ESA Science & Technology - Publication Archive

We have used vector measurements of the electron drift velocity made by the Electron Drift Instrument (EDI) on Cluster between February 2001 and March 2006 to derive statistical maps of the high-latitude plasma convection. The EDI measurements, obtained at geocentric distances between ~4 and ~20 R_E over both hemispheres, are mapped into the polar ionosphere, and sorted according to the clock-angle of the interplanetary magnetic field (IMF), measured at ACE and propagated to Earth, using best estimates of the orientation of the IMF variations. Only intervals of stable IMF are used, based on the magnitude of a "bias-vector" constructed from 30-min averages. The resulting data set consists of a total of 5862 h of EDI data. Contour maps of the electric potential in the polar ionosphere are subsequently derived from the mapped and averaged ionospheric drift vectors. Comparison with published statistical results based on Super Dual Auroral Radar Network (SuperDARN) radar and low-altitude satellite measurements shows excellent agreement between the average convection patterns, and in particular the lack of mirror-symmetry between the effects of positive and negative IMF B_y, the appearance of a duskward flow component for strongly southward IMF, and the general weakening of the average flows and potentials for northerly IMF directions. This agreement lends credence to the validity of the assumption underlying the mapping of the EDI data, namely that magnetic field lines are equipotentials. For strongly northward IMF the mapped EDI data show the clear emergence of two counter-rotating lobe cells with a channel of sunward flow between them. The total potential drops across the polar caps obtained from the mapped EDI data are intermediate between the radar and the low-altitude satellite results.

The STAFF-SC observations complemented by the data from other instruments on Cluster spacecraft were used to study the main properties of magnetospheric lion roars: sporadic bursts of whistler emissions at f~0.1-0.2f_e where f_e is the electron gyrofrequency. Magnetospheric lion roars are shown to be similar to the emissions in the magnetosheath while the conditions for their generation are much less favorable: the growth rate of the cyclotron temperature anisotropy instability is much smaller due to a smaller number of the resonant electrons. This implies a nonlinear mechanism of generation of the observed wave emissions. It is shown that the observed whistler turbulence, in reality, consists of many nearly monochromatic wave packets. It is suggested that these structures are nonlinear Gendrin's whistler solitary waves. Properties of these waves are widely discussed. Since the group velocity of Gendrin's waves is aligned with the magnetic field, these well guided wave packets can propagate through many magnetic "bottles" associated with mirror structures, without being trapped.

We report strong repeated magnetic reconnection pulses that occurred deep inside closed plasma sheet flux tubes at r <= 14Re. They have been observed with a fortuitous spacecraft constellation during three consecutive turbulent magnetic dipolarizations, accompanied by localized auroral brightenings near the equatorward edge of a wide auroral oval. The reconnection separatrix was mapped to ~64° CGLat in the ionosphere, where a very energetic and narrow energy-dispersed ion injection with unusually steep dispersion slope was observed. Reconstruction of the reconnection rate from magnetic waveforms at Cluster provided a reconnection pulse duration (~1 min) and peak strength (E_R ~ 8 mV/m) consistent with direct observations in the reconnection outflow region. The magnetic activity was rather weak, although the concurrent solar wind flow pressure was above the norm. We suggest that near-Earth reconnection events may be a phenomenon more frequent than generally thought. We also confirm that reconnection and the growth of strong turbulence in the near tail are strongly coupled together in near-Earth reconnection events.

In: Reconnection of Magnetic Fields, ed. by J. Birn and E. R. Priest, Cambridge University Press, Cambridge, ISBN-13: 978-0-521-85420-7

Cluster observations are used to illustrate the reconfiguration of an auroral potential structure encountered at the poleward boundary of the central plasma sheet within the Southern Hemisphere premidnight auroral oval. The reconfiguration from a symmetric U shape to an asymmetric S shape takes place between two consecutive crossings by Cluster spacecraft 1 and 2, moving along roughly the same orbits and separated in time by 16 minutes. During this time the plasma conditions poleward of the boundary changed dramatically. The fluxes of energetic electrons decreased, as did the intensities of the associated small-scale field-aligned currents (FACs) and the ambient plasma density. These changes were particularly pronounced in a narrow region adjacent to the boundary. The reconfiguration of the potential structure, and of the associated FAC system consistent with this, are consistent with the predictions by Marklund et al. (2004).

