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| Location and size of the global source region of whistler mode chorus |
| We use multicomponent measurements of the four Cluster spacecraft and a backward ray tracing simulation to estimate the location and size of the global source of whistler mode chorus emissions in the magnetic equatorial plane. For the first time, analysis is made in a broad range of latitudes in both hemispheres along a single Cluster orbit. Our results show that for different time intervals, the sizes of the observed portions of the global chorus source region in the equatorial plane varied between 0.4 and 1.5 Earth radii. They were found at radial distances between 4.5 and 8.2 Earth radii during 2 h of measurements. Therefore, the superposed minimum width of the global source region of whistler mode chorus in the magnetic equatorial plane is approximately 4 Earth radii. |
| Publication date: 31 Mar 2010 |
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| ULF waves associated with the periodical high speed flows in magnetotail plasma sheet |
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| Publication date: 31 Mar 2010 |
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| Cusp as a source for oxygen in the plasma sheet during geomagnetic storms |
| We have used the ion composition data from the CIS/CODIF instrument on Cluster to determine how the O+ population in the plasma sheet and the adjacent lobes changes during geomagnetic storms. The Cluster trajectory, which moves over the polar cap, into the lobe, and then into the plasma sheet on each orbit, allows us to track the changes in O+ in these regions for a prestorm orbit, main-phase orbit, and recovery phase orbit. We find that changes in the O+ density and pressure in the plasma sheet are similar to those commonly observed in the ring current during a storm. The O+ is low prestorm. It increases by about a factor of 10 just prior to or during the early main phase of the storm, and is reduced, but usually not down to prestorm levels, in the recovery phase. The lobes contain tailward streaming O+ which originates in the "cleft ion fountain". During the storms main phase, this population also increases. A detailed look at the main-phase passes shows that a significant increase in the O+/H+ ratio is observed when this lobe population reaches the plasma sheet, and the tailward streaming O+ is observed continuously as the spacecraft moves from the lobe into the plasma sheet. The enhanced O+ in the lobe and the plasma sheet is observed for many hours during the storm. The inward convection of this population is likely a significant contributor to the storm time ring current. |
| Publication date: 09 Mar 2010 |
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| The Cluster Active Archive - Studying the Earth's Space Plasma Environment |
Book published in the series "Astrophysics and Space Science Proceedings"; H. Laakso, M.G.G.T. Taylor, C.P. Escoubet (Eds.), 2010, XX, 489 p., Hardcover, ISBN: 978-90-481-3498-4, © Springer  Since the year 2000 the ESA Cluster mission has been investigating the small-scale structures and processes of the Earth's plasma environment, such as those involved in the interaction between the solar wind and the magnetospheric plasma, in global magnetotail dynamics, in cross-tail currents, and in the formation and dynamics of the neutral line and of plasmoids.
This book contains presentations made at the 15th Cluster workshop held in March 2008. It also presents several articles about the Cluster Active Archive and its datasets, a few overview papers on the Cluster mission, and articles reporting on scientific findings on the solar wind, the magnetosheath, the magnetopause and the magnetotail. The contents of the book are grouped into seven main parts:
- Part I - Products and Services of the Cluster Active Archive
- Part II - Tools for the CAA Data Analysis
- Part III - Measurement Techniques and Calibration Routines
- Part IV - Magnetospheric Missions
- Part V - Observations of Solar Wind and Magnetosheath
- Part VI - Observations of Magnetopause and Cusp
- Part VII - Observations of Magnetospheric Tail
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| Publication date: 15 Jan 2010 |
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| Oblique propagation of whistler mode waves in the chorus source region |
| Whistler mode chorus has been shown to play a role in the process of local acceleration of electrons in the outer Van Allen radiation belt. Most of the quasi-linear and nonlinear theoretical studies assume that the waves propagate parallel to the terrestrial magnetic field. We show a case where this assumption is invalid. We use data from the Cluster spacecraft to characterize propagation and spectral properties of chorus. The recorded high-resolution waveforms show that chorus in the source region can be formed by a succession of discrete wave packets with decreasing frequency that sometimes change into shapeless hiss. These changes occur at the same time in the entire source region. Multicomponent measurements show that waves in both these regimes can be found at large angles to the terrestrial magnetic field. The hiss intervals contain waves propagating less than one tenth of a degree from the resonance cone. In the regime of discrete wave packets the peak of the wave energy density is found at a few degrees from the resonance cone in a broad interval of azimuth angles. The wave intensity increases with the distance from the magnetic field minimum along a given field line, indicating a gradual amplification of chorus in the source region. |
