ESA Science & Technology - Publication Archive

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.

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.

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.

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.

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.

Observations made by a unique constellation of Cluster (at 14-16 R_E), TC2, Goes10, and LANL spacecraft (near 6.6 R_E) 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 R_E). 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 B_y component in the turbulent reconnection outflow extended far beyond the ion diffusion region and existed for several minutes. We demonstrate an apparent V_x 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.

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 R_E). 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.

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.

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 B_Z and B_Y 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.

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.

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.

We use the multi-spacecraft mission Cluster to make observational estimates of the local energy conversion across the dayside high-latitude magnetopause. The energy conversion is estimated during eleven complete magnetopause crossings under steady south-dawnward interplanetary magnetic field (IMF). We describe a new method to determine the reconnection rate from the magnitude of the local energy conversion. The reconnection rate as well as the energy conversion varies during the course of the eleven crossings and is typically much higher for the outbound crossings. This supports the previous interpretation that reconnection is continuous but its rate is modulated.

We report on Cluster observations of a thin current sheet interval under the presence of a strong |B_Y| during a fast earthward flow interval between 1655 UT and 1703 UT on 17 August 2003. The strong |B_Y| in the tail could be associated with a strong IMF |B_Y|, but the large fluctuations in B_Y, not seen in the IMF, suggest that a varying reconnection rate causes a varying transport of B_Y-dominated magnetic flux and/or a change in B_Y due to the Hall-current system. During the encounter of the high-speed flow, an intense current layer was observed around 1655:53 UT with a peak current density of 182 nA/m², the largest current density observed by the Cluster four-spacecraft magnetic field measurement in the magnetotail. The half width of this current layer was estimated to be ~290 km, which was comparable to the ion-inertia length. Its unique signature is that the strong current is mainly field-aligned current flowing close to the center of the plasma sheet. The event was associated with parallel heating of electrons with asymmetries, which suggests that electrons moving along the field lines can contribute to a strong dawn-to-dusk current when the magnetotail current sheet becomes sufficiently thin and active in a strong guide field case.

The Energetic Particles Detector and magnetometer measurements on Galileo showed that the Jovian magnetosphere undergoes reconfiguration processes which are very similar to the characteristics of a terrestrial substorm. At Jupiter the reconfiguration process occurs quasi-periodically with a repetition period of several days. In the terrestrial magnetosphere periodic substorms have been observed during magnetic storms. The comparison of the periodic magnetospheric disturbances at Jupiter and Earth shows that they are similar in dynamic features as well as in spatial distribution but have different energy sources. In the case of Earth, the well-established energy source is the solar wind. In the case of the Jovian magnetosphere, it is believed that internal energy is supplied by the fast planetary rotation and the moon Io which releases ~1000 kg s^-1 of plasma into the magnetosphere. It is established that the energy accumulation and subsequent release lead to similar features in the magnetospheres of both planets. The particle data show periodic intensity fluctuations and plasma pressure variations. In addition, recurring signatures of stretching and dipolarization are observed in the magnetic field at the terrestrial and Jovian magnetospheres. Furthermore, the release process is associated with an intensification of auroral emissions. The typical phases for terrestrial substorms like growth, expansion and recovery are also found in the periodic substorms at Jupiter. As a lesson taken from the Jovian magnetosphere it is proposed that under certain conditions periodic magnetospheric substorms at Earth can be driven by mass-loading from the plasmasphere.

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.

