Publication archive

Publication archive

Presentation from the press event marking the beginning of Cluster's operational phase - held at ESA HQ, 16 February 2001.
Published: 16 February 2001
Presentation from the press event marking the beginning of Cluster's operational phase - held at ESA HQ, 16 February 2001.
Published: 16 February 2001
The Cluster mission will be operated from ESOC. In addition to the mission challenges imposed by the simultaneous operation of four co-ordinated spacecraft, ESOC will be responsible for maintaining the software of the On-Board Data Handling (OBDH) subsystem from the time of launch until the end of the mission, which is nominally two and a half years. This on-board software maintenance capability provides a powerful means of adapting the behaviours of the four Cluster spacecraft to the real-time status of the hardware of each and to any operational difficulties that they might encounter during their operating lifetimes.
Published: 02 July 1997
The Cluster mission is designed to study the small-scale structures that are believed to be fundamental in determining the key interaction processes in cosmic plasma. The mission will be controlled from ESOC, which will also be responsible for commanding the scientific payloads of the four spacecraft, in collaboration with the Cluster Principal Investigators (PIs), and for collecting and distributing the mission results to the scientific community. To support the Cluster mission operations, ESOC has developed the Cluster Data Processing System (CDPS), the architecture of which is based on three main components.
Published: 02 July 1997
The Cluster mission was first proposed to the Agency in late 1982 and was subsequently selected, together with SOHO, as the Solar Terrestrial Science Programme (STSP), the first Cornerstone of ESA's Horizon 2000 Programme. This article gives an overview of the complex chain of events that have taken place between the loss of the original mission with the Ariane-5 launch failure and the recent approval of the recovery mission known as Cluster-II.
Published: 02 July 1997
First results are presented of an effort to model the storm-time distortion of the magnetic field in the inner magnetosphere using space magnetometer data. Strong geomagnetic storms are relatively rare events, represented by only a small fraction of the data used in the derivation of existing empirical geomagnetic field models. Hence using those models for the mapping of the storm-time magnetosphere is at most an extrapolation based on trends, obtained from quiet and moderately disturbed data. To overcome that limitation, a set of data was created, containing only clear-cut events with Dst <= -65 nT, with the goal to derive models of the inner and near geomagnetic field (R < 15 RE), representing strongly disturbed geomagnetic configurations and their evolution during the storm cycle. The final data set included about 143,000 records with 5-min average B-vectors, covering 37 major storms between 1996 and 2000. Most of the data came from GOES-8, -9, -10, Polar, and Geotail spacecraft, and two storms in February-March of 1998 were also partially covered by the data of Equator-S. In all cases, only those storms were selected for which concurrent solar wind and IMF data were available for the entire duration of the event. Interplanetary medium data were provided by Wind, ACE, and, to a lesser extent, by IMP 8 and Geotail. The inner magnetospheric field was represented using the newly developed T01 model [Tsyganenko, 2002a, 2002b], with a duskside partial ring current with variable amplitude and scale size, an essential part of the storm-time current system.
Published: 24 May 2003
On August 22, 2001 all 4 Cluster spacecraft nearly simultaneously penetrated a magnetic flux rope in the tail. The flux rope encounter took place in the central plasma sheet, betai ~ 1-2, near the leading edge of a bursty bulk flow. The "time-of-flight" of the flux rope across the 4 spacecraft yielded Vx ~ 700 km/s and a diameter of ~1 Re. The speed at which the flux rope moved over the spacecraft is in close agreement with the Cluster plasma measurements. The magnetic field profiles measured at each spacecraft were first modeled separately using the Lepping-Burlaga force-free flux rope model. The results indicated that the center of the flux rope passed northward (above) s/c 3, but southward (below) of s/c 1, 2 and 4. The peak electric currents along the central axis of the flux rope predicted by these single-s/c models were ~15-19 nA m-2. The 4-spacecraft Cluster magnetic field measurements provide a second means to determine the electric current density without any assumption regarding flux rope structure. The current profile determined using the curlometer technique was qualitatively similar to those determined by modeling the individual spacecraft magnetic field observations and yielded a peak current density of 17 nA m-2 near the central axis of the rope. However, the curlometer results also showed that the flux rope was not force-free with the component of the current density perpendicular to the magnetic field exceeding the parallel component over the forward half of the rope, perhaps due to the pressure gradients generated by the collision of the BBF with the inner magnetosphere. Hence, while the single-spacecraft models are very successful in fitting flux rope magnetic field and current variations, they do not provide a stringent test of the force-free condition.
