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| Cluster observation of a bifurcated current sheet |
| 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. |
| Publication date: 17 Jan 2003 |
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| Transverse dimensions of chorus in the source region |
| 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. |
| Publication date: 16 Jan 2003 |
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| Fast flow during current sheet thinning |
| In this paper we report Cluster observation of a fast flow event in the plasma sheet associated with a small auroral substorm intensification at 1838 UT on August 12, 2001. Cluster, located in the plasma sheet, experienced significant thinning of the current sheet associated with a high-speed Earthward flow of 900 km s-1. By using the four spacecraft magnetic field data and a Harris-type current sheet model, it was estimated that the thickness of the current sheet changes from about 1 RE before the flow observation down to 400 km, i.e., close to the ion inertia length. In the vicinity of this thin current sheet there were also signatures of enhanced current density off the center of the neutral sheet, consistent with recent Geotail results. |
| Publication date: 14 Dec 2002 |
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| Four-point Cluster application of magnetic field analysis tools: The Curlometer |
| 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. |
| Publication date: 20 Nov 2002 |
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| Fingerprints of collisionless reconnection at the separator, I, Ambipolar-Hall signatures |
| 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. |
| Publication date: 15 Oct 2002 |
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| ESA SP-1259: ESA's Report to the 34th COSPAR Meeting |
Scientific editor: B. Foing
Editor: A. Wilson
The report to the 34th COSPAR Meeting covers the missions of the Scientific Programme of ESA in the areas of astronomy, Solar System exploration and fundamental physics. |
| Publication date: 01 Oct 2002 |
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| A telescopic and microscopic view of a magnetospheric substorm on 31 March 2001 |
| On March 31, 2001 at ~0635 UT when the CLUSTER constellation was near local midnight and at ~4 RE geocentric distance, sensors observed an energetic electron injection event associated with a strong (AE ~ 1200 nT) magnetospheric substorm. Geostationary spacecraft 1991-080 located at ~20 LT also saw an abrupt electron injection event at ~0630 UT and FAST spacecraft instruments (~19 LT) detected a powerful set of magnetic field, electric field, and energetic plasma signatures at ~0637 UT. The energetic neutral atom imaging experiments onboard the IMAGE spacecraft detected an injection of substorm-produced ions in the pre-midnight sector commencing at ~0630 UT. Electron injection signatures at the four separate CLUSTER locations allow us to infer the location, speed, and direction of the substorm injection boundary. Hence, the CLUSTER (and IMAGE) telescope-microscope combination is a long-sought realization of a major magnetospheric research objective and shows the power of localized multi-point measurements from CLUSTER. |
| Publication date: 19 Sep 2002 |
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| Spatium 9: The Fourfold Way Through the Magnetosphere: The Cluster Mission |
This article is based on a talk given in November 2001 by Dr. G. Paschmann to Pro ISSI |
| Publication date: 01 Jun 2002 |
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| Coordinated ground-based and Cluster observations of large amplitude global magnetospheric oscillations during a fast solar wind speed interval |
| 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. |
| Publication date: 15 Apr 2002 |
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| Nonstationarity of strong collisionless quasiperpendicular shocks: Theory and full particle numerical simulations |
| 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. |
| Publication date: 15 Apr 2002 |
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| Polar observations of solitary waves at the Earth's magnetopause |
| 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. |
| Publication date: 13 Mar 2002 |
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| First results from the Cluster Wide-Band Plasma Wave Investigation |
| 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. |
| Publication date: 01 Dec 2001 |
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| Early results from the Whisper instrument on Cluster: an overview |
| 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. |
| Publication date: 15 Oct 2001 |
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| Rumba, Salsa, Samba and Tango in the Magnetosphere - The Cluster Quartet's First Year in Space |
| The four Cluster spacecraft were successfully launched in pairs by two Russian Soyuz rockets on 16 July and 9 August 2000. On 14 August, the second pair joined the first pair in highly eccentric polar orbits, with an apogee of 19.6 Earth radii and a perigee of 4 Earth radii. The very accurate orbital injection and low fuel consumption mean
that spacecraft operations could continue for at least two more years after the nominal two-year mission.
This is the first time that the Earth's magnetic field and its environment have been explored by a small constellation of four identical spacecraft. Preliminary results show that, as predicted, with four spacecraft we can obtain a detailed three-dimensional view of the Sun-Earth connection processes taking place at the interface between the solar wind and the Earth's magnetic field. |
| Publication date: 01 Aug 2001 |
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| The electron drift instrument on Cluster: overview of first results |
| 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. |
| Publication date: 15 Jul 2001 |
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| Use of Higher-Order Statistical Tests in the Analysis of Time Series Associated with Space Data |
| 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. |
| Publication date: 26 Jun 2001 |
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| Magnetic explosions in space |
| The magnetic field that surrounds the Earth is rarely quiet. An explanation for the explosive nature of magnetic storms is gathering support from satellite data. |
| Publication date: 29 Mar 2001 |
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| Rapid magnetic reconnection in the Earth's magnetosphere mediated by whistler waves |
| 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 |
| Publication date: 29 Mar 2001 |
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| Cluster at the Earth's Bow Shock |
| Presentation from the press event marking the beginning of Cluster's operational phase - held at ESA HQ, 16 February 2001. |
| Publication date: 16 Feb 2001 |
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| Cluster status |
| Presentation from the press event marking the beginning of Cluster's operational phase - held at ESA HQ, 16 February 2001.
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| Publication date: 16 Feb 2001 |
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