestrial Science Programme, the first 'Cornerstone' of ESA's Horizons 2000 long-term science plan.

Schmidt, R. and M. L. Goldstein (Eds.)

Proceedings of the Cluster II workshop Imperial College London UK

Four years ago, the first Cluster mission was lost when the maiden flight of Ariane-5 came to a tragic end. Today, through the combined efforts of the ESA Project Team, its industrial partners and collaborating scientific institutions, the Cluster quartet has been born again. A two-year programme of investigation into the Sun-Earth connection will begin this summer when ESA's Cornerstone mission to the magnetosphere lifts off from Baikonur. Flying in formation over the Earth's polar regions, Cluster-II will carry out the first three-dimensional exploration of near-Earth space ever attempted.

On March 24, 1995, the Geotail spacecraft observed large fluctuations of the magnetic field and plasma properties in the low-latitude boundary layer about 15 R_E tailward of the dusk meridian. Although the magnetospheric and magnetosheath magnetic fields were strongly northward, the B_z component showed strong short-duration fluctuations in which B_z could even reach negative values. We have used two-dimensional magnetohydrodynamic simulations with magnetospheric and magnetosheath input parameters specifically chosen for this Geotail event to identify the processes which cause the observed boundary properties. It is shown that these fluctuations can be explained by the Kelvin-Helmholtz instability if the k vector of the instability has a component along the magnetic field direction. The simulation results show many of the characteristic properties of the Geotail observations. In particular, the quasi-periodic strong fluctuations are well explained by satellite crossings through the Kelvin-Helmholtz vortices. It is illustrated how the interior structure of the Kelvin-Helmholtz vortices leads to the rapid fluctuations in the Geotail observations. Our results suggest an average Kelvin-Helmholtz wavelength of about 5 R_E, with a vortex size of close to 2 R_E for an average repetition time of 2.5 min. The growth time for these waves implies a source region of about 10-16 R_E upstream from the location of the Geotail spacecraft (i.e., near the dusk meridian). The results also indicate a considerable mass transport of magnetosheath material into the magnetosphere by magnetic reconnection in the Kelvin-Helmholtz vortices.

For several hours on March 24, 1995, the Geotail spacecraft remained near the duskside magnetotail boundary some 15R_E behind the Earth while the solar wind remained very quiet (V=330 km s^-1, n=14-21 cm^-3) with a very steady 11-nT northward magnetic field. Geotail experienced multiple crossings of a boundary between a dense (n=19 cm^-3), cool (T_p=40 eV), rapidly flowing (V=310 km s^-1) magnetosheath plasma and an interior region characterized by slower tailward velocities (V=100 km s^-1), lower but substantial densities (n=3 cm^-3) and somewhat hotter ions (220 eV). The crossings recurred with a roughly 3-min periodicity, and all quantities were highly variable in the boundary region. The magnetic field, in fact, exhibited some of the largest fluctuations seen anywhere in space, despite the fact that the exterior magnetosheath field and the interior magnetosphere field were both very northward and nearly parallel. On the basis of an MHD simulation of this event, we argue that the multiple crossings are due to a Kelvin-Helmholtz instability at the boundary that generates vortices which move past the spacecraft. A determination of boundary normals supports Kelvin-Helmholtz theory in that the nonlinear steepening of the waves is seen on the leading edge of the waves rather than on the trailing edge, as has sometimes been seen in the past. It is concluded that the Kelvin-Helmholtz instability is an important process for transferring energy, momentum and particles to the magnetotail during times of very northward interplanetary magnetic field.

The structure of the dissipation region in collisionless magnetic reconnection is investigated by means of kinetic particle-in-cell simulations and analytical theory. Analyses of simulations of reconnecting current sheets without guide magnetic field, which keep all parameters fixed with the exception of the electron mass, exhibit very similar large scale evolutions and time scales. A detailed comparison of two runs with different electron masses reveals very similar large scale parameters, such as ion flow velocities and magnetic field structures. The electron-scale phenomena in the reconnection region proper, however, appear to be quite different. The scale lengths of these processes are best organized by the trapping length of bouncing electrons in a field reversal region. The dissipation is explained by the electric field generated by nongyrotropic electron pressure tensor effects. In the reconnection region, the relevant electron pressure tensor components exhibit gradients which are independent of the electron mass. The similarities of the gradients as well as the behavior of the electron flow velocity can be derived from the electron trapping scale and the electron mass independence of the reconnection electric field. A further model which includes a significant guide magnetic field exhibits almost identical behavior. The explanation of this result lies in a Hall-type electric field which locally eliminates the magnetizing effect on the electrons of the guide magnetic field. The resulting electron dynamics is nearly identical to the one found in the model without guide magnetic field. --- Remainder of abstract truncated ---

A chorus generation mechanism is discussed, which is based on interrelation of ELF/VLF noise-like and discrete emissions under the cyclotron wave-particle interactions. A natural ELF/VLF noise radiation is excited by the cyclotron instability mechanism in ducts with enhanced cold plasma density or at the plasmapause. This process is accompanied by a step-like deformation of the energetic electron distribution function in the velocity space, which is situated at the boundary between resonant and nonresonant particles. The step leads to the strong phase correlation of interacting particles and waves and to a new backward wave oscillator (BWO) regime of wave generation, when an absolute cyclotron instability arises at the central cross section of the geomagnetic trap, in the form of a succession of discrete signals with growing frequency inside each element. The dynamical spectrum of a separate element is formed similar to triggered ELF/VLF emission, when the strong wavelet starts from the equatorial plane. The comparison is given of the model developed using some satellite and ground-based data. In particular, the appearance of separate groups of chorus signals with a duration 2-10 s can be connected with the preliminary stage of the step formation. BWO regime gives a succession period smaller than the bounce period of energetic electrons between the magnetic mirrors and can explain the observed intervals between chorus elements.