Published: 03 April 2003
Experimental evidence is provided for the existence of slow-mode magnetosonic solitons in the collisionless plasma at the magnetopause boundary layer. The solitons were detected by the fleet of Cluster spacecraft at the dusk flank of the magnetosphere as magnetic field depressions (up to 85%) accompanied with enhancement of the plasma density and temperature by a factor of 2. The solitons propagate 250 km/s with respect to the satellites and have perpendicular size of 1000-2000 km, which is a few ion inertial scale lengths. The comparison with numerical solutions of a theoretical model shows quantitative agreement between the model and observations.
Published: 27 February 2003
The current sheet structure and motion at XGSM = -19.5 RE, observed by Cluster/FGM during 1055-1107 UT on 29 August 2001, is examined. It is found that during the interval 1055-1102 UT the current sheet moves vertically up and down with a velocity of about 60 km s-1. During this interval the current sheet has a bifurcated structure: electric current is concentrated in two sheets with an extended layer of weak nearly uniform magnetic field in between. In the interval 1103-1107 UT the current sheet moves slowly upward and the current sheet has a Harris-type structure. By using four-spacecraft timing analysis, it is shown that the fast motion and bifurcation of the current sheet are associated with a wave-like transient propagating in the dawn-to-dusk direction.
Published: 17 January 2003
We report measurement of whistler-mode chorus by the four Cluster spacecraft at close separations. We focus our analysis on the generation region close to the magnetic equatorial plane at a radial distance of 4.4 Earth's radii. We use both linear and rank correlation analysis to define perpendicular dimensions of the sources of chorus elements below one half of the electron cyclotron frequency. Correlation is significant throughout the range of separation distances of 60-260 km parallel to the field line and 7-100 km in the perpendicular plane. At these scales, the correlation coefficient is independent for parallel separations, and decreases with perpendicular separation. The observations are consistent with a statistical model of the source region assuming individual sources as gaussian peaks of radiated power with a common half-width of 35 km perpendicular to the magnetic field. This characteristic scale is comparable to the wavelength of observed waves.
Published: 16 January 2003
For the first time, the Cluster spacecraft have collected 3-D information on magnetic field structures at small to medium scales in the Earth's dayside magnetosphere. We focus here on the first application of the Curlometer (direct estimation of the electric current density from curl(B), using measured spatial gradients of the magnetic field) analysis technique. The applicability of this multipoint technique is tested, for selected events within the data set, in the context of various mission constraints (such as position, timing, and experimental accuracy). For the Curlometer, nonconstant spatial gradients over the spacecraft volume, time dependence, and measurement errors can degrade the quality of the estimate. The estimated divergence of the magnetic field can be used to monitor (indirectly) the effect of nonconstant gradients in the case of many magnetic field structures. For others, and at highly distorted spacecraft configurations, this test may not reflect the quality of the Curlometer well. The relative scales and relative geometry between the spacecraft array and the structures present, as well as measurement errors, all are critical to the quality of the calculation. We demonstrate that even when instrumental and other errors are known to contribute to the uncertainty in the estimate of the current, a number of current signatures within the magnetosphere can be plausibly determined in direction, if not absolute size. A number of examples show consistent currents at the magnetopause, both separate from, and nearby or in the cusp region. Field-aligned currents near the polar cap boundary are also estimated reliably. We also demonstrate one example of an anomalous current arising from the effect of a highly distorted spacecraft configuration.
Published: 20 November 2002
Plasma, electric, and magnetic field data on the Polar spacecraft have been analyzed for the 29 May 1996 magnetopause traversal searching for evidence of in situ reconnection and traversal of the separator. In this paper we confine our analysis to model-free observations and intrasensor coherence of detection of the environs of the separator.