Fast reconnection is crucial to magnetospheric substorms, solar and stellar flares and fusion plasmas. Ultimate confirmation of fast reconnection must be achieved by multi-spacecraft detections of the reconnection rate itself and associated dimensions of the diffusion region. Here we report a multi-spacecraft measurement of fast reconnection rate gamma_rec ~ V_in ~ (0.07-0.15)V_A based on directly measurements of the plasma flow into the diffusion region, where V_in is the speed of reconnecting flux and V_A the characteristic Alfvén speed. It falls in the range of ~(0.03-0.2)V_A predicted in steady state reconnection simulation. The characteristic sizes for the diffusion region of the width L_z ~ 0.9 d_i (= 460 km) and the length L_x ~ (3.3-5.1)d_i (= 1680-2597 km) are measured as well. The length of the diffusion region L_x is determined for the first time based on the in situ observations. Furthermore, other features detected during the event also match the previous observation and simulation results.

Cluster crossed the magnetotail neutral sheet on four occasions between 16:38 and 16:43 UT on 08/17/2003. The four-spacecraft capabilities of Cluster are used to determine spatial gradients from the magnetic field vectors and, for the first time, full electron pressure tensors. We find that the contribution to the electric field from the Hall term (max of ~6 mV/m) pointed towards the neutral sheet, whereas that from the electron pressure divergence (max of ~1 mV/m) pointed away from the neutral sheet. The electric field contributions in this direction were closely anti-correlated. During this period Clusters 1 and 4 were sometimes above and below the neutral sheet respectively. This allowed the simultaneous observation of magnetic fields that are interpreted as two quadrants of the Hall magnetic field system. An associated field-aligned current system was detected using the curlometer and moments of the particle distributions.

We present a statistical study of four years of Cluster crossings of the mid-altitude cusp. In this first part of the study, we start by introducing the method we have used a) to define the cusp properties, b) to sort the interplanetary magnetic field (IMF) conditions or behaviors into classes, c) to determine the proper time delay between the solar wind monitors and Cluster. Out of the 920 passes that we have analyzed, only 261 fulfill our criteria and are considered as cusp crossings. We look at the size, location and dynamics of the mid-altitude cusp under various IMF orientations and solar wind conditions. For southward IMF, B_z rules the latitudinal dynamics, whereas B_y governs the zonal dynamics, confirming previous works. We show that when |B_y| is larger than |B_z|, the cusp widens and its location decorrelates from B_y. We interpret this feature in terms of component reconnection occurring under B_y-dominated IMF. For northward IMF, we demonstrate that the location of the cusp depends primarily upon the solar wind dynamic pressure and upon the Y-component of the IMF. Also, the multipoint capability of Cluster allows us to conclude that the cusp needs typically more than ~20 min to fully adjust its location and size in response to changes in external conditions, and its speed is correlated to variations in the amplitude of IMF-B_z. Indeed, the velocity in °ILAT/min of the cusp appears to be proportional to the variation in B_z in nT: V_cusp=0.024 Delta-B_z. Finally, we observe differences in the behavior of the cusp in the two hemispheres. Those differences suggest that the cusp moves and widens more freely in the summer hemisphere.

On 3 July 2001, the four Cluster satellites traversed along the dawnside magnetospheric flank and observed large variations in all plasma parameters. The estimated magnetopause boundary normals were oscillating in the z-direction and the normal component of the magnetic field showed systematic ~2-3 min bipolar variations for 1h when the IMF had a small positive b_z-component and a Parker-spiral orientation in the x,y-plane. Brief ~33 s intervals with excellent deHoffman Teller frames were observed satisfying the Walén relation. Detailed comparisons with 2-D MHD simulations indicate that Cluster encountered rotational discontinuities generated by Kelvin-Helmholtz instability. We estimate a wave length of ~6 R_E and a wave vector with a significant z-component.

