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| Low-frequency fluctuations in the magnetosheath: Double Star TC-1 and Cluster observations |
| The magnetic field variations are analyzed in the range of time periods from 4 s to 240 s in the magnetosheath observed by the Double Star TC-1 and Cluster in 2004. The characteristics of the magnetic field fluctuations are strongly controlled by the angle between the upstream interplanetary magnetic field (IMF) and the normal of the bow shock. Generally speaking, the magnetic field fluctuations in the quasi-parallel magnetosheath are more intense than those in the quasi-perpendicular ones. Almost purely compressional waves are found in the quasi-perpendicular magnetosheath. With the increase of the local plasma beta, both the magnitude and direction of the magnetic field fluctuate more intensely. There exists an inverse correlation between the local temperature anisotropy Tperp./Tparallel and the plasma beta. |
| Publication date: 14 Sep 2008 |
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| High-latitude plasma convection during Northward IMF as derived from in-situ magnetospheric Cluster EDI measurements |
| In this study, we investigate statistical, systematic variations of the high-latitude convection cell structure during northward IMF. Using 1-min-averages of Cluster/EDI electron drift observations above the Northern and Southern polar cap areas for six and a half years (February 2001 till July 2007), and mapping the spatially distributed measurements to a common reference plane at ionospheric level in a magnetic latitude/MLT grid, we obtained regular drift patterns according to the various IMF conditions. We focus on the particular conditions during northward IMF, where lobe cells at magnetic latitudes >80° with opposite (sunward) convection over the central polar cap are a permanent feature in addition to the main convection cells at lower latitudes. They are due to reconnection processes at the magnetopause boundary poleward of the cusp regions. Mapped EDI data have a particular good coverage within the central part of the polar cap, so that these patterns and their dependence on various solar wind conditions are well verified in a statistical sense. On average, 4-cell convection pattern are shown as regular structures during periods of nearly northward IMF with the tendency of a small shift toward negative clock angles. The positions of these high-latitude convection foci are within 79° to 85° magnetic latitude and 09:00-15:00 MLT. The MLT positions are approximately symmetric ±2 h about 11:30 MLT, i.e. slightly offset from midday toward prenoon hours, while the maximum (minimum) potential of the high-latitude cells is at higher magnetic latitudes near their maximum potential difference at ~-10° to -15° clock angle for the North (South) Hemisphere. - Remainder of abstract truncated - |
| Publication date: 12 Sep 2008 |
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| Wave activity inside hot flow anomaly cavities |
| Low-frequency wave properties inside two hot flow anomalies (HFAs) at different stages of evolution are, for the first time, studied applying the k-filtering technique on multipoint measurements from the Cluster satellites. The observed wave activity in an HFA cavity in an early stage of its evolution was interpreted as the combination of inherent fluctuation in the solar wind and those of a plasma component specularly reflected at the Earth's bow shock, where the amplitude of the fluctuations had been enhanced by a plasma beam instability. The wave field of a more evolved HFA was found to be less complex but contained a periodicity in the wave number distribution with a period that is suggested to come from the geometry of the HFA cavity. |
| Publication date: 23 Aug 2008 |
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| Transport of transient solar wind particles in Earth's cusps |
| An important problem in space physics still not understood well is how the solar wind enters the Earth's magnetosphere. Evidence is presented that transient solar wind particles produced by solar disturbances can appear in the Earth's mid-altitude (~5 RE geocentric) cusps with densities nearly equal to those in the magnetosheath. That these are magnetosheath particles is established by showing they have the same "flattop" electron distributions as magnetosheath electrons behind the bow shock. The transient ions are moving parallel to the magnetic field (B) toward Earth and often coexist with ionospheric particles that are flowing out. The accompanying waves include electromagnetic and broadband electrostatic noise emissions and Bernstein mode waves. Phase-space distributions show a mixture of hot and cold electrons and multiple ion species including field-aligned ionospheric O+ beams. |
| Publication date: 11 Aug 2008 |
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| A magnetic null geometry reconstructed from Cluster spacecraft observations |