Published: 16 October 2002
Whistler waves are an intrinsic feature of the oblique quasiperpendicular collisionless shock waves. For supercritical shock waves, the ramp region, where an abrupt increase of the magnetic field occurs, can be treated as a nonlinear whistler wave of large amplitude. In addition, oblique shock waves can possess a linear whistler precursor. There exist two critical Mach numbers related to the whistler components of the shock wave, the first is known as a whistler critical Mach number and the second can be referred to as a nonlinear whistler critical Mach number. When the whistler critical Much number is exceeded, a stationary linear wave train cannot stand ahead of the ramp. Above the nonlinear whistler critical Mach number, the stationary nonlinear wave train cannot exist anymore within the shock front. This happens when the nonlinear wave steepening cannot be balanced by the effects of the dispersion and dissipation. In this case nonlinear wave train becomes unstable with respect to overturning. In the present paper it is shown that the nonlinear whistler critical Mach number corresponds to the transition between stationary and nonstationary dynamical behavior of the shock wave. The results of the computer simulations making use of the 1D full particle electromagnetic code demonstrate that the transition to the nonstationarity of the shock front structure is always accompanied by the disappearance of the whistler wave train within the shock front. Using the two-fluid MHD equations, the structure of nonlinear whistler waves in plasmas with finite beta is investigated and the nonlinear whistler critical Mach number is determined. It is suggested a new more general proof of the criteria for small amplitude linear precursor or wake wave trains to exist.
Published: 16 April 2002
We present magnetospheric observations of very large amplitude global scale ULF waves, from 9 and 10 December 2000 when the upstream solar wind speed exceeded 600 km/s. We characterise these ULF waves using ground-based magnetometer, radar and optical instrumentation on both the dawn and dusk flanks; we find evidence to support the hypothesis that discrete frequency field line resonances (FLRs) were being driven by magnetospheric waveguide modes. During the early part of this interval, Cluster was on an outbound pass from the northern dusk side magnetospheric lobe into the magnetosheath, local-time conjugate to the Canadian sector. In situ magnetic fluctuations, observed by Cluster FGM, show evidence of quasi-periodic motion of the magnetosheath boundary layer with the same period as the ULF waves seen on the ground. Our observations represent the first simultaneous magnetometer, radar and optical observations of the characteristics of FLRs, and confirm the potential importance of ULF waves for magnetosphere-ionosphere coupling, particularly via the generation and modulation of electron precipitation into the ionosphere. The in situ Cluster measurements support the hypothesis that, during intervals of fast solar wind speed, the Kelvin-Helmholtz instability (KHI) can excite magnetospheric waveguide modes which bathe the flank magnetosphere with discrete frequency ULF wave power and drive large amplitude FLRs.
Published: 16 April 2002
Solitary waves have, for the first time, been identified in 3D electric field data at the subsolar, equatorial magnetopause. These nonlinear, bipolar electric field pulses parallel to the magnetic field occur both as individual spikes and as trains of spikes. The solitary waves have amplitudes up to ~25 mV/m, and velocities from ~150 km/s to >2000 km/s, with scale sizes the order of a kilometer (comparable to the Debye length). Almost all the observed solitary waves are positive potential structures with potentials of ~0.1 to 5 Volts. They are often associated with very large amplitude waves in either or both the electric and magnetic fields. Although most of the observed signatures are consistent with an electron hole mode, the events with very low velocities and the few negative potential structures may be indicative of a second type of solitary wave in the magnetopause current layer. The solitary waves may be an important source of dissipation and diffusion at the magnetopause.