The purpose of our study is to investigate the way particles are accelerated up to supra-thermal energies in the cusp diamagnetic cavities. For this reason we have examined a number of Cluster cusp crossings, originally identified by Zhang et al. (2005), for the years 2001 and 2002 using data from RAPID, STAFF, EFW, CIS, PEACE, and FGM experiments. In the present study we focus on two particular cusp crossings on 25 March 2002 and on 10 April 2002 which demonstrate in a clear way the general characteristics of the events in our survey. Both events exhibit very sharp spatial boundaries seen both in CNO (primarily single-charged oxygen of ionospheric origin based on CIS observations) and H⁺ flux increases within the RAPID energy range with the magnetic field intensity being anti-correlated. Unlike the first event, the second one shows also a moderate electron flux increase. The fact that the duskward electric field E_y has relatively low values <5 mV/m while the local wave activity is very intense provides a strong indication that particle energization is caused primarily by wave-particle interactions. The wave power spectra and propagation parameters during these cusp events are examined in detail. It is concluded that the high ion fluxes and at the same time the presence or absence of any sign of energization in the electrons clearly shows that the particle acceleration depends on the wave power near the local particle gyrofrequency and on the persistence of the wave-particle interaction process before particles escape from cusp region. Furthermore, the continuous existence of energetic O⁺ ions suggests that energetic O⁺ populations are of spatial nature at least for the eight events that we have studied so far.

A detailed analysis of successive tailward flow bursts in the near-Earth magnetotail (X~-19 R_E) plasma sheet is performed on the basis of in-situ multi-point observations by the Cluster spacecraft on 15 September 2001. The tailward flows were detected during a northward IMF interval, 2.5 h after a substorm expansion. Each flow burst (V_x<300 km/s) was associated with local auroral activation. Enhancements of the parallel and anti-parallel ~1 keV electron flux were detected during the flows. The spacecraft configuration enables to monitor the neutral sheet (B_x~0) and the level of B_x~10-15 nT simultaneously, giving a possibility to distinguish between closed plasmoid-like structures and open NFTE-like surges. The data analysis shows NFTE-like structures and localized current filaments embedded into the tailward plasma flow. 3-D shapes of the structures were reconstructed using the four-point magnetic filed measurements and the particle data.

We discuss the motion and structure of the magnetopause/boundary layer observed by Cluster in response to a joint tangential discontinuity/vortex sheet (TD/VS) observed by the Advanced Composition Explorer spacecraft on 7 December 2000. The observations are then supplemented by theory. Sharp polarity reversals in the east-west components of the field and flow B_y and V_y occurred at the discontinuity. These rotations were followed by a period of strongly northward interplanetary magnetic field (IMF). These two factors elicited a two-stage response at the magnetopause, as observed by Cluster situated in the boundary layer at the duskside terminator. First, the magnetopause suffered a large deformation from its equilibrium position, with large-amplitude oscillations of ~3-min period being set up. These are argued to be mainly the result of tangential stresses associated with DeltaV_y the contribution of dynamic pressure changes being small in comparison. This strengthens recent evidence of the importance to magnetospheric dynamics of changes in azimuthal solar wind flow. The TD/VS impact caused a global response seen by ground magnetometers in a magnetic local time range spanning at least 12 h. The response monitored on ground magnetometers is similar to that brought about by magnetopause motions driven by dynamic pressure changes. Second, Cluster recorded higher-frequency waves (~79 s). Two clear phases could be distinguished from the spectral power density, which decreased by a factor of ~3 in the second phase. Applying compressible linearized MHD theory, we show that these waves are generated by the Kelvin-Helmholtz (KH) instability. Varying the local magnetic shear at the Cluster locale, as suggested by the temporal profile of the IMF clock angle, we find that locally stability was reinstated, so that the reduced power in the second phase is argued to be due residual KH activity arriving from locations farther to the dayside.

Magnetic reconnection is one of the most fundamental processes in the magnetosphere. We present here a simple method to determine the essential parameters of reconnection such as reconnected flux and location of the reconnection site out of single spacecraft data via remote sensing. On the basis of a time-dependent reconnection model, the dependence of the reconnected flux on the magnetic field z-component B_z is shown. The integral of B_z over time is proportional to the reconnected flux and depends on the distance between the reconnection site and the actual position where B_z is measured. This distance can be estimated from analysis of magnetic field B_z data. We apply our method to Cluster measurements in the Earth's magnetotail.