ESA Science & Technology - Publication Archive

The width of the collisionless shock front is one of the key shock parameters. The width of the main shock transition layer is related to the nature of the collisionless process that balances nonlinearity and therefore leads to the formation of the shock itself. The shock width determines how the incoming plasma particles interact with the macroscopic fields within the front and, therefore, the processes that result in the energy redistribution at the front. Cluster and Themis measurements at the quasi-perpendicular part of the terrestrial bow shock are used to study the spatial scale of the magnetic ramp. It is shown that statistically the ramp spatial scale decreases with the increase of the shock Mach number. This decrease of the shock scale together with previously observed whistler packets in the foot of supercritical quasi-perpendicular shock indicates that it is the dispersion that determines the size of magnetic ramp even for supercritical shocks.

Most of the high-altitude auroral electric fields observed by CLUSTER can be classified into monopolar and bipolar structures. The observations associate monopolar electric fields with polar cap boundary arcs, while bipolar fields tend to be linked to discrete arcs within the auroral oval and to polar cap arcs. The present paper proposes an explanation for this association based on a simple model of the magnetotail configuration and kinetic model computations. The paper introduces a quasi-electrostatic model to describe the auroral current system associated with monopolar and bipolar high-altitude fields. Analytic solutions are presented. The model gives indications about the location of the up- and downward field-aligned current regions, the ionospheric and magnetospheric convection along the arc, the acceleration or deceleration of precipitating particles, and the behaviour of escaping ionospheric ions.

There have been many significant advances in understanding magnetic field reconnection as a result of improved space measurements and two-dimensional computer simulations. While reviews of recent work have tended to focus on symmetric reconnection on ion and larger spatial scales, the present review will focus on asymmetric reconnection and on electron scale physics involving the reconnection site, parallel electric fields, and electron acceleration.

The objective of this review article is to critically analyze turbulence and its role in the solar atmosphere and solar wind, as well as to provide a tutorial overview of topics worth clarification. Although turbulence is a ubiquitous phenomenon in the sun and its heliosphere, many open questions exist concerning the physical mechanisms of turbulence generation in solar environment. Also, the spatial and temporal evolution of the turbulence in the solar atmosphere and solar wind are still poorly understood. We limit the scope of this paper (leaving out the solar interior and convection zone) to the magnetized plasma that reaches from the photosphere and chromosphere upwards to the corona and inner heliosphere, and place particular emphasis on the magnetic field structures and fluctuations and their role in the dynamics and radiation of the coronal plasma. To attract the attention of scientists from both the fluid-dynamics and space-science communities we give in the first two sections a phenomenological overview of turbulence-related processes, in the context of solar and heliospheric physics and with emphasis on the photosphere-corona connection and the coupling between the solar corona and solar wind. We also discuss the basic tools and standard concepts for the empirical analysis and theoretical description of turbulence. The last two sections of this paper give a concise review of selected aspects of oscillations and waves in the solar atmosphere and related fluctuations in the solar wind. We conclude with some recommendations and suggest topics for future research.

We report in situ observations of high-frequency electrostatic waves in the vicinity of a reconnection site in the Earth's magnetotail. Two different types of waves are observed inside an ion-scale magnetic flux rope embedded in a reconnecting current sheet. Electron holes (weak double layers) produced by the Buneman instability are observed in the density minimum in the center of the flux rope. Higher frequency broadband electrostatic waves with frequencies extending up to f_pe are driven by the electron beam and are observed in the denser part of the rope. Our observations demonstrate multiscale coupling during the reconnection: Electron-scale physics is induced by the dynamics of an ion-scale flux rope embedded in a yet larger-scale magnetic reconnection process.

Cluster observations at the Earth's high-latitude magnetopause are combined with magnetic field models to demonstrate that antiparallel reconnection was occurring at the magnetopause for an event on 3 December 2001. Over a 20 min period, the reconnection line passed over the spacecraft on two occasions. In between the encounters with the reconnection line, velocity cutoffs in the ion distributions are used to determine the distance to the reconnection site. These observations are consistent with an antiparallel reconnection line whose location relative to the spacecraft depends on the orientation of the interplanetary magnetic field. Using this knowledge of the reconnection site location and a previously developed, two-spacecraft method for computing the inflow velocity into the reconnection site, the reconnection rate (V_n/V_A) is determined to be <0.08. The rate is consistent with fast reconnection and considerably higher than the reconnection rate for a component reconnection event that was determined using the same two-spacecraft method.

We show the first three dimensional (3D) dispersion relations and k spectra of magnetic turbulence in the solar wind at subproton scales. We used the Cluster data with short separations and applied the k-filtering technique to the frequency range where the transition to subproton scales occurs. We show that the cascade is carried by highly oblique kinetic Alfvén waves with w_plas below 0.1w_ci down to k_perpRho_i ~ 2. Each k spectrum in the direction perpendicular to B_o shows two scaling ranges separated by a breakpoint (in the interval [0.4, 1]k_perpRho_i): a Kolmogorov scaling k_perp^-1.7 followed by a steeper scaling k_perp^-4.5. We conjecture that the turbulence undergoes a transition range, where part of the energy is dissipated into proton heating via Landau damping and the remaining energy cascades down to electron scales where electron Landau damping may predominate.