| Publication date: 15 Dec 2009 |
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| Supermagnetosonic Jets behind a Collisionless Quasiparallel Shock |
| The downstream region of a collisionless quasiparallel shock is structured containing bulk flows with high kinetic energy density from a previously unidentified source. We present Cluster multispacecraft measurements of this type of supermagnetosonic jet as well as of a weak secondary shock front within the sheath, that allow us to propose the following generation mechanism for the jets: The local curvature variations inherent to quasiparallel shocks can create fast, deflected jets accompanied by density variations in the downstream region. If the speed of the jet is super(magneto)sonic in the reference frame of the obstacle, a second shock front forms in the sheath closer to the obstacle. Our results can be applied to collisionless quasiparallel shocks in many plasma environments. |
| Publication date: 09 Dec 2009 |
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| Cluster Active Archive: Overview |
| In the book "The Cluster Active Archive - Studying the Earth's Space Plasma Environment", Astrophysics and Space Science Proceedings, H. Laakso et al. (eds.), ISBN 978-90-481-3498-4 (Print) 978-90-481-3499-1 (Online), Springer, 2010 The four-satellite Cluster mission investigates the small-scale structures and physical processes related to interaction between the solar wind and the magnetospheric plasma. The Cluster Active Archive (CAA) (URL: http://caa.estec.esa.int) will contain the entire set of Cluster high-resolution data and other allied products in a standard format and with a complete set of metadata in machine readable format. The total amount of the data files in compressed format is expected to exceed 50 TB. The data archive is publicly accessible and suitable for science use and publication by the world-wide scientific community. The CAA aims to provide user-friendly services for searching and accessing these data and ancillary products. The CAA became operational in February 2006 and as of Summer 2008 has data from most of the Cluster instruments for at least the first 5 years of operations (2001-2005). The coverage and range of products are being continually improved with more than 200 datasets available from each spacecraft, including high-resolution magnetic and electric DC fields and wave spectra; full three-dimensional electron and ion distribution functions from a few eV to hundreds of keV; and various ancillary and browse products to help with spacecraft and event location. The CAA is continuing to extend and improve the online capabilities of the system and the quality of the existing data. It will add new data files for years 2006-2009 and is preparing for the long-term archive with complete coverage after the completion of the Cluster mission. |
| Publication date: 04 Dec 2009 |
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| Double Star: Mission, Instruments and Joint Observations |
| In the book "The Cluster Active Archive - Studying the Earth's Space Plasma Environment", Astrophysics and Space Science Proceedings, H. Laakso et al. (eds.), ISBN 978-90-481-3498-4 (Print) 978-90-481-3499-1 (Online), Springer, 2010 The Chinese National Space Agency (CNSA) Double Star (DSP) spacecraft, TC-1 and TC-2 were launched in December 2003 and July 2004 into near equatorial and polar orbits respectively. During more than 3 years of operations they have maintained a close phasing with the ESA four-spacecraft mission to produce the first, well coordinated multi-scale measurements, sampling phenomena with five and six spacecraft. In this short paper we give a brief review of the DSP mission and show its joint capability with Cluster by showing examples of use of some early and more recent analysis techniques and their application to (more than) four spacecraft. We highlight a selection of some co-ordinated events, focussing on dayside phenomena, but also with a brief discussion of a tail event. Other reviews in this special issue will deal more completely with coverage of the other regions of the magnetosphere. |
| Publication date: 04 Dec 2009 |
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| Kelvin-Helmholtz waves at the Earth's magnetopause: Multiscale development and associated reconnection |