Propagation pattern (distribution of phase velocities) is determined in three dimensions in the terrestrial magnetosheath on a statistical basis using Cluster spacecraft observations. It is found that the anti-sunward propagation dominates and that the propagation direction is toward the magnetosheath flank at smaller zenith angles, while it is toward the magnetopause at larger angles. This pattern is axially symmetric regardless of the interplanetary magnetic field direction and agrees qualitatively with the density gradient directions in a hydromagnetic flow model of the magnetosheath, suggesting that the wave refraction mechanism is more significant than the wave drift effect.

The nature of particle precipitations at dayside mid-altitudes can be interpreted in terms of the evolution of reconnected field lines. Due to the difference between electron and ion parallel velocities, two distinct boundary layers should be observed at mid-altitudes between the boundary between open and closed field lines and the injections in the cusp proper. At lowest latitudes, the electron-dominated boundary layer, named the "electron edge" of the Low-Latitude Boundary Layer (LLBL), contains soft-magnetosheath electrons but only high-energy ions of plasma sheet origin. A second layer, the LLBL proper, is a mixture of both ions and electrons with characteristic magnetosheath energies. The Cluster spacecraft frequently observe these two boundary layers. We present an illustrative example of a Cluster mid-altitude cusp crossing with an extended electron edge of the LLBL. This electron edge contains 10-200 eV, low-density, isotropic electrons, presumably originating from the solar wind halo population. These are occasionally observed with bursts of parallel and/or anti-parallel-directed electron beams with higher fluxes, which are possibly accelerated near the magnetopause X-line. We then use 3 years of data from mid-altitude cusp crossings (327 events) to carry out a statistical study of the location and size of the electron edge of the LLBL. We find that the equatorward boundary of the LLBL electron edge is observed at 10:00-17:00 magnetic local time (MLT) and is located typically between 68° and 80° invariant latitude (ILAT). The location of the electron edge shows a weak, but significant, dependence on some of the external parameters (solar wind pressure, and IMF B_Z- component), in agreement with expectations from previous studies of the cusp location.

CiteID: A10211 The Cluster spacecraft crossed the magnetopause at the duskward flank of the tail while the European Incoherent Scatter (EISCAT) radars and magnetometers observed the ionosphere during a sequence of intense substorm-like geomagnetic activity in October 2003. We attempt to estimate the local and global energy flow from the magnetosheath into the magnetotail and the ionosphere under these extreme conditions. We make for the first time direct observational estimates of the local solar wind power input using Cluster measurements. The global power input based on Cluster observations was found to be between 17 and 40 TW at the onset of the substorm intensification. However, spacecraft observations and global modelling of the magnetotail suggest that it is most probably closer to 17 TW. This is more than two times lower than the predicted parameter value (37 TW). Energy deposition in the ionosphere has been estimated locally with EISCAT and globally with the assimilated mapping of ionospheric electrodynamics (AMIE) technique. The amount of the global solar wind power input (17 TW) that is dissipated via Joule heating in the ionosphere is found to be 30%. The corresponding ratio based on empirical estimates is only 3%. However, empirical proxies seem to underestimate the magnitude of the Joule heating rate as compared to AMIE estimates (~ a factor 4) and the epsilon parameter is more than twice as large as the Cluster estimate. In summary, the observational estimates provide a good balance between the energy input to the magnetosphere and deposition in the ionosphere. Empirical proxies seem to suffer from overestimations (epsilon parameter) and underestimations (Joule heating proxies) when pushed to the extreme circumstances during the early main phase of this storm period.