A careful analysis of the wave emissions for this event has shown that Cluster 4 passed through the wave source region. Simultaneous electron particle data from the PEACE instrument in the generation region indicated the presence of a mid-energy electron population (<100 s of eV) that had a highly anisotropic temperature distribution with the perpendicular temperature 10 times the parallel temperature. To understand this somewhat rare event in which the satellite passed directly through the wave generation region and in which a free energy source (i.e., temperature anisotropy) was readily identified, a linear theory and particle in cell simulation study has been carried out to elucidate the physics of the wave generation, wave-particle interactions, and energy redistribution. The theoretical results show that for this event the anisotropic electron distribution can linearly excite obliquely propagating whistler mode waves in the upper frequency band, i.e., above 0.5f_ce. Simulation results show that in addition to the upper band emissions, nonlinear wave-wave coupling excites waves in the lower frequency band, i.e., below 0.5f_ce. The instability saturates primarily by a decrease in the temperature anisotropy of the mid-energy electrons, but also by heating of the cold electron population. The resulting wave-particle interactions lead to the formation of a high-energy plateau on the parallel component of the warm electron velocity distribution. The theoretical results for the saturation time scale indicate that the observed anisotropic electron distribution must be refreshed in less than 0.1 s allowing the anisotropy to be detected by the electron particle instrument, which takes several seconds to produce a distribution.

One of the key unresolved problems in the study of space plasmas is to explain the production of energetic electrons as magnetic field lines "reconnect" and release energy in an explosive manner. Recent observations suggest possible roles played by small-scale magnetic islands in the reconnection region, but their precise roles and the exact mechanism of electron energization have remained unclear. Here we show from two-dimensional particle-in-cell simulations that secondary islands generated in the reconnection region indeed produce energetic electrons. We found that when electrons are trapped inside the islands, they are energized continuously by the reconnection electric field prevalent in the reconnection diffusion region. Applications to observations in the Earth's magnetotail are briefly discussed.

Magnetic reconnection plays a key role in the circulation of plasma through the Earth's magnetosphere. As such, the Earth's magnetotail is an excellent natural laboratory for the study of reconnection and in particular the diffusion region. To address important questions concerning observational occurrence rates and average properties, the Cluster data set from 2001-2005 has been systematically examined for encounters with reconnection X lines and ion diffusion regions in the Earth's magnetotail. This survey of 175 magnetotail passes resulted in a sample of 33 correlated field and flow reversals. Eighteen events exhibited electric and magnetic field perturbations qualitatively consistent with the predictions of antiparallel Hall reconnection and could be identified as diffusion region encounters. The magnitudes of both the Hall magnetic and electric field were found to vary from event to event. When normalized against the inflow magnetic field and the current sheet number density the average peak Hall magnetic field was found to be 0.39 ± 0.16, the average peak Hall electric field was found to be 0.33 ± 0.18, and the average out of plane (reconnection) electric field was found to be <0.04. Good quantitative agreement was found between these results and a large, appropriately renormalized particle-in-cell simulation of reconnection. In future missions, the magnitude of the total DC electric field may be a useful tool for automatically identifying ion diffusion region encounters.

In this paper, we report observations from a Cluster satellite showing that ULF wave occurred in the outer boundary of a plasmaspheric plume on September 4, 2005. The band of observed ULF waves is between the He⁺ ion gyrofrequency and O⁺ ion gyrofrequency at the equatorial plane, implying that those ULF waves can be identified as EMIC waves generated by ring current ions in the equatorial plane and strongly affected by rich cold He+ ions in plasmaspheric plumes. During the interval of observed EMIC waves, the footprint of Cluster SC3 lies in a subauroral proton arc observed by the IMAGE FUV instrument, demonstrating that the subauroral proton arc was caused by energetic ring current protons scattered into the loss cone under the Ring Current (RC)-EMIC interaction in the plasmaspheric plume. Therefore, the paper provides a direct proof that EMIC waves can be generated in the plasmaspheric plume and scatter RC ions to cause subauroral proton arcs.

In June 2006 Venus Express crossed several times the outer boundary of Venus induced magnetosphere, its magnetosheath and its bow shock. During the same interval the Cluster spacecraft surveyed the dawn flank of the terrestrial magnetosphere, intersected the Earth's magnetopause and spent long time intervals in the magnetosheath. This configuration offers the opportunity to perform a joint investigation of the interface between Venus and Earth's outer plasma layers and the shocked solar wind. We discuss the kinetic structure of the magnetopause of both planets, its global characteristics and the effects on the interaction between the planetary plasma and the solar wind. A Vlasov equilibrium model is constructed for both planetary magnetopauses and provides good estimates of the magnetic field profile across the interface. The model is also in agreement with plasma data and evidence the role of planetary and solar wind ions on the spatial scale of the equilibrium magnetopause of the two planets. The main characteristics of the two magnetopauses are discussed and compared.

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.

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.

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.

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.

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

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.

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.