| We examine traversals on 20 November 2001 of the equatorial magnetopause boundary layer simultaneously at ~1500 magnetic local time (MLT) by the Geotail spacecraft and at ~1900 MLT by the Cluster spacecraft, which detected rolled-up MHD-scale vortices generated by the Kelvin-Helmholtz instability (KHI) under prolonged northward interplanetary magnetic field conditions. Our purpose is to address the excitation process of the KHI, MHD-scale and ion-scale structures of the vortices, and the formation mechanism of the low-latitude boundary layer (LLBL). The observed KH wavelength (>4 x 104 km) is considerably longer than predicted by the linear theory from the thickness (~1000 km) of the dayside velocity shear layer. Our analyses suggest that the KHI excitation is facilitated by combined effects of the formation of the LLBL presumably through high-latitude magnetopause reconnection and compressional magnetosheath fluctuations on the dayside, and that breakup and/or coalescence of the vortices are beginning around 1900 MLT. Current layers of thickness a few times ion inertia length ~100 km and of magnetic shear ~60° existed at the trailing edges of the vortices. Identified in one such current sheet were signatures of local reconnection: Alfvénic outflow jet within a bifurcated current sheet, nonzero magnetic field component normal to the sheet, and field-aligned beam of accelerated electrons. Because of its incipient nature, however, this reconnection process is unlikely to lead to the observed dusk-flank LLBL. It is thus inferred that the flank LLBL resulted from other mechanisms, namely, diffusion and/or remote reconnection unidentified by Cluster. |
| Publication date: 04 Dec 2009 |
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| The Cluster Mission: Space Plasma in Three Dimensions |
| In the book "The Cluster Active Archive - Studying the Earth's Space Plasma Environment", Astrophysics and Space Science Proceedings, H. Laakso et al. (eds.), ISBN 978-90-481-3498-4 (Print) 978-90-481-3499-1 (Online), Springer, 2010 Part of original abstract follows:
At the time of writing, Cluster is approaching 8 years of successful operation and continues to fulfill, if not exceed its scientific objectives. After a nominal mission lifetime of 2 years Cluster currently in its extended mission phase, up to June 2009, with a further extension request submitted for a further 3.5 years. The primary goals of the Cluster mission include three-dimensional studies of small-scale plasma structures and turbulence in the key plasma regions in the Earth's environment: solar wind and bow shock, magnetopause, polar cusps, magnetotail, and auroral zone. During the course of the mission, the relative distance between the four spacecraft is being varied to form a nearly perfect tetrahedral configuration at 100, 250, 600, 2,000, 5,000 and 10,000 km inter-spacecraft separation targeted to study scientifically interesting regions at different scales. In the last few years, the constellation strategy has moved towards a multi-scale concept, enabling two scale sizes to be investigated at the same time. In these cases, three spacecraft are separated by 10,000 km with the last spacecraft separated from this plane by varying distances from 16 km up to several 1,000 km. In this paper, we provide a brief overview of the mission concept and implementation and highlight a number of Cluster's latest science results, which include: the first observation of three dimensional (3-D) surface waves on the bow shock, the first 3-D analysis of turbulence in the magnetosheath, the discovery of magnetosonic waves accelerating electrons to MeV energies in the radiation belts, along with a number of discoveries involving magnetic reconnection. |
| Publication date: 04 Dec 2009 |
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| Reformation of an oblique shock observed by Cluster |
| On 16 March 2005, the Cluster spacecraft crossed a shock almost at the transition between the quasi-perpendicular and quasi-parallel regimes (ThetaBn = 46°) preceded by an upstream low-frequency (~0.02 Hz in the spacecraft frame) wave train observed for more than 10 mn. The wave semicycle nearest to the shock was found to grow in time, steepen and reflect an increasing fraction of the incoming ions. This gives strong indication that this pulsation is becoming a new shock front, standing ~5lambdap upstream of the main front and growing to shock-like amplitude on a timescale of ~35 Omegap. Downstream of the main shock transition, remnants of an older front are found indicating that the reformation is cyclic. This provides a unique example where the dynamics of shock reformation can be sequentially followed. The process shares many characteristics with simulations of reforming quasi-parallel shocks. |
| Publication date: 25 Nov 2009 |
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| Cluster observations of energetic electron flux variations within the plasma sheet |
| The variability of energetic electron fluxes (>40 keV) within the plasma sheet is explored using measurements from the Cluster spacecraft from 2001 through 2005. Only cases where the spacecraft remains inside the plasma sheet throughout the event are considered. Interesting cases were found using a combination of automated methods and visual inspection. Events are classified into 4 main types: (1) plasma sheet empty of energetic electrons; (2) decreasing plasma sheet energetic electron fluxes; (3) increasing plasma sheet energetic electron fluxes; and (4) sharp (rising and falling) variations in plasma sheet energetic electron fluxes during a single plasma sheet crossing. Case studies are presented for each type of event. The time it takes to fill/empty the plasma sheet of energetic electrons is quantified based on these events. Extreme events, most of which are associated with enhanced geomagnetic activity, showed that energetic electrons in the plasma sheet can vary up to several orders of magnitude. Interestingly, the energetic electron fluxes inside the plasma sheet can still undergo rapid variations when the solar wind is calm and geomagnetic activity is low. |