CiteID: L19105 We report on electron phase space distributions (PSDs) observed near the plasma sheet (PS) boundary layer (PSBL) by the Cluster electron spectrometers when the northern lobe was occupied by significant fluxes of polar rain (PR) electrons. These observations reveal the spatial structure of the electron transition layer (TL) between the polar rain electrons and the PSBL electron population accelerated during reconnection. This TL comprises overlapping spatial dispersion signatures in both energy and pitch angle, which are caused by convection of flux tubes across the magnetic separatrix during the electron time-of-flight (TOF) from the X-line combined with acceleration in the reconnection region. Analysis of this structure allows us to estimate the location of the X-line. By assuming the PSBL population arises through acceleration of the PR electrons, comparison of their PSD indicates that the electrons gain energy proportional to their initial energy.

CiteID: A09221 The use of sensor arrays in space opens up the possibility to investigate the source location of plasma waves. In order to do so, we generalize the wave telescope technique to use spherical waves instead of plane waves. The new tool determines the center of the wavefronts locally measured by the sensor array. This virtual source can be related with the position of the source generating the detected waves provided the wave propagation mode and medium properties are known. Moreover, the motion of the source region can be derived, as well as its basic geometrical characteristics. In this work we give the theoretical background for the new tool and examples of location analysis based on synthetic data. An example based on Cluster magnetic field data in the dayside magnetosheath reveals a virtual source region elongated in the magnetic field direction, moving with the plasma flow in the vicinity of the spacecraft configuration.

American Geophysical Union, Fall Meeting 2005, abstract #SM23C-06

Synoptic measurements from the DOUBLE STAR and CLUSTER spacecraft offer a unique opportunity to evaluate global models in simulating the complex topology and dynamics of the dayside merging region. We compare observations from the DOUBLE STAR TC-1 and CLUSTER spacecraft on May 8, 2004 with the predictions from a three-dimensional magnetohydrodynamic (MHD) simulation that uses plasma and magnetic field parameters measured upstream of the bow shock by the WIND spacecraft. Results from the global simulation are consistent with the large-scale features observed by CLUSTER and TC-1. We discuss topological changes and plasma flows at the dayside magnetospheric boundary inferred from the simulation results. The simulation shows that the DOUBLE STAR spacecraft passed through the dawn side merging region as the IMF rotated. In particular, the simulation indicates that at times TC-1 was very close to the merging region. In addition, we found that the bifurcation of the merging region in the simulation results is consistent with predictions by the antiparallel merging model. However, because of the draping of the magnetosheath field lines over the magnetopause, the positions and shape of the merging region differ significantly from those predicted by the model.

We investigate the nonlinear influence of the cross-tail currents carried by beamlets (substructures of PSBL ion beams) on the topology of the magnetic field, and, correspondingly, on the dispersion properties of these substructures self-consistently generated in this field. We found that some of the peculiarities of beamlet shapes found recently in CLUSTER data could be explained by taking into account the nonlinearity of the system. This model explains the steepening of local beamlets dispersion in comparison with the global dispersion of the enveloping VDIS structure. At the same time we found that velocity filter effects operating during beamlets propagation toward the Earth prevent the sign's reversal of this local dispersion.

We present an analysis of the electric and magnetic wave spectra on kinetic scales during several crossings of a reconnecting current sheet. The spectra were measured from 1 Hz or less up to 4096 Hz by the EFW, FGM and STAFF instruments onboard the Cluster spacecraft between 3 and 4 UT on 11 October 2001. During the event plasma flows of order of the local Alfvén speed reversed from tailward to earthward, suggesting that a reconnection site moved over the spacecraft. We ordered the observed electric and magnetic field wave spectrum by the position within the current sheet using the magnitude of the magnetic field B. We found that the electric and magnetic wave power decreased considerably at all frequencies towards the center of the current sheet (B ~ 0 nT). The electric energy density decreases 5 orders of magnitude from the edge of the current sheet (B = 19 nT) to the center and the magnetic energy density peaks within the current sheet (B = 13 nT) and is decreased by 2.5 orders of magnitude at the center. Within the current sheet, the electric and magnetic wave spectra were dominantly broadband electromagnetic noise (i.e., power law spectra with exponents ~ -1.4 and ~ -2.4, respectively) throughout the frequency range ~0.1-1000 Hz, spanning from MHD (i.e., ion cyclotron frequency ~0.2 Hz) to almost the electron plasma frequency (~4000 Hz). We argue that the wave activity is likely to be whistler wave turbulence and discuss the implications of these results for reconnection from wave-particle interactions.