| This paper reports for the first time the identification of a magnetic structure around a magnetic null in a magnetic reconnection region in the magnetotail. Magnetic reconnection is one of the fundamental processes in astrophysical and solar-terrestrial plasmas. Though the concept of reconnection has been studied for many years, the process that really occurs has not been fully revealed by direct measurements. In particular, the lack of a description of three-dimensional (3-D) reconnecting magnetic field from observations makes the task more difficult. The Cluster spacecraft array provide an opportunity to reconstruct the 3-D magnetic reconnection structure based on magnetic field vectors simultaneously measured at four positions. The identification of this structure comes from a new method of analysis of in situ measurements proposed here. Applying a fitting model of 10 spherical harmonic functions and a Harris current sheet function, plus a constant field, we reconstruct a 3-D magnetic field configuration around the magnetic null in an reconnection event observed by Cluster in the geo-magnetotail. |
| Publication date: 08 Aug 2008 |
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| Statistical analysis of plasmaspheric plumes with Cluster/WHISPER observations |
| Plasmaspheric plumes have been routinely observed by the four Cluster spacecraft. This paper presents a statistical analysis of plumes observed during five years (from 1 February 2001 to 1 February 2006) based on four-point measurements of the plasmasphere (outside 4 Earth radii) as it is sampled by the spacecraft in a narrow local time sector before and after perigee. Plasmaspheric plumes can be identified from electron density profiles derived from the electron plasma frequency determined by the WHISPER wave sounder onboard Cluster. As the WHISPER instrument has a limited frequency range (2-80 kHz) only plumes with densities below 80 cm-3 can be identified in this way. Their occurrence is studied as a function of several geomagnetic indices (Kp, am and Dst). Their transverse equatorial size, magnetic local time distribution, L position and density variation are discussed. Plasmaspheric plumes are observed mostly for moderate Kp and never for small Dst. They are found mainly in the afternoon and pre-midnight MLT sectors. Comparisons are also made between the density profiles of the plumes as they are crossed on the in- and outbound legs of the orbit, before and after perigee crossing, respectively. |
| Publication date: 06 Aug 2008 |
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| Diagnosis of magnetic structures and intermittency in space-plasma turbulence using the technique of surrogate data |
| Intermittency is usually identified in turbulent flows as non-Gaussian tails of the probability density functions (PDFs) of the turbulent field derivatives. Here we investigate the role of phase coherence among the Fourier modes in creating intermittency in magnetized space plasmas using the technique of surrogate data. We apply the technique to two examples: (i) synthetic data and (ii) magnetic field fluctuations recorded in the terrestrial magnetosheath by the Cluster spacecraft. We use a set of four series of data, one observed and three surrogate, and their PDFs and moments (q<=4) as discriminating statistics. We show that the technique allows for detecting coherent structures and estimating their scales. We show furthermore that the phases, but not the amplitudes, create the non-Gaussian tails of the PDFs. We show also that the surrogate data used cannot account for asymmetries of the PDFs of the observed data. This enables us to confirm a scenario of turbulent cascade of mirror structures proposed in previous publications, by showing the existence of an approximately constant energy flux in the inertial range. |
| Publication date: 05 Aug 2008 |
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| Characteristic Parameters of Drift Vortices Coupled to Alfvén Waves in an Inhomogeneous Space Plasma |
| We present detailed measurements of ion scale vortices of drift type coupled to Alfvén waves in an inhomogeneous and collisionless space magnetoplasma. The two free parameters of a dipolar vortex, intensity and spatial radius, are measured. The vortices are driven by a strong density gradient on a boundary layer with scale size of the same order as the vortex diameter. Observations of vortices off the gradient show that symmetry-breaking conditions in a real inhomogeneous plasma can lead not only to cross-field but also to cross-boundary anomalous transport of particles and energy. |
| Publication date: 04 Aug 2008 |
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| Electron trapping around a magnetic null |