| Publication date: 18 Nov 2009 |
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| Kinetic Alfvén waves turbulence in the Earth's magnetosphere |
| The numerical simulations of the model equation governing the nonlinear dynamics of kinetic Alfvén waves in the intermediate-beta plasmas are performed. When the nonlinearity arises due to the ponderomotive force driven density perturbations of kinetic Alfvén waves, the model equation turns out to be a modified nonlinear Schrödinger equation. This has been solved numerically by using appropriate boundary conditions. The coherent, damped magnetic filaments with turbulent spectra have been observed. Our results reveal the interesting change in spectral index because of the damping effect. The steeper power spectra follow ~k-3.4 scaling. Using the Fokker-Planck equation with the new velocity space diffusion coefficient, we find the distribution function of energetic electrons in these turbulent structures. These turbulent structures can be responsible for plasma heating in Earth's magnetosphere. |
| Publication date: 28 Oct 2009 |
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| Universality of Solar-Wind Turbulent Spectrum from MHD to Electron Scales |
| To investigate the universality of magnetic turbulence in space plasmas, we analyze seven time periods in the free solar wind under different plasma conditions. Three instruments on Cluster spacecraft operating in different frequency ranges give us the possibility to resolve spectra up to 300 Hz. We show that the spectra form a quasiuniversal spectrum following the Kolmogorov's law ~k-5/3 at MHD scales, a ~k-2.8 power law at ion scales, and an exponential ~exp[-sqrt(k rhoe)] at scales k rhoe~[0.1,1], where rhoe is the electron gyroradius. This is the first observation of an exponential magnetic spectrum in space plasmas that may indicate the onset of dissipation. We distinguish for the first time between the role of different spatial kinetic plasma scales and show that the electron Larmor radius plays the role of a dissipation scale in space plasma turbulence. |
| Publication date: 14 Oct 2009 |
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| Energetic electron response to ULF waves induced by interplanetary shocks in the outer radiation belt |
| Strong interplanetary shock interactions with the Earth's magnetosphere have great impacts on energetic particle dynamics in the magnetosphere. An interplanetary shock on 7 November 2004 (with the maximum solar wind dynamic pressure of ~70 nPa) was observed by the Cluster constellation to induce significant ULF waves in the plasmasphere boundary, and energetic electrons (up to 2 MeV) were almost simultaneously accelerated when the interplanetary shock impinged upon the magnetosphere. In this paper, the relationship between the energetic electron bursts and the large shock-induced ULF waves is studied. It is shown that the energetic electrons could be accelerated and decelerated by the observed ULF wave electric fields, and the distinct wave number of the poloidal and toroidal waves at different locations also indicates the different energy ranges of electrons resonating with these waves. For comparison, a rather weak interplanetary shock on 30 August 2001 (dynamic pressure ~2.7 nPa) is also investigated. It is found that interplanetary shocks or solar wind pressure pulses with even small dynamic pressure change can have a nonnegligible role in the radiation belt dynamics. |
| Publication date: 10 Oct 2009 |
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| Kinetic theory for the ion humps at the foot of the Earth's bow shock |
| The nonlinear kinetic theory is presented for the ion acoustic perturbations at the foot of the Earth's quasiperpendicular bow shock, that is characterized by weakly magnetized electrons and unmagnetized ions. The streaming ions, due to the reflection of the solar wind ions from the shock, provide the free energy source for the linear instability of the acoustic wave. In the fully nonlinear regime, a coherent localized solution is found in the form of a stationary ion hump, which is traveling with the velocity close to the phase velocity of the linear mode. The structure is supported by the nonlinearities coming from the increased population of the resonant beam ions, trapped in the self-consistent potential. As their size in the direction perpendicular to the local magnetic field is somewhat smaller that the electron Larmor radius and much larger that the Debye length, their spatial properties are determined by the effects of the magnetic field on weakly magnetized electrons. These coherent structures provide a theoretical explanation for the bipolar electric pulses, observed upstream of the shock by Polar and Cluster satellite missions. |
| Publication date: 05 Oct 2009 |
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| Plasma transport in the magnetotail lobes |