The electric-field and wave experiment (EFW) on Cluster is designed to measure the electric-field and density fluctuations with sampling rates up to 36000 samples s^-1. Langmuir probe sweeps can also be made to determine the electron density and temperature. The instrument has several important capabilities. These include (1) measurements of quasi-static electric fields of amplitudes up to 700 mV m^-1 with high amplitude and time resolution, (2) measurements over short periods of time of up to five simualtaneous waveforms (two electric signals and three magnetic signals from the seach coil magnetometer sensors) of a bandwidth of 4 kHz with high time resolution, (3) measurements of density fluctuations in four points with high time resolution. Among the more interesting scientific objectives of the experiment are studies of nonlinear wave phenomena that result in acceleration of plasma as well as large- and small-scale interferometric measurements. By using four spacecraft for large-scale differential measurements and several Langmuir probes on one spacecraft for small-scale interferometry, it will be possible to study motion and shape of plasma structures on a wide range of spatial and temporal scales. This paper describes the primary scientific objectives of the EFW experiment and the technical capabilities of the instrument.

On 4 June 1996, the maiden flight of the Ariane 5 launcher ended in failure. Only about 40 seconds after initiation of the flight sequence, at an altitude of about 3700m, the laucher veered off its flight path, broke up and exploded. Engineers from the Ariane 5 project teams of CNES and Industry immediately started to investigate the failure.

This is a report detailing their findings and suggested courses of action.

Using a common methodology to analyze data from the Active Magnetospheric Particle Tracer Explorer/Ion Release Module (AMPTE/IRM) and International Sun-Earth Explorer 2 (ISEE 2) satellites we report on the statistical properties of bursty bulk flow events (BBFs) in the inner plasma sheet (IPS). A positive correlation between BBFs and the AE index suggests that BBFs are predominantly geomagnetically active time phenomena. Earthward BBFs are more frequent close to midnight and away from Earth, up to a distance of approximately 19 R_E. Tailward BBFs are very infrequent in the IRM data set and somewhat less infrequent in the ISEE 2 data set in the region of the satellites' spatial overlap, possibly due to the more active conditions prevailing during the ISEE 2 mission in that region. However, in both data sets the ratio of tailward to earthward BBFs increases with distance from Earth; more than 20% of all BBFs are anti-sunward tailward of X = -19 R_E in the ISEE 2 data set. BBFs are responsible for 60-100% of the measured earthward transport of mass, energy and magnetic flux past the satellite in the regions of maximum occurrence rate, even though they last approximately 10-15% of the IPS observation time there. Thus BBFs represent the primary transport mechanism at those regions. The one-to-one correspondence between BBFs and substorm phase, as well as the relative contribution of BBFs to the total transport observed during substorms are questions that await further investigation based on multi instrument studies of individual events.

Observational evidence is presented indicating that beyond L = 1.7-2 plasma corotating in the plasmasphere expands continuously with a small outward directed bulk velocity perpendicular to geomagnetic field lines. A numerical simulation of plasmaspheric flux tube drift motion is presented in support of such an outward plasma expansion. The maximum expansion velocity inside the plasmasphere is determined by the maximum value of the plasma interchange velocity, which is inversely proportional to the value of the integrated Pedersen conductivity.

High-speed flows in the inner central plasma sheet (first reported by Baumjohann et al. (1990) are studied, together with the concurrent behavior of the plasma and magnetic field, by using AMPTE/IRM data from about 9 to 19 R(E) in the earth magnetotail. The conclusions drawn from the detailed analysis of a representative event are reinforced by a superposed epoch analysis applied on two years of data. The high-speed flows organize themselves in 10-min time scale flow enhancements called here bursty-bulk flow (BBF) events. Both temporal and spatial effects are responsible for their bursty nature. The flow velocity exhibits peaks of very large amplitude with a characteristic time scale of the order of a minute, which are usually associated with magnetic field dipolarizations and ion temeperature increases. The BBFs represent intervals of enhanced earthward convection and energy transport per unit area in the y-z GSM direction of the order of 5 x 10 exp 19 ergs/R(E-squared).

The occurrence rates and typical characteristics of high-speed ion flows with velocities of 400-600 km/s are determined on the basis of the analysis of a large quantity of ion moments and magnetic field measurements acquired from the AMTE/IRM satellite data. It is determined that the occurrence rates in the plasma sheet boundary layer, the outer central plasma sheet and the neutral sheet neighborhood have 4:1:2 ratio, and they increase with AE in all three regions when going from low to moderate activity; that the largest occurrence rates are near the midnight meridian at the largest radial distances accessible to IRM; and that the high-speed flow occurrence rates and ion densities are anticorrelated. It is also determined that the high-speed flows are bursty, and their angular distribution is strongly peaked in the sunward direction. No tailward high speed ion flow was detected. Correlation between the obtained results and the existing models and theories is discussed.

A statistical study on the behaviors of ion and electron moments in the central plasma sheet was carried out using tail data obtained by the three-dimensional plasma instrument on board the AMPTE/IRM satellite in 1986. Results show that the ion temperature increases with increasing magnetic activity and the ion density decreases during disturbed intervals, except in the neutral sheet neighborhood at smaller radial distances. The ion and electron temperatures (Ti and Te, respectively) in the central plasma sheet were found to be highly correlated, with the Ti/Te ratio being constant over a wide range of temperatures and about twice as large as in the distant tail. The average ion flow speeds in the central plasma sheet (below 100 km/sec) are nearly identical to those found in the plasma sheet boundary layer, but their distribution functions for these regions are quite different.