| Magnetic reconnection is an important process in astrophysical, space and laboratory plasmas. The magnetic null pair structure is theoretically suggested to be a crucial feature of the three-dimensional magnetic reconnection. The physics around the null pair, however, has not been explored in combination with the magnetic field configuration deduced from in situ observations. Here, we report the identification of the configuration around a null pair and simultaneous electron dynamics near one null of the pair, observed by four Cluster spacecraft in the geo-magnetotail. Further, we propose a new scenario of electron dynamics in the null region, suggesting that electrons are temporarily trapped in the central reconnection region including electron diffusion region resulting in an electron density peak, accelerated possibly by parallel electric field and electron pressure gradient, and reflected from the magnetic cusp mirrors leading to the bi-directional energetic electron beams, which excite the observed high frequency electrostatic waves. |
| Publication date: 29 Jul 2008 |
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| Occurrence of reconnection jets at the dayside magnetopause: Double Star observations |
| We present a statistical study on reconnection occurrence at the dayside magnetopause performed using the Double Star TC1 plasma and magnetic field data. We examined the magnetopause crossings that occurred during the first year of the mission in the 0600-1800 LT interval and we identified plasma flows, at the magnetopause or in the boundary layer, with a different velocity with respect to the adjacent magnetosheath. We used the Walén relation to test which of these flows could be generated by magnetic reconnection. For some event we observed opposite-directed reconnection jets, which could be associated with the passage of the X-line near the satellite. We analyzed the occurrence of the reconnection jets and reconnection jet reversals in relation to the magnetosheath parameters, in particular the local Alfvèn Mach number, the plasma beta, and the magnetic shear angle. We also studied the positions and velocities of the reconnection jets and jet reversals in relation to the magnetosheath magnetic field clock angle. We found that the observations indicate the presence of a reconnection line hinged near the subsolar point and tilted according to the observed magnetosheath clock angle, consistently with the component merging model. |
| Publication date: 27 Jun 2008 |
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| Cluster multispacecraft determination of AKR angular beaming |
| Simultaneous observations of AKR emission using the four-spacecraft Cluster array were used to make the first direct measurements of the angular beaming patterns of individual bursts. By comparing the spacecraft locations and AKR burst locations, the angular beaming pattern was found to be narrowly confined to a plane containing the magnetic field vector at the source and tangent to a circle of constant latitude. Most rays paths are confined within 15° of this tangent plane, consistent with numerical simulations of AKR k-vector orientation at maximum growth rate. The emission is also strongly directed upward in the tangent plane, which we interpret as refraction of the rays as they leave the auroral cavity. The narrow beaming pattern implies that an observer located above the polar cap can detect AKR emission only from a small fraction of the auroral oval at a given location. This has important consequences for interpreting AKR visibility at a given location. It also helps re-interpret previously published Cluster VLBI studies of AKR source locations, which are now seen to be only a subset of all possible source locations. The observations are inconsistent with either filled or hollow cone beaming models. |
| Publication date: 24 Jun 2008 |
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| The wave surveyor technique for fast plasma wave detection in multi-spacecraft data |
| Multi-satellite missions like Cluster allow to study the full spatio-temporal variability of plasma processes in near-Earth space, and both the frequency and the wave vector dependence of dispersion relations can be reconstructed. Existing wave analysis methods include high-resolution beamformers like the wave telescope or k-filtering technique, and the phase differencing approach that combines the correlations measured at pairs of sensors of the spacecraft array. In this paper, we make use of the eigendecomposition of the cross spectral density matrix to construct a direct wave identification method that we choose to call the wave surveyor technique. The analysis scheme extracts only the dominant wave mode but is much faster to apply than existing techniques, hence it is expected to ease survey-type detection of waves in large data sets. The wave surveyor technique is demonstrated by means of synthetic data, and is also applied to Cluster magnetometer measurements. |
| Publication date: 18 Jun 2008 |
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| Study of near-Earth reconnection events with Cluster and Double Star |