Published: 13 March 2002
The bright night-time aurorae that are visible to the unaided eye are caused by electrons accelerated towards Earth by an upward-pointing electric field. On adjacent geomagnetic field lines the reverse process occurs: a downward-pointing electric field accelerates electrons away from Earth. Such magnetic-field-aligned electric fields in the collisionless plasma above the auroral ionosphere have been predicted, but how they could be maintained is still a matter for debate. The spatial and temporal behaviour of the electric fields — a knowledge of which is crucial to an understanding of their nature — cannot be resolved uniquely by single satellite measurements. Here we report on the first observations by a formation of identically instrumented satellites crossing a beam of upward-accelerated electrons. The structure of the electric potential accelerating the beam grew in magnitude and width for about 200 s, accompanied by a widening of the downward-current sheet, with the total current remaining constant. The 200-s timescale suggests that the evacuation of the electrons from the ionosphere contributes to the formation of the downward-pointing magnetic-field-aligned electric fields. This evolution implies a growing load in the downward leg of the current circuit, which may affect the visible discrete aurorae.
Published: 13 December 2001
In this report we present the first results from the Cluster wideband plasma wave investigation. The four Cluster spacecraft were successfully placed in closely spaced, high-inclination eccentric orbits around the Earth during two separate launches in July and August 2000. Each spacecraft includes a wideband plasma wave instrument designed to provide high-resolution electric and magnetic field wave-forms via both stored data and direct downlinks to the NASA Deep Space Network.
Published: 01 December 2001
The Whisper instrument yields two data sets: (i) the electron density determined via the relaxation sounder, and (ii) the spectrum of natural plasma emissions in the frequency band 2-80 kHz. Both data sets allow for the three-dimensional exploration of the magnetosphere by the Cluster mission. The total electron density can be derived unambiguously by the sounder in most magnetospheric regions, provided it is in the range of 0.25 to 80 cm-3. The natural emissions already observed by earlier spacecraft are fairly well measured by the Whisper instrument, thanks to the digital technology which largely overcomes the limited telemetry allocation. The natural emissions are usually related to the plasma frequency, as identified by the sounder, and the combination of an active sounding operation and a passive survey operation provides a time resolution for the total density determination of 2.2 s in normal telemetry mode and 0.3 s in burst mode telemetry, respectively. Recorded on board the four spacecraft, the Whisper density data set forms a reference for other techniques measuring the electron population. We give examples of Whisper density data used to derive the vector gradient, and estimate the drift velocity of density structures. Wave observations are also of crucial interest for studying small-scale structures, as demonstrated in an example in the fore-shock region. Early results from the Whisper instrument are very encouraging, and demonstrate that the four-point Cluster measurements indeed bring a unique and completely novel view of the regions explored.
Published: 16 October 2001
EDI measures the drift velocity of artificially injected electron beams. From this drift velocity, the perpendicular electric field and the local magnetic field gradients can be deduced when employing different electron energies. The technique requires the injection of two electron beams at right angles to the magnetic field and the search for those directions within the plane that return the beams to their associated detectors after one or more gyrations. The drift velocity is then derived from the directions of the two beams and/or from the difference in their times-of-flight, measured via amplitude-modulation and coding of the emitted electron beams and correlation with the signal from the returning electrons. After careful adjustment of the control parameters, the beam recognition algorithms, and the onboard magnetometer calibrations during the commissioning phase, EDI is providing excellent data over a wide range of conditions. In this paper, we present first results in a variety of regions ranging from the polar cap, across the magnetopause, and well into the magnetosheath.
Published: 16 July 2001
Tests of hypotheses based on Higher Order Statistics (HOS) are reviewed in the particular context of the identification of non-linear processes in space plasma. The time series under study are associated with the measurement of electric or/and magnetic field components, or/and counting rates of articles. The basic principles of HOS techniques are reviewed. A general and unified procedure is suggested in order to construct statistical tests: (1) for detecting a non-gaussian or transient signal in a gaussian or non gaussian noise, (2) testing a stochastic time series for non-gaussianity (including non-linearity), (3) studying non-linear wave-wave interactions by using the kth-order coherency function. Asymptotic theory of estimates of the kth-order spectra is implemented in a digital signal processing framework. The effectiveness of the signal detection algorithms is demonstrated through computer simulations. Examples of application on the analysis of satellite data are given.
Published: 27 June 2001
4-Jul-2020 13:06 UT

ShortUrl Portlet

Shortcut URL

https://sci.esa.int/p/BAlaNqw