| The Earth's magnetosphere is populated by particles originating from the solar wind and the terrestrial ionosphere. A substantial fraction of the plasma from these sources are convected through the magnetotail lobes. In this paper, we present a statistical study of convective plasma transport through the Earth's magnetotail lobes for various geomagnetic conditions. The results are based on a combination of density measurements from the Electric Field and Waves Experiment (EFW) and convection velocities from the Electron Drift Instrument (EDI) on board the Cluster spacecraft. The results show that variations in the plasma flow is primarily attributed to changes in the convection velocity, whereas the plasma density remains fairly constant and shows little correlation with geomagnetic activity. During disturbed conditions there is also an increased abundance of heavier ions, which combined with enhanced convection, cause an accentuation of the mass flow. The convective transport is much slower than the field aligned transport. A substantial amount of plasma therefore escape downtail without ever reaching the central plasma sheet. |
| Publication date: 28 Sep 2009 |
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| On the triggering of auroral substorms by northward turnings of the interplanetary magnetic field |
| Some studies over the last decade have indicated that the instability responsible for substorm expansion phase onset may require an external trigger such as a northward turning of the interplanetary magnetic field (IMF). Statistical investigations have lead to contrasting interpretations regarding the relationship between proposed solar wind triggers and substorm onsets identified from geomagnetic data. We therefore present the results of a study into the possible triggering of 260 substorms between 2001-2005, exploiting data from the Cluster and IMAGE satellite missions. We find that only a small fraction (<25%) of the substorms studied are associated with northward turnings of the IMF. However, the majority of the observed onsets are associated with a growth phase characterised using a subset of the criteria employed to define northward-turning IMF triggers. Based upon a case-by-case investigation and the results of an analysis using the statistics of point processes, we conclude that northward-turning structures in the IMF, while sometimes coinciding with the initial phase of individual substorms, are not required to trigger the magnetospheric instability associated with substorm expansion phase onset. |
| Publication date: 25 Sep 2009 |
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| A mechanism for electrostatic solitary structures in the Earth's magnetosheath |
| Electrostatic solitary waves (ESWs) have been observed in the Earth's magnetosheath region by Cluster. A mechanism for the generation of these structures in terms of electron-acoustic solitons and double layers is discussed. The model simulates the magnetosheath plasma by a four-component plasma system consisting of core electrons, two counterstreaming electron beams, and one type of ions. The analysis is based on the fluid equations and the Poisson equation, and employs the Sagdeev pseudopotential techniques to investigate the solitary waves. The electric field amplitudes, the time durations, and the propagation speeds of the solitary structures predicted by the model are in good agreement with the observed electric fields, pulse widths, and speeds of the electrostatic bipolar pulses. |
| Publication date: 24 Sep 2009 |
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| The Earth's Plasmasphere - A Cluster and Image Perspective |
Darrouzet, F., De Keyser, J., Pierrard, V. (Eds.)
2009, IV, 296 pages, 100 illus., 60 in colour, Hardcover.
ISBN: 978-1-4419-1322-7 (Print), 978-1-4419-1323-4 (Online)
Reprinted from Space Science Reviews journal, Vol. 145/1-2, 2009.
This book reviews the state of the art in plasmaspheric science based on the modern observations provided by ESA's Cluster and NASA's IMAGE spacecraft. The plasmasphere, discovered at the beginning of the space age, has remained largely unexplored territory. Now, with innovative observational techniques, new light is being shed on this key region of the magnetosphere. This book sketches the emerging overall picture of a highly structured plasma, sculpted by the ever-changing electromagnetic fields that result from the interaction of the solar wind with the magnetosphere.
The Earth's Plasmasphere, written by an international group of scientists representative of the world-wide community, is aimed at researchers and graduate students with an interest in magnetospheric physics, space plasma physics and geophysics.
Table of Contents
Preface (J. L. Burch & C. P. Escoubet)
Foreword (F. Darrouzet, J. De Keyser, & V. Pierrard)
CLUSTER and IMAGE: New Ways to Study the Earth's Plasmasphere (J. De Keyser, et al.)
Plasmaspheric Density Structures and Dynamics: Properties Observed by the CLUSTER and IMAGE Missions
(F. Darrouzet, et al.)
Electric Fields and Magnetic Fields in the Plasmasphere: A Perspective from CLUSTER and IMAGE (H. Matsui, et al.)
Advances in Plasmaspheric Wave Research with CLUSTER and IMAGE Observations (A. Masson, et al.)
Recent Progress in Physics-Based Models of the Plasmasphere (V. Pierrard, et al.)
Augmented Empirical Models of Plasmaspheric Density and Electric Field Using IMAGE and CLUSTER Data (B. W. Reinisch, et al.)
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| Publication date: 21 Aug 2009 |
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