| Observations made by a unique constellation of Cluster (at 14-16 RE), TC2, Goes10, and LANL spacecraft (near 6.6 RE) have allowed us to study the details of three reconnection events in the middle of a thick plasma sheet with the reconnection X-line located unusually close to Earth (10-12 RE). We use mapping along field lines with magnetospheric models adapted to magnetic field observations to confirm that the reconnection region mapped onto localized auroral brightenings. Using simultaneous observations in the inflow and outflow regions, we describe an encounter with a localized tailward Alfvénic jet produced by a short isolated reconnection pulse. A good correlation between intense E and ion [BV] indicates that the concurrent strong turbulence could not destroy the frozen-in ion behavior in the reconnection outflow. We find that a steady quadrupole-like distribution of the magnetic By component in the turbulent reconnection outflow extended far beyond the ion diffusion region and existed for several minutes. We demonstrate an apparent Vx flow reversal, formed owing to the reappearance (switch-on) of reconnection at another location, rather than to a continuous motion of the active X-line. Using the Liouville mapping technique, we show that the acceleration of outflow electrons, after the particles passed a potential drop of 180 V, is consistent with Fermi/betatron acceleration. We also suggest another interpretation of the energetic particle bursts at the onsets, to emphasize the role of seed population and explain the sudden burst as a consequence of changing magnetic topology. |
| Publication date: 05 Jun 2008 |
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| Downward current electron beam observed by Cluster and FAST |
| We report observations from a conjunction of FAST and Cluster during an interval of downward current at an MLT of 3-4 h on field lines mapping to the PSBL. Both spacecraft see upgoing electrons with an energy of a few hundred eV, suggesting substantial acceleration has occurred below FAST's altitude of 3200 km. At Cluster, isolated bursts of electrons are seen, and modeling indicates that the current mapped from the ionosphere exists as a collection of current filaments at Cluster (4-5 RE). The current filaments are aligned with the background magnetic field and have a perpendicular scale at Cluster of about 100 km (which maps to 10-20 km in the ionosphere), and is similar to the mapped width observed by FAST. The electron beams are quasi-steady during a Cluster spacecraft transit time of 1 min. The field aligned current densities at FAST and Cluster are of the order of a few microAms-2 and 0.05 microAm-2, respectively, and j/B is conserved along a current filament. |
| Publication date: 03 Jun 2008 |
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| High kinetic energy jets in the Earth's magnetosheath: Implications for plasma dynamics and anomalous transport |
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| Publication date: 30 May 2008 |
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| Magnetosheath Plasma Turbulence and Its Spatiotemporal Evolution as Observed by the Cluster Spacecraft |
| We study the plasma turbulence, at scales larger than the ion inertial length scale, downstream of a quasiparallel bow shock using Cluster multispacecraft measurements. We show that turbulence is intermittent and well described by the extended structure function model, which takes into account the spatial inhomogeneity of the cascade rate. For the first time we use multispacecraft observations to characterize the evolution of magnetosheath turbulence, particularly its intermittency, as a function of the distance from the bow shock. The intermittency significantly changes over the distance of the order of 100 ion inertial lengths, being increasingly stronger and anisotropic away from the bow shock. |
| Publication date: 21 May 2008 |
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| Multispacecraft observation of electron beam in reconnection region |
| On the 18th of August 2002, during a crossing of the near-Earth plasma sheet Cluster observed an ion flow burst, caused by a near-Earth reconnection event. Cluster observed a tailward bulk flow which reverse to earthward flow, indicating a close passage of the diffusion region. We show that reversals in BZ and BY are consistent with reconnection. During the event, a short duration burst of electrons in the range of a few keV up to more than 100 keV are observed streaming away from the reconnection region. The accelerated electrons were aligned with the magnetic field, streaming tailward, and were observed simultaneously by all four spacecraft located on both the northern and southern side of the current sheet. The four Cluster spacecraft, separated by ~3700 km, observe the electrons for a time period of ~60 s, indicating the burst to be a temporal rather than localized feature. A second burst of tailward accelerated electrons observed for ~40 s was observed by Cluster 1 and 2 upon entering the earthward outflow region. The second beam thus appear to be directed toward the X-line. The flux levels of the energetic electron bursts exceed those of the ambient plasma sheet by a factor 2-4. In general, the highest energetic electron fluxes during this event were observed in the earthward outflow region. Observations indicate that reconnection operates on closed plasma sheet field lines in this event and does not reach lobe field lines. |