New results on a steady state Vlasov theory of current sheets, which generalizes the Harris (1962) model by assuming anisotropic and nongyrotropic plasmas and using the invariant of particle motion in regions of strong gradients, are presented with the aim to explain multiprobe observations of thin current sheets in the geomagnetotail and laboratory experiments, including the effects of current sheet embedding and bifurcation. The dynamics of these sheets is explored using a full particle code with more realistic mass ratio and anisotropy parameters than those used in our earlier works. The results relevant to 2001 CLUSTER observations, with the sheet thickness appreciably exceeding the thermal ion gyroradius, include ion distributions and pressure tensor components, which reveal the important role of nongyrotropic effects on the structure of these sheets. Their flapping motions are distinguished by north-south asymmetry of current profiles, quasi-rectangular shape of the flapping waves, and their small propagation speed, suggesting an explanation of their propagation toward the flanks of the tail sheet. The main effect of the ion anisotropy on the sheets with thickness less than the thermal ion gyroradius, relevant to 2003 CLUSTER observations and laboratory experiments, is their charging, which may limit their minimum thickness, while their structure can be modified by electron anisotropy. Other distinctive features of these sheets are three-peaked current density profiles, found both in simulations and in the steady state theory, the north-south asymmetry of flapping sheets, and the shape of flapping waves, which is drastically different from the case of thicker sheets.

On May 10, 2002 the Cluster spacecraft (SC) encountered a ~450 km (five magnetosheath thermal proton gyro-radii) wide high-latitude magnetopause (MP). Magnetic field observations indicate the crossing of a ~130 km thick MP current sheet (CS) located inside a magnetic hole. Proton flux measurements diagnose a dense boundary layer (BL) directly attached to the MP and an additional rare BL located earthwards from the MP. Enhanced magnetic fluctuations are found near the local proton-cyclotron frequency Omega_cp (0.4-2 Hz). Applying the phase-differencing technique we obtained a wavelength of 150-250 km and the propagating direction earthward perpendicular to the MP. Inside the MP the pitch-angle proton distribution demonstrates the presence of a transverse population. The formation of the two BLs can be understood by enhanced collisionless diffusion of magnetosheath protons due to wave-particle interaction, while higher-energy protons (W_p > 300 eV) penetrate into the BLs also via finite gyro-radius effect.

Energetic electrons (E >= 30 keV) travelling along and perpendicular to the magnetic field lines have been observed in the magnetotail at L~17:00 and 22:00 MLT during the recovery phase of a storm-time substorm on 7 October 2002. Three-dimensional electron distributions of the full unit sphere obtained from the IES/RAPID sensor system demonstrated a rather complicated and random behavior of the energetic electrons. Occasionally these electrons were appearing to travel parallel, perpendicular, or in both directions, relative to the magnetic field direction, forming in this way bi-directional, perpendicular-peaked, and mixed distributions. The electron enhancements occurred while the Cluster spacecraft were on closed field lines in the central plasma sheet approaching the neutral sheet from the northern tail lobe. Magnetic field and energetic particle measurements have been used from geosynchronous and Cluster satellites, in order to describe the general context of the event and then give a possible interpretation regarding the occurrence of the electron anisotropies observed by the IES/RAPID spectrometer on board Cluster. According to geosynchronous measurements an electron dispersionless ejection is very well correlated with a dipolar re-configuration of the magnetic field. The latter fact supports the idea that electrons and, in general, particle ejections at geosynchronous altitude are directly related to electric fields arising from field dipolarization caused by current disruption. Also, having as a main objective the understanding of the way 3-D electron distributions are formed, we have analyzed electron energy spectra along and perpendicular to the magnetic field direction, demonstrating the fact that the electron population consists of two distinct components acting independently and in a random manner relative to each other.