| Publication date: 10 May 2008 |
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| Reconnection at the dayside magnetopause: Comparisons of global MHD simulation results with Cluster and Double Star observations |
| This study uses two conjunctions between Cluster and Double Star TC-1 spacecraft together with global magnetohydrodynamic (MHD) simulations to investigate the large-scale configuration of magnetic reconnection at the dayside magnetopause. Both events involve southward interplanetary magnetic fields with significant By components. The first event occurred on 8 May 2004, while both spacecraft were exploring the dawn flank of the magnetosphere; TC-1 was skimming the magnetopause whereas Cluster was exploring higher latitudes. Results from a global MHD simulation show the formation of an equatorial merging line in the morning sector and suggest that the three-dimensional geometry of the merging region is mostly a radial juxtaposition of planes displaying X-type reconnection geometries. The second conjunction was on 6 April 2004. During this event, Cluster was located at high latitudes and close to the noon-midnight meridian, while TC-1 was exploring the dawnside at low latitudes. Analysis of the simulation reveals that both antiparallel and component merging occurred simultaneously. Three-dimensional rendering of the parallel electric field indicates that component merging initiated in the subsolar magnetopause. Simulation runs carried out using different parameters in the model suggest that the spread of the merging region depends on the local resistivity. The subsolar-merging region grows with increasing resistivity values and becomes patchy when a resistivity threshold is used. However, the region of component merging appears to remain spatially constrained to the subsolar region where stronger parallel electric fields occur and no clear connection with the antiparallel-merging regions is found for the range of parameters surveyed. |
| Publication date: 08 May 2008 |
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| Effect of a northward turning of the interplanetary magnetic field on cusp precipitation as observed by Cluster |
| The immediate effect of the rotation of the interplanetary magnetic field (IMF) from southward to northward on cusp precipitation has been rarely observed by a polar orbiting satellite in the past. The four Cluster spacecraft observed such an event on 23 September 2004 as they were crossing the polar cusp within 2-16 min from each other. Between the first three and the last spacecraft crossing the cusp, the IMF rotated from southward to northward with a dominant By (GSM) component. For the first time we can examine the changes in the particle precipitation immediately after such IMF change. The first two spacecraft observed typical IMF-southward ion dispersion, while the last one observed both an IMF-southward-like dispersion in the boundary layer and an IMF-northward dispersion in the cusp. After the IMF turning, the cusp is shown to have grown in size in both the poleward and equatorward directions. A three-dimensional magnetohydrodynamic simulation is used to determine the locations of the sources of the ions and the topology of the magnetic field during the event. |
| Publication date: 02 May 2008 |
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| Electron density estimations derived from spacecraft potential measurements on Cluster in tenuous plasma regions |
| Spacecraft potential measurements by the EFW electric field experiment on the Cluster satellites can be used to obtain plasma density estimates in regions barely accessible to other type of plasma experiments. Direct calibrations of the plasma density as a function of the measured potential difference between the spacecraft and the probes can be carried out in the solar wind, the magnetosheath, and the plasmashere by the use of CIS ion density and WHISPER electron density measurements. The spacecraft photoelectron characteristic (photoelectrons escaping to the plasma in current balance with collected ambient electrons) can be calculated from knowledge of the electron current to the spacecraft based on plasma density and electron temperature data from the above mentioned experiments and can be extended to more positive spacecraft potentials by CIS ion and the PEACE electron experiments in the plasma sheet. This characteristic enables determination of the electron density as a function of spacecraft potential over the polar caps and in the lobes of the magnetosphere, regions where other experiments on Cluster have intrinsic limitations. Data from 2001 to 2006 reveal that the photoelectron characteristics of the Cluster spacecraft as well as the electric field probes vary with the solar cycle and solar activity. The consequences for plasma density measurements are addressed. Typical examples are presented to demonstrate the use of this technique in a polar cap/lobe plasma. |
| Publication date: 02 May 2008 |
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